Ruthenium(II) complexes of Schiff base derived from cycloalkylamines as pre-catalysts for ROMP of norbornene and ATRP of methyl methacrylate

Article abstract of DOI:10.1016/j.jorganchem.2017.09.043

Ruthenium(II) complexes of Schiff base derived from cycloalkylamines (cycloalkyl = cyclopentyl (1a), cyclohexyl (1b), cycloheptyl (1c), and cyclooctyl) (1d) were synthesized: [RuCl(CyPen-Salen)(PPh₃)₂] (2a), [RuCl(CyHex-Salen)(PPh

Full text of DOI:10.1016/j.jorganchem.2017.09.043

Accepted Manuscript
Ruthenium(II) complexes of Schiff base derived from cycloalkylamines as pre-
catalysts for ROMP of norbornene and ATRP of methyl methacrylate
Maria Beatriz A. Afonso, Thais R. Cruz, Yan F. Silva, João Clécio A. Pereira, Antonio
E.H. Machado, Beatriz E. Goi, Benedito S. Lima-Neto, Valdemiro P. Carvalho-Jr
PII:
S0022-328X(17)30574-0
10.1016/j.jorganchem.2017.09.043
JOM 20118
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 4 August 2017
Revised Date: 18 September 2017
Accepted Date: 28 September 2017
Please cite this article as: M.B.A. Afonso, T.R. Cruz, Y.F. Silva, Joã.Clé.A. Pereira, A.E.H. Machado,
B.E. Goi, B.S. Lima-Neto, V.P. Carvalho-Jr, Ruthenium(II) complexes of Schiff base derived from
cycloalkylamines as pre-catalysts for ROMP of norbornene and ATRP of methyl methacrylate, Journal of
Organometallic Chemistry (2017), doi: 10.1016/j.jorganchem.2017.09.043.
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ACCEPTED MANUSCRIPT
Ruthenium(II) complexes of Schiff base derived from cycloalkylamines as pre-
catalysts for ROMP of norbornene and ATRP of methyl methacrylate
Maria Beatriz A. Afonsoa, Thais R. Cruza, Yan F. Silvaa, João Clécio A. Pereirab, Antonio E.
H. Machadoc, Beatriz E. Goia, Benedito S. Lima-Netob, Valdemiro P. Carvalho-Jra*
aFaculdade de Ciências e Tecnologia, UNESP Univ Estadual Paulista, CEP 19060-900,
Presidente Prudente, SP, Brazil
bInstituto de Química de São Carlos, Universidade de São Paulo, CEP 13560-970, São
Carlos, SP, Brazil
cInstituto de Química, UFU Universidade Federal de Uberlândia, CEP 38400-902,
Uberlândia, MG, Brazil.
* Correspondence to: V. P. Carvalho Jr (e-mail: valdemiro@fct.unesp.br)
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Abstract
Ruthenium(II) complexes of Schiff base derived from cycloalkylamines (cycloalkyl =
cyclopentyl (1a), cyclohexyl (1b), cycloheptyl (1c), and cyclooctyl) (1d) were synthesized:
[RuCl(CyPen-Salen)(PPh3)2] (2a), [RuCl(CyHex-Salen)(PPh3)2] (2b), [RuCl(CyHep-
Salen)(PPh3)2] (2c), and [RuCl(CyOct-Salen)(PPh3)2] (2d). The Schiff base-RuII complexes
1
13
31
2a-d were characterized by elemental analysis, FTIR, UV-Vis, H-, C- and P-NMR, and
cyclic voltammetry. The complexes 2a-d were evaluated as catalytic precursors for ROMP of
norbornene (NBE) and for ATRP of methyl methacrylate (MMA). The syntheses of
polynorbornene (polyNBE) via ROMP with complexes 2a-d as pre-catalysts were evaluated
under different reaction conditions ([HCl]/[Ru], [EDA]/[Ru], [NBE]/[Ru], and temperature).
The highest yields of polyNBE were obtained with [NBE]/[HCl]/[Ru] = 5000/25/1 molar ratio
in the presence of 5 µL of EDA for 60 minutes at 50 °C. MMA polymerization via ATRP was
conducted using the complexes 2a-d in the presence of ethyl-α-bromoisobutyrate (EBiB) as
initiator. The catalytic tests were evaluated as a function of the reaction time using the initial
molar ratio of [MMA]/[EBiB]/[Ru] = 1000/2/1 at 85 °C. The linear correlation of
ln([MMA]0/[MMA]) and time indicates that the concentration of radicals remains constant
during the polymerization and that the ATRP of MMA mediated by 2a-d proceeds in a
controlled manner. Molecular weights increased linearly with conversion, however, the
experimental molecular weights were higher than the theoretical ones.
Keywords: Schiff base, ruthenium complexes, ROMP, ATRP; norbornene, methyl
methacrylate
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1. Introduction
Schiff bases have been playing an important part in the development of coordination
chemistry. Schiff base metal complexes have been studied extensively because of their
attractive chemical and physical properties and their wide range of applications in numerous
scientific areas [1-4]. Concurrently, complexes bearing Schiff base ligands are recognized as
homogeneous or heterogeneous catalysts in various organic reactions. Schiff base complexes
play a central role in various homogeneous catalytic reactions and the activity of these
complexes varies with the type of ligands, coordination sites and metal ions. Furthermore,
such complexes have recently attracted much attention for oxidation, epoxidation,
hydrogenation, miscellaneos, and polymerization reactions [5,6].
In particular, notable works were conducted in ethylene polymerization reactions
catalyzed by various metal complexes containing Schiff bases ligands. Aluminum complexes
of a series of tridentate Schiff base ligands were found to catalyze the polymerization of
ethylene [7]. A number of pyridyl bis(imide) complexes and phenoxy imine complexes are
used as catalysts in the polymerization of ethylene [8,9]. Pyridine bis(imine) complexes of
iron(III) and cobalt(II) show significant activity in the polymerization of ethylene and
copolymerization of ethylene with 1-hexene [10]. The salicylaldiminato complexes of
zirconium were found to be effective catalysts in ethylene polymerization and promoted
radical decomposition in certain cases [11]. Poly(methylmethacrylate) was prepared in
presence of Cr(III) and Ni(II) salen complexes as catalysts for the controlled radical
polymerization of methyl methacrylate (MMA) monomer [12]. Verpoort et al. reported a
detailed discussion on catalytic activity in the atom transfer radical polymerization (ATRP)
and ring opening metathesis polymerization (ROMP) of various substrates using Schiff bases
Ru catalysts [13]. The critical points of these works showed that the efficiencies of catalysts
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were directly affected by the steric and electronic properties of the ligands. Therefore, the
efforts in the easy synthesis of new catalysts and investigation of their activity in ROMP and
ATRP reactions are an ongoing interest for the catalysis community.
Herein, we report the facile preparation and evaluation of novel ruthenium(II)
complexes of bidentade Schiff bases derived from cycloalkylamines, where the cycloalkyl is
cyclopentyl (2a), cyclohexyl (2b), cycloheptyl (2c), and cyclooctyl (2d) (Fig. 1), as pre-
