Organometallic-mediated radical polymerization using well-defined Schiff base cobalt(II) complexes

Article abstract of DOI:10.1080/00958972.2018.1527322

A series of new cobalt(II) complexes of Schiff base derived from salicylaldehyde and different cycloalkylamines (cycloalkyl = cyclopentyl-1a, cyclohexyl-1b, and cycloheptyl-1c) was synthesized: [Co(CyPen-Salicyl)₂] (2a), [Co(CyHex-Salicyl)

Full text of DOI:10.1080/00958972.2018.1527322

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Organometallic-mediated radical polymerization
using well-defined Schiff base cobalt(II) complexes
Yan F. Silva, Beatriz A. Riga, Victor M. Deflon, Jhonathan R. Souza, Leonardo
H. F. Silva, Antonio E. H. Machado, Pedro Ivo S. Maia, Valdemiro P. Carvalho-
Jr & Beatriz E. Goi
To cite this article: Yan F. Silva, Beatriz A. Riga, Victor M. Deflon, Jhonathan R. Souza, Leonardo
H. F. Silva, Antonio E. H. Machado, Pedro Ivo S. Maia, Valdemiro P. Carvalho-Jr & Beatriz E. Goi
(2018): Organometallic-mediated radical polymerization using well-defined Schiff base cobalt(II)
complexes, Journal of Coordination Chemistry, DOI: 10.1080/00958972.2018.1527322
To link to this article: https://doi.org/10.1080/00958972.2018.1527322
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Organometallic-mediated radical polymerization using well-defined
Schiff base cobalt(II) complexes
YAN F. SILVAa, BEATRIZ A. RIGAa, VICTOR M. DEFLONb, JHONATHAN R. SOUZAc,
LEONARDO H. F. SILVAc, ANTONIO E. H. MACHADOc, PEDRO IVO S. MAIAd,
VALDEMIRO P. CARVALHO-JRa and BEATRIZ E. GOI*a
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, Universidade Federal de Uberlândia, P.O. Box 593, Uberlândia 38400-089,
Minas Gerais, Brazil
dDepartamento de Química, Universidade Federal do Triangulo Mineiro, CEP 38025-440, Uberaba, MG, Brazil
A series of new cobalt(II) complexes of Schiff base derived from salicylaldehyde and different
cycloalkylamines (cycloalkyl = cyclopentyl-1a, cyclohexyl-1b, and cycloheptyl-1c) was
synthesized: [Co(CyPen-Salicyl)2] (2a), [Co(CyHex-Salicyl)2] (2b), and [Co(CyHep-Salicyl)2]
(2c). The bis(phenoxyiminato)Co(II) complexes (2a-2c) have been fully characterized by FTIR
and UV-Vis spectroscopy, elemental analysis, cyclic voltammetry, computational methods, and
two of the complexes were further studied by single crystal X-ray crystallography. The X-ray
structure analysis of 2a-b shows that the geometry around the metal atom is a distorted
tetrahedron, confirming the spectroscopic data. Electrochemical studies suggest that the redox
potential of 2a-2c are sensitive to the substituent group, decreasing in order cyclopentyl >
cyclohexyl > cycloheptyl. Complexes 2a-2c were used as controlling agents for the
polymerization of vinyl acetate (VAc) initiated by AIBN, according to a cobalt-mediated radical
polymerization (CMRP) mechanism. The VAc polymerization mediated by 2a-2c suggests that
the level of control can be slightly tuned by the substitution of the cycloalkyl group on the Schiff
base ligand. Complex 2b showed the smaller discrepancy between observed and calculated
molecular weight, and narrower molecular weight distribution.
Keywords: Schiff base; Cobalt(II) complexes; Polymerization; Vinyl acetate; CMRP
*Corresponding author. Email: beatriz_goi@fct.unesp.br

