Palladium(II) complexes containing N,N′-bidentate imine ligands derived from picolinaldehyde and substituted anilines: Synthesis, structure and polymerisation of methyl methacrylate

Accepted Manuscript

Palladium(II) complexes containing N,N'-bidentate imine ligands derived from

picolinaldehyde and substituted anilines: Synthesis, structure and polymerisa-

tion of methyl methacrylate

Suhyun Park, Jaegyeong Lee, Jong Hwa Jeong, Hyosun Lee, Saira Nayab

PII:

S0277-5387(18)30269-9

https://doi.org/10.1016/j.poly.2018.05.031

POLY 13178

DOI:

Reference:

Polyhedron

To appear in:

Received Date:

Accepted Date:

17 April 2018

14 May 2018

Please cite this article as: S. Park, J. Lee, J.H. Jeong, H. Lee, S. Nayab, Palladium(II) complexes containing N,N'-

bidentate imine ligands derived from picolinaldehyde and substituted anilines: Synthesis, structure and

polymerisation of methyl methacrylate, Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.2018.05.031

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Revised

Palladium(II) complexes containing N,N'-bidentate imine ligands derived

from picolinaldehyde and substituted anilines: Synthesis, structure and

polymerisation of methyl methacrylate

Suhyun Park^a, Jaegyeong Lee^a, Jong Hwa Jeong^a, Hyosun Lee^a,*and Saira Nayab^b,*

^aDepartment of Chemistry and Green-Nano Materials Research Centre, Kyungpook National

University, 80 Daehakro, Bukgu, Daegu, 41566, Republic of Korea

*E-mail address: hyosunlee@knu.ac.kr (H. Lee)

^bDepartment of Chemistry, Shaheed Benazir Bhutto University, Sheringal Dir (U), 18000,

Khyber Pakhtunkhwa, Islamic Republic of Pakistan

*E-mail address: sairanayab07@yahoo.com (S. Nayab)

----------------------------------

^*Corresponding author. Tel: +82-53-950-5337; fax: +82-53-950-6330

E-mail address: hyosunlee@knu.ac.kr (H. Lee)

E-mail address: sairanayab07@yahoo.com (S. Nayab)

1

Abstract

Palladium(II) complexes, L_nPdCl₂(L_n= L_A– L_I), with N,N'-bidentate imine ligands

derived from picolinaldehyde and substituted anilines have been synthesized and structurally

characterized. Molecular structures revealed a distorted square plane geometry around Pd(II)

centre in L_nPdCl₂(L_n= L_A– L_C) obtained via coordination with pyridine and imine nitrogens

and two chloro ligands. Pd(II) complexes L_nPdCl₂(L_n= L_A– L_I) initiate polymerisation of

methylmethacrylate (MMA) in the presence of modified methylaluminoxane (MMAO). The

complex L_IPdCl₂(of which the ligand was N-furfuryl substituted) showed the highest

catalytic activity for the polymerisation of MMA with an activity of 7.08 × 10⁴

g

PMMA/mol·Pd·h at 60 °C. All the complexes yielded syndio-rich poly(methyl methacrylate)

(PMMA) ([rr] = 0.70). Notably, the substituents on the imine moiety of the iminopyridine

fragments affects the activities towards MMA polymerization, whereas the stereoslectivities

remained unchanged.

Keywords: N,N'-bidentate imine ligands, Palladium(II) complexes, Square planar geometry,

Methyl methacrylate polymerisation, Syndiotacticity

2

1. Introduction

Metal-mediated polymerisation of methyl methacrylate (MMA) has received much

attention recently due to its ability to promptly produce high molecular weight (M_W)

poly(methylmethacrylate) (PMMA) with improved stereoregularity and optical properties [1].

There has been growing interest in the development of late transition metal initiators for the

polymerisation of polar vinyl monomers such as MMA [2-5]. Late transition metal catalysts

are preferred for polymerisation of polar monomers due to their reduced oxophilicity and

more functional group tolerance [6,7]. To date several reports have been documented on

iminopyridines and N-substituted-2-iminoalkylpyridines based late transition metal

complexes for MMA polymerisation. For instance, the Kim group demonstrated that late

transition metal complexes such as (α-diimine)nickel(II), (pyridyl bis-imine) iron(II) and

(pyridyl bis-imine)cobalt(II) proved to be effective catalysts in combination with MAO to

yield syndiotactic poly(methyl methacrylate) (PMMA) [3a]. The Gibson group studied well-

controlled polymerisation of both styrene and MMA with iminopyridine based iron

complexes [8]. Similarly, recently we have reported imino-pyridyl based Pd(II), Zn(II), Cd(II)

and Pt(II) initiators with a variety of pendant groups for MMA polymerisation. These

catalysts were effective and afforded high molecular weight PMMAs with mediocre to high

syndiotacticities [9]. In particular, palladium was the most active metal when MMA

polymerisation by any other metal was performed [10].

The catalyst performance (activity and stereoregularity) in the polymerisation can be

controlled through ligand modification as well as varying the identity of the central metal.

Thus, appropriate choice of ligand framework and metal centre is crucial for controlled

polymerisation. Schiff bases containing pyridine moiety such as pyridyl-imines [11],

bispyridine-imines [12], 2,6-bis(imino)pyridines ligands [13] and N-substituted 2-

3

iminoalkylpyridines [14,15] contributed significantly for their applications as ligands for a

variety of transition metal-based initiators [16-19] owing to their convenient synthesis and

thermal stability. Additionally, the asymmetry in N-substituted 2-iminopyridines ligands

allow easy optimization of the ligand scaffold by installing various pendant groups and to

fine tune the steric and electronic properties.