catalysts for ROMP of norbornene (NBE) and ATRP of methyl methacrylate (MMA) under
different conditions of temperature, reaction time, and monomer concentration. Ethyl
diazoacetate (EDA) was used as carbene source for ROMP and ethyl 2-bromoisobutyrate
(EBiB) was used as initiator for ATRP. The goal was to observe the ring size influence and its
effects on catalytic activity of the studies complexes, discussing the σ-donor ability and steric
hindrance, obtaining resources to understand the factors that influence the efficiency of both
reactions. Moreover, base Schiff ligands bound to ruthenium impart good stability and
tolerance towards various organic functionalities, air and moisture, widening thus the area of
their applications.
Fig. 1. Illustration of the Schiff base ruthenium(II) complexes (2a-d).
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2. Experimental
2.1. General remarks
All reactions and manipulations were performed under nitrogen atmosphere following
standard Schlenk techniques. 1,2-dichloroethane (DCE) was dried with CaCl2 overnight,
filtered, distilled and degassed by three vacuum–nitrogen cycles under nitrogen before use.
Methyl methacrylate (MMA) was washed with 5% NaOH solution, dried over anhydrous
MgSO4, vacuum distilled from CaH2 and stored under nitrogen at −18 ºC before use.
RuCl3.xH2O,
2,2,6,6-tetramethyl-1-piperidinoxyl
(TEMPO),
tetrabutylammonium
hexafluorophosphate (n-Bu4NPF6), norbornene (NBE), ethyl diazoacetate (EDA),
cyclopentylamine, cyclohexylamine, cycloheptylamine, cyclooctylamine, salicylaldehyde,
and ethyl 2-bromoisobutyrate (EBiB) were used as acquired. The [RuCl2(PPh3)3] complex
was prepared following the literature and its purity was checked by satisfactory elemental
analysis and spectroscopic examination (31P{1H} and 1H-NMR, FTIR and EPR) [14].
2.2. Analyses
Elemental analyses were performed with a Perkin-Elmer CHN 2400 at the Elemental
Analysis Laboratory of Institute of Chemistry - USP. ESR measurements from solid sample
were conducted at 77 K using a Bruker ESR 300C apparatus (X-band) equipped with a TE102
cavity and an HP 52152A frequency counter. The FTIR spectra in CsI pellets were obtained
on a Bomem FTIR MB 102. Electronic spectra were recorded on a Varian model Cary 500
NIR spectrophotometer, using 1 cm path length quartz cells. The 1H and 31P{1H}NMR spectra
were obtained in CDCl3 at 298 K on a Bruker DRX-400 spectrometer operating at 400.13 and
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161.98 MHz, respectively. The obtained chemical shifts were reported in ppm relative to
TMS or 85% H3PO4. Conversion was determined from the concentration of residual monomer
measured by gas chromatography (GC) using a Shimadzu GC-2010 gas chromatograph
equipped with a flame ionization detector and a 30 m (0.53 mm I.D., 0.5 µm film thickness)
SPB-1 Supelco fused silica capillary column. Anisole was added to polymerization and used
as an internal standard. Analysis conditions: injector and detector temperature, 250 ºC;
temperature program, 40 ºC (4 min), 20 ºC min−1 until 200 ºC, 200 ºC (2 min). The molecular
weights and the molecular weight distribution of the polymers were determined by gel
permeation chromatography using a Shimadzu Prominence LC system equipped with a LC-
20AD pump, a DGU-20A5 degasser, a CBM-20A communication module, a CTO-20A oven
at 40 ºC and a RID-10A detector equipped with two Shimadzu column (GPC-805: 30 cm, Ø =
8.0 mm). The retention time was calibrated with standard monodispersed polystyrene using
HPLC-grade THF as an eluent at 40 ºC with a flow rate of 1.0 mL min−1. Electrochemical
measurements were performed using an Autolab PGSTAT204 potentiostat with a stationary
platinum disk and a wire as working and auxiliary electrodes, respectively. The reference
electrode was Ag/AgCl. The measurements were performed at 25 ºC ± 0.1 in CH2Cl2 with 0.1
mol L−1 of n-Bu4NPF6.
2.3. General Procedure for the Preparation of Schiff-Base Ligands (1a-d)
To prepare the Schiff base ligands 1a-d, a solution of salicylaldehyde in methanol was
slowly added over a solution of the respective cycloalkylamine in methanol. The mixture was
stirred at room temperature for 16 h and the product was obtained as a yellowish orange oil.
Any modifications are described below for each reaction.
Schiff-Base Ligand 1a: Salicylaldehyde (0.48 g, 4.0 mmol), cyclopentylamine (0.34 g,
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4.0 mmol), and methanol (15 mL) afforded 0.60 g (80%) of the title compound as a yellow
oil. Refractive index 1.5626; (a) UV–Vis: λmax(n) (nm), εmax(n) [M−1 cm−1]: λmax(1) (317), εmax(1)
1
[9600]; (b) IR (KBr): νx (cm–1): νC=N (1629), νC–O (1277); (c) H NMR: (CDCl3, 400
3
3
MHz): 13.8 (s, 1H, OH), 8.32 (s, 1H, CH=N), 7.27-7.31 ((dd, JH,H = 1.6 Hz, dd, JH,H
3
=0.8 Hz, 1H, salicyl-ring), 7.22-7.24 (dd, JH,H = 1.6 Hz, 1H, salicyl-ring), 6.93-6.96
3
3
3
(dt, JH,H = 6 Hz, JH,H = 0.4 Hz, 1H, salicyl-ring), 6.85-6.88 (td, JH,H = 0.8 Hz 1H,
salicyl-ring), 3.75-3.82 (m, 1H, CHPentyl), 1.90-1.99 (m, 2H, CH2Pentyl), 1.81-1.88 (m, 2H,
CH2Pentyl), 1.65-1.77 (m, 4H, CH2Pentyl),13C NMR (CDCl3) δ 162.3, 161.3, 131.8, 130.9,
118.4, 116.9, 70, 34.7, 24.5.
Schiff-Base Ligand 1b: Salicylaldehyde (0.48 g, 4.0 mmol), cyclohexylamine (0.39 g,
4.0 mmol), and methanol (15 mL) afforded 0.68 g (85%) of the title compound as a yellow
oil. Refractive index 1.5678; (a) UV–Vis: λmax(n) (nm), εmax(n) [M−1 cm−1]: λmax(1) (317), εmax(1)
1
[9700]; (b) IR (KBr): νx (cm–1): νC=N (1629), νC–O (1274), (c) H NMR: (CDCl3, 400
3
3
MHz): 13.83 (s, 1H, OH), 8.37 (s, 1H, CH=N), 7.31-7.27 (dd, JH,H = 1.6 Hz, dd, JH,H
3
=0.8 Hz, 1H, salicyl-ring), 7.23-7.25 (dd, JH,H = 1.6 Hz, 1H, salicyl-ring), 6.94-6.98
3
3
3
(dt, JH,H = 6 Hz, JH,H = 0.4 Hz, 1H, salicyl-ring), 6.85-6.89 (td, JH,H = 0.8 Hz, 1H,
salicyl-ring), 3.22-3.30 (m, 1H, CHHexyl), 1.80-1.87 (m, 4H, CH2Hexyl), 1.50-1.70 (m, 3H,
CH2Hexyl), 1.27-1.45 (m, 3H, CH2Hexyl); 13C NMR (CDCl3) δ 162.1, 161.4, 131.9, 131.1,
118.9, 118.3, 117.04, 77.3, 77.02, 76.7, 67.4, 25.5, 24.3.
Schiff-Base Ligand 1c: Salicylaldehyde (0.48 g, 4.0 mmol), cycloheptylamine (0.45 g,
4.0 mmol), and methanol (15 mL) afforded 0.69 g (80%) of the title compound as a yellow
oil. Refractive index 1.5652; (a) UV–Vis: λmax(n) (nm), εmax(n) [M−1 cm−1]: λmax(1) (316), εmax(1)
1
[9600]; (b) IR (KBr): νx (cm–1): νC=N (1621), νC–O (1270); (c) H NMR: (CDCl3, 400
3
3
MHz): 13.85 (s, 1H, OH), 8.3 (s, 1H, CH=N), 7.25-7.32 (dd, JH,H = 1.6 Hz, dd, JH,H