1. Introduction
Schiff bases play an important role in inorganic chemistry as they can easily form stable
complexes with most transition metal ions. This superabundant interest comes from the
numerous and various applications of Schiff base metal complexes. Schiff bases and their metal
complexes have large potential applications in various fields including optical [1-5], magnetic
[6] and supramolecular materials [7-9] as well as chemical probes [10, 11] and catalysis
[4, 12-15].
Schiff base complexes have been used as highly efficient catalysts for cyclopropanation,
oxidation, epoxidation, Diels-Alder, and polymerization reactions [4, 12-21]. In particular, Schiff
base cobalt complexes are an interesting class of compounds for mediating the polymerization of
polar monomers including VAc (vinyl acetate) due to their less oxophilic nature relative to early
transition metals. This monomer can be polymerized exclusively via a radical mechanism [22],
so its “living” polymerization may only be considered using modern “living”/controlled radical
polymerization (CRP) methodologies based on the reversible equilibrium between growing
radicals and dormant species [23-27]. N2O2 Schiff base cobalt-mediated radical polymerization
has been extended to VAc [28-30]. In a recent report, Peng and coworkers described the cobalt-
mediated radical polymerization (CMRP) of both vinyl acetate and methyl acrylate using a
Schiff base cobalt complex as controlling agent [29]. They reported a good control over the two
polymerizations with the formation of polymers with low dispersities. Debuigne and coworkers
replace the bidentate acac ligand for a tetradentate Salen type ligand in a one-pot process for
improving the block copolymerization of vinyl acetate and n-butyl acrylate [31].
It is well known that CMRP, while technically belonging to the organometallic-mediated
radical polymerization (OMRP) class, has interesting advantages such as enabling the synthesis
of complex architecture polymers [32-36] and the obtention of polymers with well-defined
properties [29, 36-39]. Moreover, it is by far the most efficient technique to synthesize controlled
polymers of VAc, which is a less activated monomer (LAM) capable of generating more reactive
radicals [38, 40-42]. The success of this technique is mostly attributed to its mechanism and the
reversible formation of a Co-carbon covalent bond. This reactional mechanism is based on a
temporary reversible deactivation of the growing radicals, which coordinate to the metal center
to reduce the instant concentrations of radicals, minimizing the possibility of irreversible
terminations to occur [43]. Depending on the source and concentration of radicals, the

mechanism follows two different pathways, degenerative transfer (DT) or reversible termination
(RT) [44-46]. In the former case, the radical source is exclusively the initiator, and the radical
concentration is determined by the initiator concentration. Contrarily, in the RT pathway the
radical species enter the reaction dissociated from the organo-cobalt complex (CoIII—P), the
dormant specie, and the radical concentration is controlled by the equilibrium established
between active and dormant species [29, 44, 47, 48]. These two CMRP mechanisms are
illustrated in scheme 1.
Scheme 1. Degenerative transfer (DT) and reversible termination (RT) of CMRP reaction.
Besides the importance of the mechanism upon the control, the chosen cobalt complex
plays an important role mediating the activation/deactivation equilibrium. We believe that the
structures of metal complexes can be modified in different ways to achieve a better
understanding over the functioning of the mediating systems and for development of more
versatile and efficient controlling agents for CMRP. Here, in contrast to extensive research done
with N2O2 tetradentate Schiff-base cobalt complexes as mediators in CMRP, we report the
synthesis, characterization and X-ray structure of a series of NO bidentate Schiff base Co(II)
complexes derived from different cycloalkylamines and their application on VAc polymerization
via the CMRP mechanism.