As a continuation of our efforts towards the synthesis of an effective MMA catalyst,

we herein report the synthesis and X-ray structures of Pd(II) complexes with N,N'-bidentate

imine ligands derived from picolinaldehyde and substituted anilines.These complexes were

assessed for MMA polymerisation in the presence of modified methylaluminoxane (MMAO).

Further, the effects of structural variation of the ligand on the activities of catalysts and

polymer microstructure are described.

2. Experimental

2.1. Physical measurement

2-Pyridinecarboxaldehyde, 2,6-diethylaniline, 2,4,6-trimethylaniline, 4-nitroaniline, 4-

bromoaniline, aniline, 2,6-dimethylaniline, p-anisidine, 4-fluoroaniline, furfurylamine,

magnesium sulfate (MgSO₄) and methyl methacrylate (MMA) were purchased from Aldrich.

PdCl₂was purchased from Kojima Chemicals and Pd(MeCN)₂Cl₂was prepared according to

the literature method [20]. Anhydrous solvents such as acetonitrile (MeCN), ethanol (EtOH),

dimethylformamide (DMF), diethyl ether (Et₂O) and dichloromethane (CH₂Cl₂) were

purchased from Merck and used without further purification. Modified methylaluminoxane

(MMAO) was purchased from Tosoh Finechem Corporation as 5.90 % weight aluminum of a

toluene solution and used without further purification. Elemental analysis (C, H, N) of the

4

prepared complexes were carried out on an elemental analyzer (EA 1108; Carlo-Erba, Milan,

1

Italy). Melting point was measured by electro-thermal apparatus, IA 9100. H NMR (500

MHz) and ¹³C NMR (125 MHz) spectra were reported on a Bruker Avance Digital 500 NMR

spectrometer; chemical shifts were recorded in ppm units (δ) relative to SiMe₄as the internal

standard. Infrared (IR) spectra were recorded on Bruker FT/IR-Alpha (neat) and the data are

reported in reciprocal centimeters (cm^-1). The weight average molecular weight (M_n) and

molecular weight distribution (M_w/M_n) of the obtained poly(methylmethacrylate) (PMMA)

were carried out using gel permeation chromatography (GPC) in THF (Alliance e2695;

Waters Corp., Milford, MA). Glass transition temperature (T_g) was determined using a

thermal analyzer (DSC 4000; PerkinElmer).

2.2. Preparation of ligands and palladium(II) complexes

(E)-2,6-Diethyl-N-((pyridin-2-yl)methylene)aniline (L_A), (E)-2,4,6-trimethyl-N-

((pyridin-2-yl)methylene)aniline (L_B), (E)-4-nitro-N-((pyridin-2-yl)methylene)aniline (L_C),

(E)-4-bromo-N-((pyridin-2-yl)methylene)aniline

yl)methylene)aniline (L_E), (E)-2,6-dimethyl-N-((pyridin-2-yl)methylene)aniline (L_F), (E)-4-

methoxy-N-((pyridin-2-yl)methylene)aniline (L_G), (E)-4-fluoro-N-((pyridin-2-

yl)methylene)aniline (L_H) and (E)-1-(furan-2-yl)-N-(pyridin-2-ylmethylene)methanamine (L_I)

were prepared as reported previously [16c,21-23,35]. N-((pyridin-2-

chloride

(L_D),

(E)-N-((pyridin-2-

yl)methylene)anilinepalladium(II)

(L_EPdCl₂)

(L_FPdCl₂)

2,6-dimethyl-N-((pyridin-2-

4-methoxy-N-((pyridin-2-

yl)methylene)anilinepalladium(II) chloride (L_GPdCl₂) 4-fluoro-N-((pyridin-2-yl)methylene)

anilinepalladium(II)chloride

(L_HPdCl₂)

(furan-2-yl)-N-((pyridin-2-

5

yl)methylene)methanaminepalladium(II) chloride (L_IPdCl₂)

were synthesized using

reported synthetic protocol [24-28].

2.2.1. 2,6-Diethyl-N-((pyridin-2-yl)methylene)anilinepalladium(II) chloride (L_APdCl₂)

L_A(1.00 g, 4.20 mmol) was added to a solution of Pd(MeCN)₂Cl₂(1.09 g, 4.20

mmol) in CHCl₃(50 mL), and the resulting mixture was stirred at room temperature for 14 h.

The resultant orange precipitate was washed with cold CHCl₃(30 mL × 2), followed by

washing with Et₂O (30 mL × 3). The precipitate was dried in vacuo to get final product

(1.15 g, 65.9 %). Analysis calculated for C₁₆H₁₈Cl₂N₂Pd (%): C, 46.2; H, 4.36; N, 6.74.

Found: C, 46.7; H, 4.35; N, 6.73. mp (^oC): 318. ¹H NMR (DMSO-d₆, 500 MHz): δ 9.08 (dd,

3

1H, J = 5.6, 1.0 Hz, –PryH–), 8.77 (s, 1H, –N=CH–PyrH–), 8.43 (td, 1H, J = 7.8, 1.0 Hz, –

PryH–), 8.20 (dd, 1H, J = 7.6, 0.8 Hz, –PyrH–), 8.00 (ddd, 1H, J = 7.8, 1.0 Hz, –PyrH–), 7.29

(t, 1H, J = 7.63 Hz, –(MeCH₂)₂ArH–), 7.15 (d, 2H, J = 7.6 Hz, –(MeCH₂)₂ArH–), 2.77 – 2.61

(m, 4H, –(MeCH₂)₂ArH–), 1.21 (t, 6H, J = 7.5 Hz, –(MeCH₂)₂ArH–). ¹³C NMR (DMSO-d₆,