=
0.8 Hz, 1H, salicyl-ring), 7.22-7.25 (dd, 3JH,H = 1.2 Hz, 1H, salicyl-ring), 6.95-6.98 (dt,
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3
3
3JH,H = 6.4 Hz, JH,H = 0.4 Hz, 1H, salicyl-ring), 6.85-6.89 (td, JH,H = 0.8 Hz, 1H,
salicyl-ring), 3.37-3.46 (m, 1H, CHHeptyl), 1.82-1.90 (m, 2H, CH2Heptyl), 1.72-1.82 (m, 4H,
CH2Heptyl), 1.50-1.70 (m, 6H, CH2Heptyl); 13C NMR (CDCl3) δ 161.66, 161.39, 131.92, 131.02,
118.91, 118.29, 116.99, 77.34, 77.08, 76.83, 70.12, 36.40, 28.54, 24.24.
Schiff-Base Ligand 1d: Salicylaldehyde (0.48 g, 4.0 mmol), cyclooctylamine (0.51 g,
4.0 mmol), and methanol (15 mL) afforded 0.82 g (90%) of the title compound as a yellow
oil. Refractive index 1.5631; (a) UV–Vis: λmax(n) (nm), εmax(n) [M−1 cm−1]: λmax(1) (318), εmax(1)
1
[10000]; (b) IR (KBr): νx (cm–1): νC=N (1624), νC–O (1278); (c) H NMR: (CDCl3, 400
3
3
MHz): 13.85 (s, 1H, OH), 8.3 (s, 1H, CH=N), 7.27-7.32 (dd, JH,H = 1.2 Hz, dd, JH,H
=
0.8 Hz, 1H, salicyl-ring), 7.22-7.25 (dd, 3JH,H = 1.2 Hz, 1H, salicyl-ring), 6.93-6.97 (dt,
3
3JH,H = 6.4 Hz, 1H, salicyl-ring), 6.84-6.88 (td, JH,H = 0.8 Hz, 1H, salicyl-ring), 3.40-
3.45 (m, 1H, CHOctyl), 1.75-1.90 (m, 6H, CH2Octyl), 1.45-1.70 (m, 8H, CH2Octyl); 13C NMR
(CDCl3) δ 161.7, 161.4, 131.9, 131, 118.9, 118.3, 117.04, 77.3, 77, 76.8, 70, 51.4, 35.6,
33.47, 27.54, 25.48, 23.6.
2.3. General Procedure for the Preparation of Schiff-Base Ru Complexes (2a-d)
Synthesis of the ruthenium(II) Schiff base complexes (2a-d) was accomplished
according to the following procedure: To a solution of Schiff base 1a-d in methanol was
added dropwise a solution of NaOH in methanol and the reaction mixture was stirred for 2 h
at room temperature. The deprotonated ligand mixture was transferred by cannula to a 50-mL
three-necked flask fitted with a reflux condenser containing the [RuCl2(PPh3)3] precursor,
stirred mixture was refluxed for 4 h. A yellow precipitate was then filtered and washed with
methanol and ethyl ether and then dried in a vacuum.
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Complex 2a: [RuCl2(PPh3)3] complex (0.30 g, 0.31 mmol), Schiff base 1a (0.070 g,
0.37 mmol), NaOH (0.18 g, 0.45 mmol), and methanol (20 mL) afforded 0.25 g (80%) of the
title complex as a yellow solid: anal. calculated for C49H48ClNOP2Ru was 68.01 C, 5.59 H
and 1.62% N; found: 68.34 C, 5.55 H and 1.60% N. UV–Vis: λmax(n) (nm), εmax(n) [M−1 cm−1]:
λmax(1) (252), εmax(1) [10020], λmax(2) (370), εmax(2) [625], λmax(3) (422), εmax(3) [240]; IR (KBr):
1
νx (cm–1): νC=N (1618), νC–O (1355); H NMR: (CDCl3, 400 MHz): 7.30-7.70 (m, 12H:
metha-PPh3 and 1H: CH=N), 7.30-7.70 (m, 6H, para-PPh3), 7.21-7.30 (m, 12H, ortho-
3
3
PPh3), 6.63-6.68 (m, 1H, salicyl-ring), 6.4-6.5 (dd, JH,H = 1.6 Hz, dd, JH,H = 1.2 Hz,
1H, salicyl-ring), 6.04-6.10 (m, 1H, salicyl-ring), 5.85-5.80 (m, 1H, salicyl-ring), 3.85-
3.92 (m, 1H, CHPentyl), 1.60-1.80 (m, 3H, CH2Pentyl), 1.29-1.38 (m, 4H, CH2Pentyl), 1.07-1.15
(m, 1H, CH2Pentyl),13C NMR (CDCl3) δ 166.12, 160.83, 135.16, 135, 134.84, 134.26, 134.21,
134.16, 132.12, 132.04, 131.93, 131.91, 129, 128.53, 128.44, 127.62, 127.59, 127.55, 123.36,
121.99, 111.80, 75.92, 32.39, 23.43; 31P{1H} NMR (CDCl3: δ, ppm): 43.15 (s). EPR: no
signal was observed.
Complex 2b: [RuCl2(PPh3)3] complex (0.30 g, 0.31 mmol), Schiff base 1b (0.075 g,
0.37 mmol), NaOH (0.18 g, 0.45 mmol), and methanol (20 mL) afforded 0.20 g (75%) of the
title complex as a yellow solid: anal. calculated for C50H50ClNOP2Ru was 68.29 C, 5.73 H
and 1.59% N; found: 68.41 C, 5.64 H and 1.61% N. UV–Vis: λmax(n) (nm), εmax(n) [M−1 cm−1]:
λmax(1) (252), εmax(1) [10050], λmax(2) (369), εmax(2) [1766], λmax(3) (420), εmax(3) [585]; IR (KBr):
1
νx (cm–1): νC=N (1618), νC–O (1342); H NMR: (CDCl3, 400 MHz): 7.30-7.80 (m, 12H:
metha-PPh3 and 1H: CH=N), 7.30-7.80 (m, 6H, para-PPh3), 7.08-7.30 (m, 12H, ortho-
PPh3), 6.57-6.67 (m, 1H, salicyl-ring), 6.38-6.51 (dd, 3JH,H = 1.2 Hz, dd, 3JH,H = 1.6 Hz,
1H, salicyl-ring), 5.99-6.12 (m, 1H, salicyl-ring), 5.76-5.89 (dd, J = 0.8 Hz, 1H, salicyl-
ring), 3.18-3.33 (m, 1H, CHHexyl), 1.44-1.57 (m, 4H, CH2Hexyl), 0.78-1.05 (m, 4H, CH2Hexyl),
0.65-0.78 (m, 2H, CH2Hexyl); 13C NMR (CDCl3) δ 166.03, 160.74, 135.26, 135.10, 134.94,
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134.22, 131.98, 128.99, 127.59, 123.40, 122.28, 111.68, 77.24, 76.99, 76.73, 73.17, 33.20,
26.03, 25.84; 31P{1H} NMR (CDCl3: δ, ppm): 43.37 (s). EPR: no signal was observed.
Complex 2c: [RuCl2(PPh3)3] complex (0.30 g, 0.31 mmol), Schiff base 1c (0.080 g,
0.37 mmol), NaOH (0.18 g, 0.45 mmol), and methanol (20 mL) afforded 0.23 g (85%) of the
title complex as a yellow solid: nal. calculated for C51H52ClNOP2Ru was 68.56 C, 5.87 H and
1.57% N; found: 69.19 C, 5.64 H and 1.64% N; UV–Vis: λmax(n) (nm), εmax(n) [M−1 cm−1]:
λmax(1) (246), εmax(1) [9500], λmax(2) (370), εmax(2) [1268], λmax(3) (423), εmax(3) [599]; IR (KBr):
1
νx (cm–1): νC=N (1610), νC–O (1337); H NMR: (CDCl3, 400 MHz): 7.30-7.69 (m, 12H:
metha-PPh3 and 1H: CH=N), 7.30-7.69 (m, 6H, para-PPh3), 6.92-7.30 (m, 12H, ortho-
3
3
PPh3), 6.60-6.73 (d, JH,H = 1.6 Hz, 1H, salicyl-ring), 6.47-6.56 (d, JH,H = 1.6 Hz, 1H,
3
salicyl-ring), 6.09-6.20 (m, 1H, salicyl-ring), 5.73-5.85 (d, JH,H = 0.8 Hz, 1H, salicyl-
ring), 3.63-3.77 (m, 1H, CHHeptyl), 1.37-1.64 (m, 4H, CH2Heptyl), 1.27-1.36 (m, 4H, CH2Heptyl),
1.03-1.15 (m, 4H, CH2Heptyl); 13C NMR (CDCl3) δ 166.20, 161.6, 135.47, 135.30, 135.14,
134.24, 134.19, 134.14, 134.03, 132.13, 132.05, 131.96, 131.88, 128.97, 128.68, 128.52,
128.42, 127.64, 127.60, 127.56, 123.52, 122.78, 111.75, 75.0, 33.32, 26.6, 25.85, 25.02;
31P{1H} NMR (CDCl3: δ, ppm): 42.48 (s). EPR: no signal was observed.
Complex 2d: [RuCl2(PPh3)3] complex (0.30 g, 0.31 mmol), Schiff base 1d (0.085 g,
0.37 mmol), NaOH (0.18 g, 0.45 mmol), and methanol (20 mL) afforded 0.21 g (75%) of the
title complex as a yellow solid: anal. calculated for C52H54ClNOP2Ru was 68.82 C, 6.00 H
and 1.54% N; found 69.03 C, 6.21 H and 1.62% N. UV–Vis: λmax(n) (nm), εmax(n) [M−1 cm−1]:
λmax(1) (262), εmax(1) [10000], λmax(2) (377), εmax(2) [1528], λmax(3) (426), εmax(3) [822]; IR (KBr):
1
νx (cm–1): νC=N (1611), νC–O (1336); H NMR: (CDCl3, 400 MHz): 7.23-7.72 (m, 12H:
metha-PPh3 and 1H: CH=N), 7.23-7.72 (m, 6H, para-PPh3), 6.95-7.23 (m, 12H, ortho-
3
PPh3), 6.60-6.67 (m, 1H, salicyl-ring), 6.52-6.58 (d, JH,H = 1.6 Hz, 1H, salicyl-ring),
6.09-6.16 (m, 1H, salicyl-ring), 5.76-5.80 (m, 1H, salicyl-ring), 3.62-3.76 (m, 1H,
10