2. Experimental
2.1. General remarks
All reagents were purchased from Aldrich Chemical Co. All reactions and manipulations were
performed under a nitrogen atmosphere by using standard Schlenk techniques. Vinyl acetate
(VAc) was washed with 5% NaOH solution, dried over anhydrous MgSO4, and degassed by
several freeze-thaw cycles before being distilled from CaH2 and stored at 18 C under nitrogen.
CoCl2 was dried in a reaction flask carefully under reduced pressure (0.5 torr) by heating with a
hair dryer for 2 h immediately before use. Cyclopentylamine, cyclohexylamine,
cycloheptylamine, salicylaldehyde, tetrabutylammonium hexafluorophosphate (n-Bu4NPF6) and
2,2′-Azobis(2-methylpropionitrile) solution (AIBN) (0.2 M in toluene) were used as acquired.
Complexes 2b and 2c were prepared following the literature and their purity was checked by
satisfactory elemental analysis and spectroscopic examination (FTIR and UV-Vis) [49, 50]. The
synthesis and characterization of the ligands 1a-1c and complexes 2a-2c are reported in the
Supporting Information.
2.2. Analyses
Elemental analyses were performed in a Perkin-Elmer CHN 2400 at Elemental Analysis
Laboratory at Institute of Chemistry - USP. The FTIR spectra in KBr pellets were obtained on a
1
Shimadzu IRAffinity-1 FT-IR spectrometer. H and 13C{1H} NMR spectra were obtained in
CDCl3 at 298 K on an Agilent MR 400 Ultrashield spectrometer operating at 400.13 and
100.61 MHz, respectively. The obtained chemical shifts were reported in ppm relative to the
high frequency of TMS. Conversion was determined from the concentration of residual
monomer measured by gas chromatography (GC) using a Shimadzu GC-2010 gas
chromatograph equipped with flame ionization detector and a 30 m (0.53 mm I.D., 0.5 m film
thickness) SPB-1 Supelco fused silica capillary column. Anisole was added in polymerization
and used as an internal standard. Analysis conditions: injector and detector temperature, 250 C;
temperature program, 40 C (4 min), 20 C/min until 200 C, 200 C (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 modulo, a CTO-20A oven
at 40 C and a RID-10A detector, equipped with two Shimadzu columns (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 C with a flow rate of 1.0 mL/min. UV–vis measurements
were performed on a Cary 400 UV–vis spectrophotometer (Varian) using 1 cm path length
quartz cells. Toluene solutions of the complexes of 0.2 mmol L-1 concentrations were used for
these measurements.
2.3. Synthesis of ligands 1a-1c
To prepare the Schiff base ligands 1a-1c, a solution of salicylaldehyde (4 mmol; 0.49 g) in
methanol (20 mL) was slowly added over a solution of the appropriate amine (4 mmol; 0.34,
0.39 and 0.45 g for cyclopentylamine, cyclohexylamine and cycloheptylamine, respectively) in
the same solvent (20 mL). The mixture was stirred at room temperature for 16 h and the product
was obtained as a yellowish orange oil.
1a: Yield: 88%; Refractive index 1.5626; (a) UV–Vis: max(n) (nm), max(n) [M1 cm1]:
1
max(1) (317), max(1) [5320]; (b) IR (KBr, cm–1): C=N (1626), C–O (1277); (c) H NMR:
3
(CDCl3, 400 MHz): 13.68 (s, 1H, OH), 8.34 (s, 1H, CH=N), 7.28 (ddd, Jb,a = 3Jb,c = 7.5 Hz,
3
4
4Jb,d = 1.9 Hz, 1H, salicyl-ring), 7.23 (dd, Jd,c = 7.5 Hz, Jd,b = 1.9 Hz, 1H, salicyl-ring),
3
4
3
6.94 (dd, Ja,b = 7.5 Hz, Ja,c = 1.1 Hz, 1H, salicyl-ring), 6.86 (ddd, 3Jc,d = Jc,b = 7.5 Hz,
4Jc,a = 1.1 Hz, 1H, salicyl-ring), 3.75-3.82 (m, 1H, CHPentyl), 1.95-1.81 (m, 4H, CH2Pentyl), 1.65-
1.76 (m, 4H, CH2Pentyl). 13C NMR (CDCl3)  162.3, 161.3, 131.8, 130.9, 118.4, 116.9, 70, 34.7,
24.5.
1b: Yield: 94%; Refractive index 1.5678; (a) UV–Vis: max(n) (nm), max(n) [M1 cm1]:
1
max(1) (316,8), max(1) [7770]; (b) IR (KBr, cm–1): C=N (1629), C–O (1274), (c) H NMR
3
3
(CDCl3, 400 MHz): 13.82 (s, 1H, OH), 8.36 (s, 1H, CH=N), 7.28 (ddd, Jb,c = 7.5 Hz, Jb,a =
4
3
4
7.2 Hz, Jb,d = 1.9 Hz, 1H, salicyl-ring), 7.23 (dd, Jd,c = 7.5 Hz, Jd,b = 1.9 Hz, 1H,
3
4
salicyl-ring), 6.94 (dd, Ja,b = 7.2 Hz, Ja,c = 1.1 Hz, 1H, salicyl-ring), 6.86 (ddd, 3Jc,b =
3Jc,d = 7.5 Hz, 4Jc,a = 1.1 Hz, 1H, salicyl-ring), 3.20-3.27 (m, 1H, CHHexyl), 1.78-1.86 (m, 4H,
CH2Hexyl), 1.50-1.68 (m, 3H, CH2Hexyl), 1.43-1.25 (m, 3H, CH2Hexyl). 13C NMR (CDCl3)  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.
1c: Yield: 91%; Refractive index 1.5652; (a) UV–Vis: max(n) (nm), max(n) [M1 cm1]:
1
max(1) (316.2), max(1) [10000]; (b) IR (KBr, cm–1): C=N (1630), C–O (1270); (c) H NMR
3
3
(CDCl3, 400 MHz): 13.82 (s, 1H, OH), 8.36 (s, 1H, CH=N), 7.28 (ddd, Jb,c = 7.5 Hz, Jb,a =