125 MHz): δ 174.52 (d, J = 185 Hz, –N=CH–PyrC–), 154.97 (s, ipso-N=C–PyrC–), 150.44 (d,

J = 190 Hz, –PyrC–), 144.51 (s, 1C, ipso-(MeCH₂)₂ArC–), 141.48 (d, J = 171 Hz, –PyrC–),

135.58 (s, ipso-(MeCH₂)₂ArC–), 129.89 (d, J = 172 Hz, –PyrC–), 129.80 (d, J = 172 Hz, –

PyrC–), 127.91 (d, J = 162 Hz, p-(MeCH₂)₂ArC–), 125.51 (d, J = 160.76 Hz, m-

(MeCH₂)₂ArC–), 24.04 (t, J = 127 Hz, –(MeCH₂)₂ArC–), 13.90 (q, J = 127 Hz, –

(MeCH₂)₂ArC–). IR (solid neat; cm^-1): 2997 w(C-H), 1591 m(C=N), 1464 w(C=C), 595

w(Pd–N).

2.2.2. 2,4,6-Trimethyl-N-((pyridin-2-yl)methylene)anilinepalladium(II) chloride (L_BPdCl₂)

L_BPdCl₂was prepared according to a similar procedure as described for L_APdCl₂

except utilizing L_B(0.70 g, 3.12 mmol) and Pd(MeCN)₂Cl₂(0.809 g, 3.12 mmol) in MeCN

6

(50 mL) to yield an orange solid (1.18 g, 94.2 %). Analysis calculated for C₁₅H₁₆Cl₂N₂Pd (%):

C, 44.9; H, 4.02; N, 6.97. Found: C, 44.8; H, 3.96; N, 6.91. mp (^oC): 293. ¹H NMR (DMSO-

d₆, 500 MHz): δ 9.06 (dd, 1H, J = 5.6, 0.9 Hz, –PyrH–), 8.63 (s, 1H, –N=CH–Pyr–), 8.42 (td,

1H, J = 7.8, 1.0 Hz, –PyrH–), 8.16 (dd, 1H, J = 7.7, 0.7 Hz, –PyrH–), 7.98 (ddd, 1H, J = 7.8,

1.1 Hz, –PyrH–), 6.92 (s, 2H, –(Me)₃ArH–), 2.24 (s, 3H, –(Me)₃ArH–), 2.23 (s, 6H, –

(Me)₃ArH–). ¹³C NMR (DMSO-d₆, 125 MHz): δ 174.64 (d, J = 185 Hz, –N=CH–PyrC–),

155.13 (s, ipso-PyrC–) 150.18 (d, J = 189 Hz, –PyrC–), 143.40 (s, ipso-(Me)₃ArC–), 141.24

(d, J = 171 Hz, –PyrC–), 136.50 (s, ipso-(Me)₃ArC–), 129.80 (s, ipso-(Me)₃ArC–), 129.67 (d,

J = 172 Hz, –PyrC–), 129.50 (d, J = 173 Hz, –PyrC–), 128.12 (d, J = 163 Hz, m-(Me)₃ArC–),

20.53 (q, J = 126 Hz, –(Me)₃ArC–), 18.12 (q, J = 127.16 Hz, –(Me)₃ArC–). IR (solid neat;

cm^-1): 2991 w(C-H), 1585 m(C=N), 1468 w(C=C), 597 w(Pd–N).

2.2.3. 4-Nitro-N-((pyridin-2-yl)methylene)anilineepalladium(II) chloride (L_CPdCl₂)

L_CPdCl₂was prepared according to a similar procedure as described for L_APdCl₂

except utilizing L_C(0.80 g, 3.52 mmol) and Pd(MeCN)₂Cl₂(0.913 g, 3.52 mmol) in MeCN

to give orange solid (1.38 g, 96.9 %). Analysis calculated for C₁₂H₉Cl₂N₃O₂Pd (%): C, 35.6;

H, 2.24; N, 10.39. Found: C, 35.9; H, 2.29; N, 10.37. mp (^oC): 301. ¹H NMR (DMSO-d₆, 500

MHz): δ 9.07 (dd, 1H, J = 5.6, 1.2 Hz, –PyrH–), 8.87 (s, 1H, –N=CH–Pyr–), 8.43 (td, 1H, J =

7.8, 1.5 Hz, –PyrH–), 8.34 (m, 2H, –NO₂ArH–), 8.25 (dd, 1H, J = 7.8, 1.6 Hz, Hz, –PyrH–),

7.98 (dd, 1H, J = 7.7, 0.8 Hz, –PyrH–), 7.68 (m, 2H, –NO₂ArH–). ¹³C NMR (DMSO-d₆, 125

MHz): δ 174.10 (d, J = 187 Hz, –N=CH–PyrC–), 155.64 (s, ipso-NO₂ArC–), 151.64 (s, ipso-

PyrC–), 150.18 (d, J = 189 Hz, PyrC–), 146.75 (s, ipso-NO₂ArC–), 141.39 (d, J = 172 Hz,

PyrC), 130.00 (d, J = 175 Hz, –PyrC–), 129.56 (d, J = 174 Hz, –PyrC–), 125.53 (d, J = 171

Hz, –NO₂ArC–), 123.71 (d, J = 171 Hz, –NO₂ArC–). IR (solid neat; cm^-1): 3101 w(C-H),

7

1581 m(C=N), 1481 w(C=C), 686 w(Pd–N).