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CHOctyl), 1.38-1.50 (m, 4H, CH2Octyl), 1.30-1.36 (m, 4H, CH2Octyl), 1.05-1.25 (m, 6H,
CH2Octyl); 13C NMR (CDCl3) δ 166.13, 161.78, 135.12, 134.19, 134.03, 132.12, 132.04,
131.93, 128.99, 128.53, 128.44, 127.61, 123.50, 122.80, 111.81, 77.25, 77.00, 76.74, 74.85,
31
33.31, 26.57, 25.84, 25.01; P{1H} NMR (CDCl3: δ, ppm): 42.57 (s). EPR: no signal was
observed.
4.3. ROMP procedure
In a typical ROMP experiment, 1.1 µmol of complex was dissolved in CHCl3 (2 mL)
with an appropriate amount of monomer (NBE, 5.5 mmol) and additive (HCl, 27.5 µmol),
followed by addition of carbene source (EDA, 43 µmol). Usually the solution gelled for 1-2
min, but the reaction mixture was stirred for 60 min at 25 or 50 ºC in a silicon oil bath. At
room temperature, 5 mL of methanol was added and the polymer was filtered, washed with
methanol and dried in a vacuum oven at 40 ºC up to constant weight. The reported yields are
average values from catalytic runs performed at least three times with 10% error at the most.
The isolated polyNBEs were dissolved in THF for GPC data.
4.4. ATRP procedure
In a typical ATRP experiment, 12.3 µmol of complex was placed in a Schlenk tube
containing a magnet bar and capped by a rubber septum. Air was expelled by three vacuum–
nitrogen cycles before appropriate amounts of monomer (MMA, 12.3 mmol), initiator (EBiB,
24.6 µmol), and DCE (1 mL) were added. All liquids were handled with dried syringes under
nitrogen. The tube was capped under N2 atmosphere using Schlenk techniques, then the
reaction mixture was immediately immersed in an oil bath previously heated to the desired
11