4
3
4
7.2 Hz, Jb,d = 1.9 Hz, 1H, salicyl-ring), 7.23 (dd, Jd,c = 7.5 Hz, Jd,b = 1.9 Hz, 1H,
3
4
salicyl-ring), 6.94 (dd, Ja,b = 7.5 Hz, Ja,c = 1.1 Hz, 1H, salicyl-ring), 6.86 (ddd, 3Jc,b =
3Jc,d = 7.5 Hz, 4Jc,a = 1.1 Hz, 1H, salicyl-ring), 3.37-3.46 (m, 1H, CHHeptyl), 1.74-1.85 (m, 4H,
13
CH2Heptyl), 1.50-1.68 (m, 8H, CH2Heptyl). C NMR (CDCl3)  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.
2.4. Crystal structure determination
Red crystals of 2a and 2b were grown by slow evaporation from a MeOH solution at room
temperature. The data collections were performed using Mo-Kα radiation (λ = 71.073 pm) on a
BRUKER APEX II Duo diffractometer. Standard procedures were applied for data reduction and
absorption correction [51]. The structure was solved with SHELXS97 using direct methods [52]
and all non-hydrogen atoms were refined with anisotropic displacement parameters with
SHELXL2014 [52]. Hydrogens were placed at idealized positions using the riding model option
of SHELXL2014 [52]. The absolute structure showed a flack parameter value of 0.011(17) and
was refined as a 2-component inversion twin. More details on data collections and structure
determination are in table 1.
2.5. Theoretical calculations
The structures of the compounds under study were optimized using the density functional theory
(DFT) at the level of the hybrid functional PBE0 [53], implemented in Gaussian 09 [54], using
the TZVP triple-zeta basis set [55]. The optimizations and calculation of the vibrational
frequencies were done without any symmetry constraints. As the Co(II) complexes under study
are high-spin pseudotetrahedral compounds (S = 3/2) [56, 57], all calculations were done
considering a multiplicity equal to 4. After optimization, in all cases, <S2> = S(S+1) = 3.75,
suggesting the desirable non-existence of spin contamination in the generated orbital-based wave
functions. For the Co(III) complexes formed by the coupling between the Co(II) complex and the
vinyl acetate radical, the multiplicity was defined as being unitary. There are several previous
studies which have reported DFT calculations of CMRP dormant species models [22, 58-67].
Here, in order to reproduce the experimental conditions, the simulations involving these
compounds were also done under condition of solvation. For this, the previous structure was
reoptimized using the model IEFPCM [68] to build a dielectric continuum with the

characteristics of dichloromethane, in a self-consistent reaction field procedure [54]. The
structure of the product of coupling between the Co(II) complexes and the vinyl acetate radical
was optimized only under solvation condition. The electronic spectra were calculated for the first
sixty electronic states also considering the molecules solvated in dichloromethane (IEFPCM),
using the same DFT functional and basis set previously described.
2.6. CMRP procedure
In a typical CMRP experiment, 0.02 mmol of Schiff base Co(II) complex (8.7, 9.3 and 9.8 mg
for 2a, 2b and 2c, respectively) and 0.065 mmol of AIBN (10 mg) were placed in a 50 mL
Schlenk flask and degassed by three vacuum/nitrogen cycles. Dry, degassed vinyl acetate (5 mL,
54 mmol) was then added using a syringe under nitrogen. The red mixture was stirred and heated
at 65 °C to start the polymerization. Samples were withdrawn at different reaction times and
analyzed by GC to determine monomer conversion and by SEC-THF for molecular parameters
using PS calibration, after addition of TEMPO to neutralize radicals.
3. Results and discussion
3.1. Synthesis and Characterization
The Schiff bases were prepared in high yields by condensation of salicylaldehyde with
stoichiometric amounts of the desired amine in methanol; the corresponding products (1a-1c)
were obtained under mild conditions (scheme 2). Confirmation of these products was
demonstrated by spectroscopic methods. The azomethine proton (CH=N) resonance is observed
within the range of 8.3–8.4 ppm for N-cycloalkyl salicylaldimines (1a-1c), as the electron-
withdrawing phenyl group de-shields the azomethine proton and shifts the resonance to relatively
low field (figure S1 and table S1). The CH=N resonance appears as a broad singlet for all
compounds in CDCl3, providing further evidence that in solution these compounds exist mainly
in the enol-imino tautomeric form. In all cases, 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 [69]. All cobalt(II) Schiff base complexes (2a-2c) were successfully synthesized
in good yields in a single step from the metallation of the corresponding ligands with CoCl2
under mild conditions by stirring under reflux (scheme 2) and were characterized by FTIR and
UV-Vis spectroscopy (see discussion in the SI), elemental analysis, cyclic voltammetry and two