2.2.4. 4-Bromo-N-((pyridin-2-yl)methylene)anilinepalladium(II) chloride (L_DPdCl₂)

L_DPdCl₂was prepared according to a similar procedure as described for L_APdCl₂

except utilizing L_D(1.00 g, 3.83 mmol) and Pd(MeCN)₂Cl₂(0.99 g, 3.83 mmol) in MeCN to

yield a yellowish orange solid (1.59 g, 94.7 %). Analysis calculated for C₁₂H₉BrCl₂N₂Pd (%):

C, 32.9; H, 2.07; N, 6.39. Found: C, 33.3; H, 2.06; N, 6.57. mp (^oC): 310. ¹H NMR

(DMSO-d₆, 500 MHz): δ 9.05 (dd, 1H, J = 5.5 Hz, –PyrH–), 8.76 (s, 1H, –N=CH–PyrH–),

8.40 (td, 1H, J = 7.8, 1.2 Hz, –PyrH–), 8.21 (dd, 1H, J = 7.8, 1.6 Hz, –PyrH–), 7.95 (m, 1H, –

PyrH–), 7.67 (m, 2H, –BrArH–), 7.38 (m, 2H, –BrArH–). ¹³C NMR (DMSO-d₆, 125 MHz): δ

172.98 (d, J = 185 Hz, –N=CH–PyrC–), 155.75 (s, –PyrC–), 150.09 (d, J = 190 Hz, PyrC–),

146.02 (s, –ArC–), 141.32 (d, J = 170 Hz, –PyrC–), 131.10 (d, J = 169 Hz, –BrArC–), 129.62

(d, J = 172 Hz, –PyrC–), 129.21 (d, J = 172 Hz, –PyrC–), 126.16 (d, J = 167 Hz, –BrArC–),

121.50 (s, ipso-BrArC–). IR (solid neat; cm^-1): 3074 w(C-H), 1585 m(C=N), 1480 w(C=C),

574 w(Pd–N).

2.3. X-Ray crystallographic studies

A yellowish cubic-shaped crystal of L_APdCl₂was picked up with paraton-N oil and

mounted on a Bruker SMART CCD diffractometer equipped with a graphite-monochromated

Mo-Kα (λ = 0.71073 Å) radiation source under nitrogen cold stream (200(2) K). Data

collection and integration were performed with SMART (Bruker, 2000) and SAINT-Plus

(Bruker, 2001) software packages [29]. Semi-empirical absorption corrections based on

equivalent reflections were applied by SADABS [30]. Structures were solved by direct

8

methods and refined using a full-matrix least-squares method on F²using SHELXTL or

SHELXTL-2016 [31]. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms

were added to their geometrically ideal positions. Similarly, the X-ray-quality single crystal

was coated with paratone-N oil and the diffraction data measured at 100(2) K with

synchrotron radiation (λ = 0.61000 Å) for L_BPdCl₂and 100(2) K with synchrotron radiation

(λ = 0.62073 Å) for L_CPdCl₂on an ADSC Quantum-210 detector at 2D SMC with a silicon

(111) double crystal monochromator (DCM) at the Pohang Accelerator Laboratory, South

Korea. The ADSC Q210 ADX program [32] was used for data collection (detector distance is

63 mm, omega scan; ∆ω = 1 º, exposure time is 1 sec per frame) and HKL3000sm (Ver. 703r)

[33] was used for cell refinement, reduction and absorption correction. (T_min= 0.868, T_max

=

0.962). Structures were solved by direct methods, and refined by full-matrix least-squares

refinement using the SHELXL-2014 [34] computer program. The positions of all non-

hydrogen atoms were refined with anisotropic displacement factors. All hydrogen atoms were

placed using a riding model, and their positions were constrained relative to their parent

atoms using the appropriate HFIX command in SHELXL-2014, SHELXL-2016 computer

program. Crystallographic data and structural refinements for the Pd(II) complexes L_nPdCl₂

(L_n= L_A– L_C) are summarized in Table S1.

2.4. Catalytic activity for MMA polymerisation

Methyl methacrylate (MMA) was dried over molecular sieves and distilled under

reduced pressure before use. In a Schlenk line, a complex (15.0 µmol, 6.20 mg for L_APdCl₂,

6.00 mg for L_BPdCl₂, 6.10 mg for L_CPdCl₂, 6.60 mg for L_DPdCl₂, 5.40 mg for L_EPdCl₂,

5.80 mg for L_FPdCl₂, 5.80 mg for L_GPdCl₂, 5.70 mg for L_HPdCl₂and 5.50 mg for L_IPdCl₂)

9

was dissolved in dried toluene (10 mL) followed by the addition of modified

methylaluminoxane (MMAO) (5.90 wt% in toluene, 3.80 mL, 7.50 mmol and

[MMAO]₀/[M(II) catalyst]₀=500) as a co-catalyst. The solution was stirred for 20 min at

60 °C. MMA (5.0 mL, 47.1 mmol and [MMA]₀/[M(II) catalyst]₀= 3100) was added to the

above reaction mixture and stirred for 2 h to obtain a viscous solution. For termination MeOH

(2 mL) was added to polymerisation solution. The reaction mixture was poured into a large

quantity of MeOH (500 mL), and 35.0 % HCl (5 mL) was injected to remove the remaining

co-catalyst (MMAO). The resultant PMMA was obtained by filtration following by washing

with MeOH (250 mL × 2) and dried under vacuum at 60 °C for 12 h.