ACCEPTED MANUSCRIPT
temperature. The polymerizations were conducted at 85 ºC. The samples were removed from
the tube after certain time intervals using degassed syringes. The polymerization was stopped
when the reaction mixture became very viscous. The reported conversions are average values
from catalytic runs performed at least twice.
12

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3. Results and discussion
3.1. Synthesis and characterization
The bidentate Schiff bases (1a-d) were readily prepared by condensation of
salicylaldehyde with four different cycloalkylamines in methanol (Scheme 1). When 1 mol of
salicylaldehyde and 1 mol of amine were reacted, the corresponding products were obtained
under mild conditions; confirmation of these products was demonstrated by spectroscopic
data. As the ligands predominantly exist in the keto tautomeric form in the solid state. Prior to
the complexation step an equivalent amount of NaOH in methanol was added to the ligands to
convert this keto form to enolate form. This renders the coordination of enolate oxygen.
Equimolar reactions between [RuCl2(PPh3)2] with the corresponding deprotonated Schiff
bases (1a-d) led to the ruthenium(II) Schiff base complexes 2a-d in high yields (Scheme 1).
Scheme 1. Synthesis protocol of Schiff base 1a-d and their ruthenium complexes 2a-d.
13

ACCEPTED MANUSCRIPT
1
In the H NMR spectra in CDCl3 for the synthesized ligands (1a-d) and their
complexes (2a-d) are given in Section 2. The peaks in the range 1.0-2.0 ppm for ligands 1a-d
and 0.65-2.0 ppm for complexes 2a-d, as multiplets, are assignable to the CH2 groups
hydrogens from the N-cycloalkyl substituent. Additionally, the peaks at range 6.84-7.32 ppm
for ligands 1a-d and 5.73-6.73 for complexes 2a-d, as multiplets, are assignable to the protons
1
of aromatic –CH groups. In the H NMR spectra of ligands 1a-d and complexes 2a-d, the
chemical shifts observed around 8.3 ppm for free ligands and at 7.22-7.50 for complexes 2a-d
as singlets are assigned to the proton of azomethine (−N=CH−) [15]. The peak due to the
azomethine showed a high field shift compared to the free Schiff base after complexation with
the metal ion indicating coordination through the azomethine nitrogen atom. A singlet for OH
has a distinct down-field resonance at 13.8 ppm, characteristic for the acidic proton involved
in a strong intramolecular hydrogen bond in the ligands 1a-d [15]. These signals did not
appear in the complexes 2a-d as expected. The 1H NMR spectra for the complexes 2a-d were
dominated by multiplets between 6.92-7.80 ppm due to the phenyl protons of two PPh3
ligands coordinated in the Ru center which are upfield relative to the multiplets around 6.92-
13
7.80 ppm from the aromatic signals for the Schiff base. In the C NMR spectra, the carbon
peaks between 23.6-161.6 ppm for ligands 1a-d and 23.4-166.2 ppm for complexes 2a-d were
observed. 31P NMR spectroscopy confirmed the presence of the PPh3 ligands and their
magnetic equivalence revealed that the two PPh3 ligands are trans-positioned to each other in
the complexes 2a-d since only a singlet around 43 ppm was found for all complexes, thereby
minimizing steric repulsion. This orientation is typical for ruthenium Schiff base complexes
containing the trans-[Ru(PPh3)2] core [16]. The FTIR spectra of the ligands 1a-d were
compared with that of the complexes 2a-d in order to confirm the coordination of ligand to
the ruthenium metal. The infrared spectra of free ligands show the characteristic ν(O−H)
absorption bands around 2678 cm−1 which disappears after complexation, the absorption
14

ACCEPTED MANUSCRIPT
corresponding to the ν(C=N) vibration is around 1625 cm−1 in the ligands, and it is shifted
approximately 12 cm−1 to a lower wavenumber in the spectra of the complexes 2a-d
confirming the coordination of azomethine nitrogen to the metal. The coordination of
phenolic oxygen of the Schiff bases is supported by the appearance of new bands in 497-499
cm−1 range due to ν(Ru−O) stretching in the ruthenium complexes [17]. In addition, these
complexes exhibit one strong band in the range 414-417 cm−1, which may be due to ν(Ru−N)
stretching suggesting coordination of azomethine nitrogen atoms [17].
Electronic spectra of ligands 1a-d and their complexes 2a-d have been recorded in the
200–700 nm range in CHCl3 and their corresponding data are given in Table 1. The formation
of the complexes 2a-d was also confirmed by electronic spectra (Fig. 2). In the electronic
spectra of the free ligands and their complexes, the wide range bands were observed due to
either the π→π* and n→π* of C=N chromophore or charge-transfer transition arising from π
electron interactions between the metal and ligand, which involves either a metal-to-ligand
electron transfer [18]. The electronic spectra of the ligands 1a-d in CHCl3 (Fig. 3) showed
strong absorption bands in the ultraviolet region (316–318 nm), that could be attributed
respectively to the π→π* and n→π* transitions in the benzene ring or azomethine (–C=N)
groups [19]. In the electronic spectra of the complexes 2a-d, these bands show hypsochromic
shifts relative to their free ligands, and they may be hidden under the electronic transition of
the PPh3 ligands. This displacement of the absorption bands of the complexes 2a-d most
likely originate from the metalation which increases the conjugation and delocalization of the
whole electronic system and results in the energy change of the intra-ligand transitions of the
conjugated chromophore. These results clearly indicate that the ligand coordinates to metal
center, which are in accordance with the results of the other spectroscopic data. Furthermore,
the absorption bands in the visible region are observed at between 360 and 426 nm as a low
intensity bands These bands are considered to arise from the MLCT transition [20].
15

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Complex 2a
1.0
0.8
0.6
0.4
0.2
0.0
Complex 2b
Complex 2c
Complex 2d
200
250
300
350
400
450
500
550
600
Wavelength (nm)
Fig. 2. Electronic spectra of the complexes 2a-d in degassed CH2Cl2 solution at room
temperature ([Ru] =0.1 mmol L−1).
The electrochemical activity of the complexes 2a-d was studied by cyclic voltammetry
in scan rate of 100 mV s−1 in CH2Cl2 solution containing 0.1M n-Bu4NPF6 supporting
electrolyte in the potential range 0 to 1.1 V. The cyclic voltammograms of 2a-d are shown in
Fig. 3 and the voltammetric data are summarized in Table 2. On scanning anodically and
reversing the scan direction, similar anodic waves which may be attributed to the RuII/III redox
couple and the redox-active phenolate moieties were observed between 0.41-0.60 V and 0.7-
1.0, respectively, for all Schiff base Ru complexes (Fig. 3). Less intense corresponding
cathodic peaks were observed; and this may be attributed to the instability and transient nature
of the RuIII ions in solution. Overall there is a clear shift in the redox potentials towards more
negative values as the electron-donating ability of the cycloalkyl substituents is increased
(Octyl > Heptyl > Hexyl > Pentyl). Modulation of the electron-donating ability, as well as the
16