3. Results and Discussion

3.1. Synthesis and chemical properties

Ligands (L_n= L_A– L_I) were obtained in yields of 60 –87% from the condensation

reaction of X-aniline and furfurylamine (X = 2,6-dimethyl, 2,6-diethyl, 4-nitro, 4-bromo,

2,4,6-trimethyl, 4-methoxy, 4-fluoro) with 2-pyridinecarboxaldehyde in an appropriate

solvent as reported previously [16c,21-23,35]. These ligands with variety of substituents at

imine moiety were used in the current study to investigate their steric and electronic

properties of resultant Pd(II) complexes.

The new mononuclear Pd(II) complexes L_nPdCl₂(L_n= L_A– L_D) (62 – 97 % yield)

were obtained by treating ligands (L_n= L_A– L_I) with Pd(MeCN)₂Cl₂in 1:1 ratio at ambient

1

temperature (Scheme 1). The synthesized complexes were characterized by H NMR, ¹³C

1

NMR and IR spectroscopy along with X-ray diffraction. In H NMR spectra of Pd(II)

complexes, L_nPdCl₂(L_n= L_A– L_D), singlet peaks between 8.63 and 8.87 ppm were

10

indicative of imine moiety [35]. Similarly in ¹³C NMR spectra the imine carbon appeared in

range of 174.64 – 172.98 ppm consistent with the reported imino-pyridyl Pd(II) complexes

[36]. The absorption band for imine functionality (> C=N-) in IR spectra is typically between

1581 and 1591cm^-1. Absorption bands at 686 – 574 cm⁻¹are due to formation of Pd–N bonds

in L_nPdCl₂(L_n= L_A− L_D). Elemental analyses of newly synthesized complexes were

consistent with the proposed structures shown in Scheme 1 and confirmed the purity of the

isolated complexes L_nPdCl₂(L_n= L_A− L_D). The reported synthetic protocol has been

followed for the synthesis of L_nPdCl₂(L_n= L_E– L_I) [24-28].

3.2. Description of X-ray crystal structures

Crystals suitable for the X-ray diffraction studies of [L_nPdCl₂] (L_n= L_A– L_C) were

obtained from Et₂O diffusion into DMF solution of complexes. L_APdCl₂and L_BPdCl₂

crystallized in the monoclinic system with P2₁/c space group, whereas L_CPdCl₂crystallized

in the triclinic system with P-1 space group. The ORTEP drawings of L_nPdCl₂(L_n= L_A– L_C)

are shown in Fig. 1-3 along with their selected bond lengths and angles. L_APdCl₂and

L_BPdCl₂were found to be solvent-free and existed in a four coordinated monomeric form.

We failed to get X-ray quality crystals of L_DPdCl₂.

Pd–N_pyridineand Pd–N_iminebond lengths in L_nPdCl₂(L_n= L_A– L_C) lie in the range of

2.0294(1) - 2.038(2) Å and 2.0219(1) - 2.039(2) Å, respectively, similar to the Pd–N bond

lengths of square planar pyridyl-imine Pd(II) complexes [17a,19a,25,27,28]. The Pd–Cl bond

lengths in 2.2724(9) - 2.2896(5) Å range compared well with the average bond length of

2.290(13) Å observed for similar Pd(II) complexes [37]. The Pd–N_iminelength increased in

order of L_BPdCl₂< L_APdCl₂< L_CPdCl₂, ranging in length from 0.003 – 0.016 Å. The Pd–

11

N_pyridinebond lengths of L_APdCl₂and L_BPdCl₂were found to be longer than Pd–N_imine

lengths. In contrast, the Pd–N_pyridinelength was shorter than Pd–N_iminein L_CPdCl₂(Fig. 3).

The double bond N(2)–C(6) distances of 1.298(8) Å (L_APdCl₂), 1.2899(2) Å (L_BPdCl₂) and

1.291(3) Å (L_CPdCl₂) were in the range of accepted >C=N- bond distances [25,38]. The

C(5)–C(6) bond distances of the complexes L_nPdCl₂(L_n= L_A– L_C) were in the ranged

1.441(8) – 1.432(3) Å, reflecting delocalized π-electrons and the bond lengths of the

synthesized palladium complexes were slightly affected by the N-phenyl substituent group.

The central palladium atom in L_nPdCl₂(L_n= L_A– L_C) was four coordinated and

adopted distorted square planner geometry by coordinating to the nitrogen atoms of pyridyl

and imine moieties of N,N-bidentate ligands and two chloro ligands. The N_Pyridine–Pd(1)–

N_iminebond angles in L_nPdCl₂(L_n= L_A– L_C) ranged from 80.31(5)° to 80.6(2)° as observed

in reported pyridyl-imine palladium systems [18b,25,28,38]. The Cl(1)–Pd(1)–Cl(2) angles

were 91.71(7)° L_APdCl₂, 91.20(2)° L_BPdCl₂and 90.05(3)° L_CPdCl₂and were slightly

affected by the substituents of the imine moiety. The N(1)–Pd(1)–Cl(2) and N(2)–Pd(1)–Cl(1)

angles for the complexes L_nPdCl₂(L_n= L_A– L_C) were nearly linear in the range of 173.90(6)°

– 175.47(6)° [28]. Interestingly, the planes of the N-phenyl group and the plane of palladium

and pyridine were perpendicular in L_APdCl₂and L_BPdCl₂(each 84.3°, 79.6°). Moreover, the

plane of the N-nitrophenyl group and the plane of palladium and pyridine in L_CPdCl₂were

only slightly twisted by ca. 57.2° rather than 84.3° for L_APdCl₂. In addition, we also

observed the degree of distortion of N-((pyridin-2-yl)methylene)amine ligands and Pd(II)

complexes by the new 4-coordinate structure index (τ₄). The new geometric parameters for

the four coordinate compounds (τ₄) are presented in Table S2 as improved simple metrics for

quantitatively evaluating the geometry of the four coordinate complexes and compounds [39].