ACCEPTED MANUSCRIPT
steric effect of the Schiff base ligands is subsequently shown to have an effect on the activity
on ROMP and ATRP of the complexes 2a-d.
Table 1. Infrared and electronic absorption data for base Schiff ligands 1a-d and their
ruthenium complexes 2a-d.
FTIR (cm−1)
UV-Vis (nm)
Compounds
Ligand
Complex
Ligand
Complex
416 ν(Ru-N)
499 ν(Ru-O)
1355 ν(C-O)
1618 ν(C=N)
417 ν(Ru-N)
497 ν(Ru-O)
1342 ν(C-O)
1618 ν(C=N)
416 ν(Ru-N)
497 ν(Ru-O)
1337 ν(C-O)
1610 ν(C=N)
414 ν(Ru-N)
498 ν(Ru-O)
1336 ν(C-O)
1611 ν(C=N)
1277 ν(C-O)
1629 ν(C=N)
2734 ν(O-H)
2952 ν(C-H)
1274 ν(C-O)
1629 ν(C=N)
2663 ν(O-H)
2925 ν(C-H)
1270 ν(C-O)
1621 ν(C=N)
2662 ν(O-H)
2910 ν(C-H)
1278 ν(C-O)
1624 ν(C=N)
2651 ν(O-H)
2904 ν(C-H)
317 (π→π*)
252 (π→π*)
313 (n→π*)
370 (MLCT)
422 (MLCT)
252 (π→π*)
313 (n→π*)
369 (MLCT)
420 (MLCT)
246 (π→π*)
273 (n→π*)
370 (MLCT)
423 (MLCT)
262 (π→π*)
320 (n→π*)
377 (MLCT)
426 (MLCT)
1a/2a
1b/2b
1c/2c
1d/2d
317 (π→π*)
316 (π→π*)
318 (π→π*)
17

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8
6
4
2
0
Complex 2a
Complex 2b
Complex 2c
Complex 2d
-2
-4
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Potential (V)
Fig. 3. Cyclics voltammograms of 2a-d in CH2Cl2 at 25 ºC. [Ru] = 10 mM; [n-Bu4NPF6] =
0.1 M. Scanning anodically from 0.0 up to 1.1 V at scan rates of 100 mV s−1.
Table 2. Cyclic voltammetrya results for complexes 2a-d.
CV
Complex
Epa (V)
0.599
0.568
0.529
0.515
Epc (V)
0.462
0.483
0.408
0.433
E1/2 (V)
0.530
0.525
0.468
0.474
∆Ep (V)
0.137
0.085
0.121
0.082
2a
2b
2c
2d
a Conditions: CH2Cl2, n-Bu4NPF6 (supporting electrolyte, 0.1 mol L−1), [Ru] = 5 mmol L−1, scan rate = 100 mV
s−1), platinum disk and wire (working and auxiliary electrode), Ag/AgCl (reference electrode). E1/2 is the half-
potential for the complex; ∆Ep is the cathodic-anodic peak separation.
18

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3.2. ROMP reactions
The reactivity of the complexes 2a-d as catalytic precursors was tested on ROMP of
NBE in CHCl3 with [NBE]/[Ru] = 5000, volume of EDA = 5 µL at 25 °C for 60 min (Fig. 4).
In general, the complexes 2a-d presented low yields of polyNBE, the complex 2a with lower
yield of 4% and 2d with higher yield equal to 11% (Table 3). ROMP of NBE with complexes
2a-d was also evaluated at 50 °C under the same conditions. The increase of temperature
produced higher yields of polyNBE, with maximum yields of up to 25% and PDI values
between 1.1 and 2.3. As the temperature increased to 50 °C, the yields practically doubled
with a significant increase in Mn values of the order from 103 to 104 g mol−1 in relation to 25
°C. When comparing the reactivity at 25 and 50 ºC, the main difference was in the yields of
polymer, which the catalytic activity at 50 °C was always higher than that observed at 25 °C
for all complexes. Thus, the improvement in the catalytic activity of the complexes 2a-d at 50
ºC shows that the induction period was favored.
Fig. 4. Dependence of yield on the [HCl]/[Ru] molar ratio for ROMP of NBE with 2a-d;
[NBE]/[Ru] = 5000 and 5 µL of EDA in CH2Cl2 at 25 and 50 ºC for 60 min.
19

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In order to optimize the induction period of the complexes 2a-d in the ROMP
mechanism, a chemical activation of the catalytic precursors was attempted with the use of
HCl acid. This strategy has already been applied for the activation of ruthenium(II) Schiff
bases catalysts, which the acid protonates the N-atom of the azomethine group [13c]. This
causes the partial labilization of the Schiff base with generation of a vacancy at the ruthenium
center, and the initiation step occurs as monomer is added. The ROMP of NBE catalyzed by
the complexes 2a-d in the presence of HCl is shown in Fig. 4, where the [HCl]/[Ru] ratio was
investigated to find the optimal concentration of acid.
Table 3. Yield values and SEC data from the ROMP of NBE with 2a-d at 25 and 50 °C
;
[NBE]/[Ru] = 5000 and 5 µL of EDA with 1.1 µmol of complex in CH2Cl2 for 60 min.
Complex [HCl]/[Ru]
25 °C
50 °C
Yield (%) Mn (103) PDI
Yield (%) Mn (103) PDI
0
10
25
50
150
300
0
10
25
4
4.1
38.2
49.2
78.6
60.2
5.4
1.2
1.5
1.2
1.2
1.4
2.9
1.1
3.8
3.8
1.1
3.2
1.0
2.0
2.6
2.3
3.8
3.1
-
8
98.8
94.0
87.3
62.1
97.0
15.0
86.0
19.3
49.2
76.4
54.4
76.5
68.0
60.0
32.5
70.8
71.0
94.6
96.1
48.4
72.6
83.7
90.1
7.7
2.3
3.0
1.3
1.2
3.7
2.0
1.1
1.5
3.2
1.1
1.2
1.1
1.3
3.6
1.6
1.5
1.1
1.1
1.1
3.1
1.1
1.3
2.8
1.1
2a
2b
2c
2d
22
29
21
16
6
34
43
38
24
15
10
37
43
17
18
13
15
32
43
23
18
11
25
35
44
45
36
2
4
8.7
25
28
17
18
14
8
26
31
14
11
0
11
31
34
21
13
0
46.0
68.0
79.5
56.4
9.6
50
150
300
0
10
25
2.4
37.2
64.5
48.9
61.2
-
54.0
56.6
10.8
23.1
34.5
-
50
150
300
0
10
25
50
150
300
1.1
2.6
3.0
1.4
2.7
-
20

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The catalytic activity of the complexes 2a-d was sensitive with variation of the
[HCl]/[Ru] molar ratio at 25 and 50 °C (Table 3). At 25 °C, a considerable increase in the
yields of polyNBE as increasing the [HCl]/[Ru] ratio up to 25 was observed, followed by a
drop for [HCl]/[Ru] ≥ 50. When polymerization was performed at 50 °C in the presence of
acid, a similar profile was observed, although, higher values of polyNBE yields were
achieved with Mn values near the order of 105 g mol-1. It is rationalized that this increase in
the catalytic activity of 2a-d can be explained by the protonation of N-atom of the Schiff base
azomethine group, it creates a vacant site at the ruthenium center to formation of active
species. In addition, the steric hindrance in the different Schiff bases plays a decisive role in
the reactivity in the complexes 2a-d. Besides that, the order of reactivity of the complexes in
ROMP increases from 2a to 2d. The lability of the azomethine group becomes more favored
as the steric hindrance increases, as cycloalkyl substituent is increased from cyclopentyl (2a)
to cyclooctyl (2d). However, excess HCl acid ([HCl]/[Ru] ≥ 50) in the mixture conducted a
decreasing in the polyNBE yields. Perhaps, with a very excessive amount of HCl, the
chlorides should compete for a coordination site and be able to coordinate to the ruthenium
center, causing the degradation of the complex in solution by full release of the Schiff base of
the coordinating sphere.
ROMP of NBE with the complexes 2a-d was conducted varying the volume of EDA
(Fig. 5), the polymerizations were carried out by keeping the optimal concentration of acid
found ([HCl]/[Ru] = 25) at 25 ° C. No formation of polymer was observed in the absence of
EDA. An increase in polyNBE yields is observed by increasing the volume of EDA up to 5
µL. For higher volumes of EDA (≥ 10 µL), a decreasing in yields is observed. In general, the
Mn values also follow this trend with an increase of the molecular weights up to 5 µL
followed by decreases to higher volumes of EDA (≥ 10 µL) with PDI values ranging from 1.1
21

ACCEPTED MANUSCRIPT
to 3.0. (Table 4). The increase of polyNBE yields with increasing volume of EDA up to 5 µL
is indicative of the coordination of EDA with associative character. However, it should be
noted that a very excessive amount of EDA (≥ 10 µL) provokes a decreasing in the yields
values, probably due to competition with the monomer for coordination onto the initiator
active sites. Thus, it worth to mention that the optimum EDA amount used as a carbene
source was of 5 µL for the complexes 2a-d. Considering that these complexes have the same
profile when reacted with EDA, it is possible to affirm that the four complexes have the same
pathway in the formation of Ru carbene in the induction period.
50
Complex 2a
Complex 2b
Complex 2c
45
40
Complex 2d
35
30
25
20
15
10
5
0
0.0
2.5
5.0
7.5
10.0
12.5
15.0
17.5
20.0
Volume of EDA (µL)
Fig. 5. Dependence of yield on the volume of EDA for ROMP of NBE with 2a-d;
[NBE]/[HCl]/[Ru] = 5000/25/1 with 1.1 µmol of complex in CH2Cl2 at 25 ºC for 60 min.
22

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Table 4. Yield values and SEC data from the ROMP of NBE with 2a-d at 25 °C
;
[NBE]/[HCl]/[Ru] = 5000/25/1 with 1.1 µmol of complex in CH2Cl2 for 60 min.
Complex
Volume of EDA
Yield (%)
Mn (103)
5.6
PDI
1.1
1.2
1.2
1.2
1.3
1.3
1.2
1.2
1.8
1.2
1.5
1.4
1.3
1.2
2.3
1.4
1.2
1.1
1.1
1.1
3.0
1.1
1.2
1.2
1
3
5
10
15
20
1
3
5
10
15
20
1
3
5
10
15
20
1
3
5
10
15
20
2
2
22
8
3
5
2a
6.8
49.2
7.9
6.8
8.1
4
6.8
2b
2c
2d
15
28
6
3
3
71.2
68.0
21.8
7.0
8.9
7.2
62.2
64.5
7.4
7.7
7.4
3
14
31
6
4
2
4
5.9
15
36
14
8
60.7
10.8
76.5
19.6
7.9
6
The yields increase when increasing the [NBE]/[Ru] molar ratio starting from 3000
with yields of 5, 8, 12, and 15% with 2a, 2b, 2c, and 2d, respectively, reaching yields at least
twice higher at 5000 for all complexes (Table 5). ROMP is a process governed by
thermodynamic equilibrium, where the increase of monomer concentration favors the
thermodynamic of polymerization providing higher polymer production [21].
23

ACCEPTED MANUSCRIPT
Table 5. Yield values and SEC data from the ROMP of NBE with 2a-d at 25 °C
;
[HCl]/[Ru] = 25/1 and 5 µL of EDA with 1.1 µmol of complex in CH2Cl2 for 60 min.
Complex
[NBE]/[Ru]
3000
Yield (%)
Mn (103)
70.4
PDI
1.2
1.2
1.3
1.8
1.4
2.3
1.2
3.0
5
22
8
28
12
31
15
36
2a
5000
3000
5000
3000
5000
3000
5000
49.2
73.4
68.0
72.7
64.5
69.8
10.8
2b
2c
2d
When comparing the catalytic activity of the complexes in ROMP reactions, it is
interesting to point out that the reactivity follows this order: 2a < 2b < 2c < 2d. The electronic
and steric characteristics of the Schiff base ligands were able to tune the catalytic activity of
complexes for ROMP of NBE, highlighting the importance of the Schiff base as ancillary
ligand. However, it is reasonable to attribute that the steric effects on the Schiff base play a
determinant role for the release of the azomethine group, rate-determining step for the
formation of in-situ active species. Furthermore, this induction period can be faster as using
acid (HCl).
From the UV-Vis experiments, the kinetic constants of the reaction of the complexes
2a-d with HCl acid ([HCl]/[Ru] = 25) was calculated (Fig. 6). The deprotonation reaction of
the azomethine group was pseudo-first order with apparent rate constants (kobs) of 2.7 × 10−4,
4.5 × 10−4, 6.4 × 10−4, and 2.5 × 10−3 s−1 to the complex 2a, 2b, 2c, and 2d, respectively. A
decrease in the kobs values with the increase of the cycloalkyl substituent is observed; it
confirms that the reaction between the complexes 2a-d with the HCl acid is kinetically
favored as the steric hindrance is increased.
24