In Table S2, the τ₄value of [L_nPdCl₂] (L_n= L_A– L_C, L_Fand L_I) showed a square planar

12

distorted by 0.0753 – 0.0819.

3.3. MMA polymerisation

The catalytic capabilities of synthesized Pd(II) complexes were assessed towards the

polymerisation of MMA in the presence of MMAO. The polymerisation was conducted at

60 °C and all the complexes L_nPdCl₂(L_n= L_A– L_I) polymerized MMA yielding PMMA with

T_g(glass transition temperature) ranging from 120 to 131 °C. The resultant polymers (PMMA)

were isolated as a white solid and characterized by GPC in THF using standard polystyrene

as the reference. The major polymer characteristics are summarized in Table 1.

Metal complexes with appropriate ligand architecture are vital for accomplishing a high

degree control of polymerisation process due to the fact that metal complexes influence all

steps of polymerisation such as initiation, propagation, and termination [8,40,41]. It is evident

from polymerisation data that substituents on imine moiety influence the catalytic efficacy

which was determined by the relative amounts of PMMA product. The presence of sterically

demanding substituents around the metal centre is presumed to have a negative effect on

MMA activation as the steric bulk might inhibit monomer access to the metal centre [42]. For

instance, L_APdCl₂with a 2,6-diethylphenyl substituent on the imine moiety exhibited lower

catalytic activity compared with L_BPdCl₂with a 2,6-dimethylphenyl substituent on the imine

moiety (Table 1; entries 4 & 5). The iminopyridine-based Pd(II) complexes showed activity

in the order of L_CPdCl₂> L_BPdCl₂> L_APdCl₂, depending on the steric bulk provided by the

ortho substituents on phenyl group of imine moiety (Fig 1-3) around the metal centre. Further,

the orientation of the imine substituents to the xy-plane of the pyridine ring and metal centre

also influence the activity towards MMA polymerisation. L_APdCl₂with the 2,6-

13

diethylphenyl group that is perpendicular and slightly twisted toward the xy plane of the N-

((pyridin-2-yl)methylene)amine-Pd moiety by 84.3° has greater steric hindrance and

2,4,6-

exhibited lower activity than L_BPdCl₂, in which the distortion angle of the

trimethylphenyl

group towards the xy plane of the N-((pyridin-2-yl)methylene)amine-Pd

moiety was by 79.6°. Further L_BPdCl₂exhibited lower activity than L_CPdCl₂, in which the

4-nitrophenyl

distortion angle of the

group towards the xy plane of the N-((pyridin-2-

yl)methylene)amine-Pd moiety was by 57.2°. It can be seen that ortho ethyl substituent

2,6-

receives some degree of steric hindrance compared to other ligands. Thus, both of

diethylphenyl

substituted ring and metallacyclic ring are almost perpendicular. Further the

was calculated as

distance of C(15)-Pd(1) and C(9)-Pd(1) in L_APdCl₂

3.755 and 4.025 Å to

2,6-diethylphenyl

give obvious steric hindrance of

substituent ring than phenyl substituent

ring in the N-((pyridin-2-yl)methylene)amine-Pd(II) complexes.

To evaluate the total steric hindrance by bulky ligands to metal centre, the program

"SambVca" [43] was used to compare and predict and quantitatively calculate the

topographic steric map of the Pd(II) complexes. Fig. S1 shows ball and stick model, space-

filling model and topographic steric map of L_nPdCl₂(L_n= L_A–L_C, L_F, L_I) to provide a three

dimensional bulk ligand. Only considering total steric factor by topographic steric map of

[L_nPdCl₂] (L_n= L_A– L_C), [L_CPdCl₂] (buried volume % is 35.4), which has a smaller steric

hindrance than L_APdCl₂(buried volume % is 39.9), has higher polymerisation activity

(L_CPdCl₂: 4.79 × 10⁴g PMMA/mol·Pd·h > L_APdCl₂: 4.00 × 10⁴g PMMA/mol·Pd·h).

Similarly L_APdCl₂(buried volume % is 39.9, 4.00 × 10⁴g PMMA/mol·Pd·h) < L_BPdCl₂

(buried volume % is 39.8, 4.30 × 10⁴g PMMA/mol·Pd·h) < L_CPdCl₂(buried volume % is

35.4, 4.79 × 10⁴g PMMA/mol·Pd·h). When L_nPdCl₂was used as a polymerisation catalyst,

L_IPdCl₂exhibited the highest catalytic activity (7.08 × 10⁴g PMMA/mol·Pd·h). It is

14

evident from Fig. S1 that L_CPdCl₂(buried volume % is 35.4, 4.79 × 10⁴

g

PMMA/mol·Pd·h) with lesser steric bulk exhibited lower activity than L_IPdCl₂(buried

volume % is 35.9, 7.08 × 10⁴g PMMA/mol·Pd·h). These catalytic systems clearly showed

steric and electronic effects of Pd(II) complexes during MMA polymerisation. For instance,

L_FPdCl₂showed better catalytic activity (buried volume % is 39.8, 5.44 × 10⁴

g

PMMA/mol·Pd·h) than L_BPdCl₂(buried volume % is 39.8, 4.30 × 10⁴g PMMA/mol·Pd·h)

with identical steric bulk (Fig. S1). Thus, both steric and electronic effects of initiators are

needed to correlate the polymerisation activity and structure of L_nPdCl₂(L_n= L_A– L_I)

complexes. It is evident from polymerisation data (Table 1) that L_EPdCl₂despite of having

unsubstituted phenyl group at imine moiety showed a profound loss of catalytic activity

under same experimental conditions. Moreover, the lowest catalytic activity of L_DPdCl₂

(Table 1; entry 7) compared to rest of its analogous, i.e. L_nPdCl₂(L_n= L_A– L_Cand L_E– L_I),

was due to its poor solubility in the reaction media. Hence the solubility of the catalytic

species in the reaction solution also influences the catalytic activities.

In comparison to our previously reported Pd(II) complexes bearing N-substituted 2-

iminoalkylpyridines,

N,N-di(2-picolyl)cycloalkylamine,

and

N-substituted

2-

iminomethylpyridine- and 2-iminomethylquinoline coordinated palladium(II) [9c-d], the

current system exhibited better polymerisation control under same experimental control.

Similarly, the Pd(II) complexes in the current study are faster (activity 7.08 × 10⁴g

PMMA/mol·Pd·h) and yielded higher molecular weight PMMA (11.0 ×10⁵g/mol) with

moderate stereoselectivities compared with Zn(II) initiators bearing N-cycloalkyl 2-

iminomethylpyridine- and 2-iminomethylquinoline-based ligands (activity 3.33 × 10⁴g

PMMA/mol·Zn·h) [9a].

The blank polymerisation of MMA was processed with PdCl₂and Pd(MeCN)₂Cl₂

15

in presence of MMAO or MAO only under identical experimental conditions (Table 1;

entries 1-3). It is evident that the synthesized Pd(II) complexes garnered higher activities with

better polymerisation control and better selectivities compared to starting material. Thus, the

activity of these Pd(II) complexes toward MMA polymerisation is influenced by electronic

and steric effect of ligand framework which may induce steric hindrance and electronic

differences on the palladium centre. This result is comparable with previous Pd(II) complexes

containing the N,N′-bidentate N-substituted 2-iminomethylpyridines and N,N′,N-tridentate

N,N-di(2-picolyl)cycloalkylamine, which showed N-substitution’s steric effects and

electronic effects of ligands. The PDIs of the Pd(II) complexes in the current system are in

the range 2.16 – 3.10 at 60 °C (Table 1). In general, the PDI range narrowed as the molecular

weight of PMMA increased [44,45]. L_EPdCl₂yielded the narrowest PDI and the highest

molecular weight of PMMA at 60 °C (Table 1; entry 8).

The tacticity of PMMA was determined in the range around syndiotactic (rr, δ 0.85),

1

heterotactic (mr, δ 1.02), and isotactic (mm, δ 1.21), based on H NMR data [46].

Unexpectedly, the stereoselectivity and PDIs were not strongly affected by the substituents

attached to imine moiety in our current study. All the Pd(II) complexes L_nPdCl₂(L_n= L_A–

L_I) yielded syndiotactic PMMA. Syndiotacticity was not sufficiently high to offer a

mechanism for the coordination polymerisation but was similar for all the synthesised Pd(II)

complexes. In addition, experiments using 2,6,6,6-tetramethylpiperidinyloxy (TEMPO), a 5-

equiv. radical inhibitor during the MMA polymerisation, did not significantly reduce the yield

of PMMA and showed a yield reduction of at least 2 % up to 5 % supporting the mechanism

of coordination polymerisation is not a radical mechanism [47]. Fe(II) complexes bearing pyr

idylmethylamine [5a,8] polymerized MMA with moderate activity and syndiotacticity.

S i m i l a r l y , Cu(II) complexes bearing N-(2-furanylmethyl)-N-(1-3,5-dimethyl-1H-

16

pyrazolylmethyl)-N-(phenylmethyl)amines yielded syndiotactic PMMA (rr = 0.78) with only

30% conversion [48]. Recently, lanthanide metal organosamarium(II) complex with bispy

rrolylaldiminato ligand produces isotactic PMMA with an mm value of 98% [49].

Moreover, it is evident from polymerization data that steric hindrance of substituents at the

imine moiety significantly affected the activity of these complexes towards MMA

polymerisation. However, slight the molecular weights of resultant PMMAs have been

increased slightly with decrease of the steric bulk at imine moiety. We are currently carrying

out further investigations to improve the catalytic activity in terms of ligand framework

variations and the resultant stereoregularities.

4. Conclusions

A series of Pd(II) complexes, [L_nPdCl₂] (L_n= L_A–L_I) have been synthesized and

structurally characterized. The coordination geometry around the Pd(II) centres in

iminopryridine-Pd(II) complexes were distorted square-planar. L_IPdCl₂exhibited high

activity (7.08 ×10⁴g PMMA/mol·Pd·h) in the presence of MMAO at 60 °C towards MMA

polymerisation. Moreover, it has been observed that steric hindrance of substituents at the

imine moiety plays a major role in determining the catalytic efficacy of these complexes for

MMA polymerisation. The catalytic activity of complexes with less steric hindrance is higher

than complexes with sterically demanding substituents at imine moiety. However, the

syndiotacticity and molecular weights of resultant PMMs were not significantly affected by

ligand architecture.

5. Supplementary materials

17

CCDC 1837589–1837591 contain the supplementary crystallographic data for complexes

L_APdCl₂, L_BPdCl₂and L_CPdCl₂, respectively. The data can be obtained free of charge via

http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic

Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:

deposit@ccdc.cam.ac.uk.

Acknowledgments

This research was supported by the National Research Foundation of Korea (NRF), funded

by the Ministry of the Education, Science, and Technology (MeST) (Grant No.

2017R1A2B4002100).

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23

Scheme 1. Synthetic route of Pd(II) complexes bearing picolinaldehyde based imine ligands,

L_nPdCl₂(L_n= L_A– L_I).