ACCEPTED MANUSCRIPT
Complex 2a
1.0
Complex 2b
1.0
0.8
0.6
0.4
0.2
0.0
5.0
4.8
4.6
4.4
4.2
4.0
3.8
3.6
3.4
3.2
3.0
4.5
4.0
3.5
3.0
2.5
0.8
0.6
kobs = 4.47 x 10-4 s-1
kobs = 2.70 x 10-4 s-1
0.4
0
20
40
60
80
100 120 140
10 15 20 25 30 35 40 45 50 55
Time (min)
Time (min)
0.2
0.0
200
300
400
500
600
700
200
300
400
500
600
700
λ
(nm)
λ
(nm)
Complex 2c
Complex 2d
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
4.0
3.8
3.6
3.4
3.2
3.0
2.8
2.6
7.0
6.5
6.0
5.5
5.0
4.5
kobs = 6.41 x 10-4 s-1
kobs = 2.49 x 10-3 s-1
10 15 20 25 30 35 40 45 50 55
Time (min)
2
4
6
8
10 12 14 16 18
Time (min)
200
300
400
500
600
700
200
300
400
500
600
700
λ
(nm)
λ
(nm)
Fig. 6. Time-dependent UV-vis absorption spectra of 1, 2, and 3 in the presence of HCl in
CH2Cl2 at 25 °C; [Ru] =0.1 mmol L−1); [HCl]/[Ru] = 25. Insert: Dependence of ln(Ainf-At) on
the reaction time at 365 nm.
Based on this, it is possible to infer that the difference in the reactivity of the studied
complexes is directly related to the steric characteristics of the Schiff base ligands, which are
modulated by their substituents. Upon metal-carbene formation, a PPh3 leaves the complex,
followed by the coordination of NBE to the carbene-Ru species. In order to confirm this
proposal, experiments in the presence of excess PPh3 (20 equiv.) at 25 °C for 60 min with
[NBE]/[HCl]/[Ru] = 5000/25/1 and 5 µL of EDA for the complexes 2a-d, which no formation
of polymer was observed. This procedure confirms that the ROMP reaction did not occur,
although the carbene complex formation took place. The ROMP will only occur when the
PPh3 molecule undergoes discoordination from the metal center (Scheme 2).
25

ACCEPTED MANUSCRIPT
Scheme 2. Illustration of possible reaction steps for ROMP of NBE with the complexes 2a-d.
3.3. ATRP reactions
The complexes 2a-d have properties that make them promising reagents for use as
ATRP catalysts. They provide reversible or quasi-reversible RuII/RuIII couples at easily
accessible potentials, as shown by the electrochemical data. They have either a vacant
coordination site, which makes it possible for a halide ligand to enter the coordination sphere.
Thus, MMA polymerization via ATRP with complexes 2a-d were performed as a function of
time using EBiB as initiator with [MMA]/[EBiB]/[Ru] = 1000/2/1 molar ratio at 85 ° C. The
MMA conversion values increase exponentially as a function of time in all cases (Fig. 7).
MMA polymerization with 2a achieved a maximum conversion of 47% of polyMMA and,
when catalyzed by 2b, the conversion was increased by 20%, reaching approximately 70% in
17 h. However, a decrease in conversion values was observed for the complexes 2c-d under
the same conditions.
26

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1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
70
60
50
40
30
20
10
0
0
2
4
6
8
10
12
14
16
18
0
2
4
6
8
10
12
14
16
18
Time (h)
Time (h)
Fig. 7. Dependence of conversion and ln([MMA]0/[MMA]) on the reaction time for ATRP of
MMA with 2a (■), 2b (●), 2c (▲) and 2d (▼); [MMA]/[EBiB]/[Ru] = 1000/2/1 with 12.3
µmol of complex in CH2Cl2 at 85 ºC.
Kinetics studies of MMA polymerization mediated by the complexes 2a-d show a
linear correlation of ln([MMA]0/[MMA]) as a function of time (Fig. 7), with a pseudo-first
order rate constant (kobs) equal to 1.07 × 10−5, 1.78 × 10−5, 1.08 × 10−5 and 1.15 × 10−5 s−1 for
2a, 2b, 2c and 2d, respectively. The linear semilogarithmic plot of ln[MMA]0/[MMA]t versus
time and the linear increase of molecular weight with conversion, in conjunction with
moderate PDIs, illustrates a certain level of control imparted by the complexes 2a-b (Fig. 7
and 8). However, in repeated kinetic experiments molecular weights were observed to be
somewhat higher than the theoretical values. This can be attributed to the number of growing
radical chains being lower than expected, resulting in an effective increase in the monomer
concentration (f = 0.30-0.35). On the other hand, the molecular weight of polyMMA obtained
with 2c-d showed non-dependence of the molecular weight on the conversion, coupled with
PDI of ca. 2.0 clearly illustrating the lack of control during the polymerization. As observed,
MMA polymerization suggests that the level of control can be slightly tuned by the
27

ACCEPTED MANUSCRIPT
substitution pattern of the ancillary ligand in the complexes 2a-d, as more sterically hindered
substituents were incorporated into the base Schiff ligand, polymerization control decreased.
100
2.4
2.2
90
80
70
60
50
40
30
20
10
0
2.0
1.8
1.6
1.4
1.2
1.0
0
10
20
30
40
50
60
70
80
0
10
20
30
40
50
60
70
80
Conversion (%)
Conversion (%)
Fig. 8. Dependence of Mn and PDI on the conversion for ATRP of MMA with 2a (■), 2b (●),
2c (▲) and 2d (▼), [MMA]/[EBiB]/[Ru] = 1000/2/1 with 12.3 µmol of complex in CH2Cl2 at
85 ºC.
4. Conclusion
The Schiff bases ligands 1a-d and their respective complexes 2a-d were successfully
synthesized. The Schiff base-RuII complexes 2a-d were characterized by FTIR, UV-Vis, 1H-,
13C- and 31P-NMR, and cyclic voltammetry. Complexes 2a-d were moderately active as
catalytic precursors in ROMP of NBE and their catalytic activity was improved in the
presence of the HCl acid using [NBE]/[HCl]/[Ru] = 5000/25/1 ratio in the presence of 5 µL of
EDA for 60 minutes. The kinetic studies were determinate to explain the reactivity difference
between the complexes 2a-d against the ROMP reactions, it follows this order: 2a < 2b < 2c
< 2d. The catalytic activity of the complexes 2a-d suggests that the steric effects on the Schiff
base play a determinate role for the release of the azomethine group, rate-determining step for
28

ACCEPTED MANUSCRIPT
the formation of in-situ active species, and this induction period can be faster as using acid
(HCl).
MMA polymerization mediated by complexes 2a-d was performed using
[MMA]/[EBiB]/[Ru] = 1000/2/1 molar ratio at 85 °C. A linear correlation of
ln([MMA]0/[MMA]) as a function of time mediated by complexes 2a-d indicates some level
of control in the polymerization as compared to conventional radical polymerization.
However, better control levels were achieved with the complexes 2a-b, in which the
molecular weights increased linearly with the conversion with narrow polydispersity. On the
other hand, complexes 2c-d showed low efficiency in the control of MMA polymerization,
evidenced by non-dependence of the molecular weight on the conversion and broad PDIs. It is
believed that the steric hindrance of the Schiff base played a decisive role in the
reactivity/efficiency against the controlled polymerization of MMA.
Acknowledgements
The authors are indebted to the financial support from FAPESP (Proc. 2013/10002-0).
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