Table 1. Polymerisation of MMA by L_nPdCl₂(L_n= L_A– L_I) in the presence of MMAO.

e

Entry

Catalyst^a

Yield^bActivity^c

T_g^d

Tacticity

M_w

PDI^f

(g/mol-

(g)

(℃)

%mm %mr

%rr

(g/mol)×10⁵

9.27

Cat·h)×10⁴

g

PdCl₂

12.6

18.8

8.97

25.5

27.5

1.97

2.93

1.40

4.00

4.30

129

10.2

7.60

37.2

9.82

8.33

23.5 67.7

22.8 69.6

10.9 51.9

21.5 68.7

22.2 69.4

1.58

2.20

2.09

3.10

2.33

1

2

3

4

5

g

Pd(MeCN)₂Cl₂

131

120

121

120

0.61

6.78

11.0

11.2

MMAO^h

L_APdCl₂

L_BPdCl₂

24

30.7

14.8

22.9

34.9

22.9

26.2

45.4

4.79

2.30

3.33

5.44

3.58

4.09

7.08

121

122

124

121

128

123

122

8.82

8.33

10.3

9.68

10.3

9.09

8.82

23.5 67.6

25.0 66.7

20.5 69.2

22.6 67.7

20.7 69.0

21.2 69.7

23.5 67.7

11.0

8.39

12.5

8.76

10.8

12.1

8.45

2.38

2.31

2.16

2.24

2.96

2.40

2.52

6

7

L_CPdCl₂

L_DPdCl₂

j

8

L_EPdCl₂

i

9

L_FPdCl₂

j

L_GPdCl₂

10

11

12

L_HPdCl₂

i

L_IPdCl₂

a

[Pd(II) catalyst]₀= 15 µmol, and [MMA]₀/[MMAO]₀/[Pd(II) catalyst]₀= 3100:500:1, polymerisation temp.

= 60 °C and time = 2 h.

^bYield defined a mass of dried polymer recovered/mass of monomer used.

^cActivity is calculated (g of PMMA)/(mol·Pd·h).

^dT_gis glass transition temperature which is determined by a thermal analyzer.

e

Determined by gel permeation chromatography (GPC) eluted with THF at room temperature by filtration

with polystyrene calibration.

^fM_nrefers the number average of molecular weights of PMMA.

^gIt is a blank polymerisation in which PdCl₂and Pd(MeCN)₂Cl₂were also activated by MMAO.

^hIt is a blank polymerisation which was done solely by MMAO.

ⁱComplexes of L_FPdCl₂and [L_IPdCl₂are reported in our earlier work [25,28], however, data in Table 1

comes from this work.

^jComplexes of L_EPdCl₂, L_FPdCl₂, L_GPdCl₂, L_HPdCl₂and L_IPdCl₂are reported in the literature [24-28].

25

Fig. 1. ORTEP drawing of L_APdCl₂with thermal ellipsoids at 30% probability. All hydrogen

atoms are omitted for clarity. Bond lengths (Å) and angles (°), Pd(1)–N(2) 2.023(5), Pd(1)–

N(1) 2.035(5), Pd(1)–Cl(2) 2.273(2), Pd(1)–Cl(1) 2.283(2); N(2)–Pd(1)–N(1) 80.6(2), N(2)–

Pd(1)–Cl(2) 93.67(2), N(1)–Pd(1)–Cl(2) 174.27(2), N(2)–Pd(1)–Cl(1) 174.23(2), N(1)–

Pd(1)–Cl(1) 94.02(2), Cl(2)–Pd(1)–Cl(1) 91.71(7).

26

Fig. 2. ORTEP drawing of L_BPdCl₂with thermal ellipsoids at 70% probability. All hydrogen

atoms are omitted for clarity. Bond lengths (Å) and angles (°), Pd(1)–N(2) 2.0219(1), Pd(1)–

N(1) 2.0294(1), Pd(1)–Cl(2) 2.2850(6), Pd(1)–Cl(1) 2.2896(5); N(2)–Pd(1)–N(1) 80.31(5),

N(2)–Pd(1)–Cl(2) 94.25(4), N(1)–Pd(1)–Cl(2) 174.53(3), N(2)–Pd(1)–Cl(1) 174.55(4), N(1)–

Pd(1)–Cl(1) 94.24(4), Cl(2)–Pd(1)–Cl(1) 91.20(2).

27

Fig. 3. ORTEP drawing of L_CPdCl₂with thermal ellipsoids at 30% probability. All hydrogen

atoms are omitted for clarity. Bond lengths (Å) and angles (°), Pd(1)–N(2) 2.039(2), Pd(1)–

N(1) 2.038(2), Pd(1)–Cl(2) 2.2724(9), Pd(1)–Cl(1) 2.2871(9), N(2)–Pd(1)–N(1) 80.35(9);

N(2)–Pd(1)–Cl(2) 95.53(6), N(1)–Pd(1)–Cl(2) 175.47(6), N(2)–Pd(1)–Cl(1) 173.90(6), N(1)–

Pd(1)–Cl(1) 94.15(7), Cl(2)–Pd(1)–Cl(1) 90.05(3).

28

Graphical Abstract (Pictorial)

29

Article Doi

Palladium(II) complexes containing N,N′-bidentate imine ligands derived from picolinaldehyde and substituted anilines: Synthesis, structure and polymerisation of methyl methacrylate

DOI: 10.1016/j.poly.2018.05.031

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Article abstract of DOI:10.1016/j.poly.2018.05.031

Full text of DOI:10.1016/j.poly.2018.05.031

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