of the complexes were further studied by single crystal X-ray crystallography. Additional
characterizations of 2a and 2b were necessary to complete the early presented studies [49, 50], to
better understand their reactivity in the current purpose.
Scheme 2. Synthesis of ON donor Schiff bases 1a-c and their cobalt(II) complexes 2a-c.
The X-ray structure analysis of 2a and 2b confirmed the spectroscopic data. The crystal
data and refinement details are listed in table 1. The compounds crystallized as discrete, neutral
molecules in the non-centrosymmetric space group Cc and centrosymmetric space group P21/c
for 2a and 2b, respectively. Figure 1 illustrates ellipsoid representation for one of the molecules
in the asymmetric unit of 2a as a representative of the cobalt(II) complexes. The asymmetric unit
containing two molecules of 2a as well as the molecular structure of 2b are given in figures S2
and S3, respectively. Selected bond lengths and angles are presented in table 2. Both molecules
contained in the asymmetric unit of 2a present considerable disorder with 51.6 and 60.6 percent
occupations for major components of the cyclopentyl group of one of the ligands. Attempts on
the acquisition of better quality crystals of 2a were not successful, however, good R values were
obtained. Each ligand is deprotonated and coordinates to Co(II) ion via the phenolate and imine
donor atoms in the usual trans configuration in which the cobalt(II) ion is tetracoordinated
arranged in a tetrahedral geometry. Bond angles in 2a-b confirm the tetrahedral geometry around
the cobalt atom, with the N(1)CoN(2) (116.62(16)/123.08(17) for 2a and 118.15(6) for 2b)
and O(1)CoO(2) angles (111.66(17)/116.60(16) for 2a and 120.93(7) for 2b). The C−O
(1.297(6)–1.313(2) Å) and C−N bonds (1.281(2)–1.289(2) Å) are within the ranges between
carbon−oxygen and carbon−nitrogen single and double bonds. This reflects a considerable
delocalization of π-electron density within the six-membered rings.

The theoretical geometric parameters (bond lengths and angles) in table 2 show good
agreement with the measured data in X-ray crystallography, despite some bond angles presenting
an expressive discrepancy. This may be due to the fact that the theoretical data are related to a
free molecule, whereas the experimental ones reflect intermolecular interactions in the crystalline
reticulum. The solvation of the compound induces a minimal increase in these values.
3.2. Polymerization of VAc
To study the controlling ability of 2a-2c on the VAc polymerization, the preliminary tests using
each of the three complexes as controlling agent were investigated with AIBN as the initiator
under the [VAc]/[AIBN]/[Co] molar ratio of 542/3.25/1 at 55 °C for 12 h. As is evident from the
data presented in figure 2, the VAc polymerization mediated by 2a-2c suggests that the level of
control can be slightly tuned by the substitution of the cycloalkyl group on the Schiff base
ligand. VAc polymerizations in the presence of 2a-2c, the conversions increase from 35 to 75%
with increasing CH2 groups on the cycloalkyl ring from cyclopentyl up to cycloheptyl. The VAc
polymerization mediated by 2a and 2b showed an induction period up to 2 h, which has been
reported in CMRP [29, 31] and attributed to the time required to form the organocobalt(III)
species in the medium. It proves that the polymerization mediated by 2a-b must take place by
degenerative transfer. Conversely, the absence of an induction time with 2c suggests a reversible
termination mechanism.
The linear semi-logarithmic plot of ln[VAc]0/[VAc]t versus time and the linear increase
of molecular weight with conversion, in conjunction with moderate Đ values, are consistent with
a certain degree of control for the polymerization imparted by these complexes (figure 3).
However, in repeated kinetic experiments, molecular weights were observed to be somewhat
higher or lower than the theoretical values. The molecular weight distribution curves were
monomodal throughout the polymerization (figure S4). This can be attributed to the number of
growing radical chains being higher or lower than expected, resulting in an effective
increase/decrease in the monomer concentration. All these results are consistent with a controlled
polymerization, although the controllability is far from ideal when compared with that of
reported for Co systems [23-30].
As the substituents became more sterically hindered (from cyclopentyl to cycloheptyl),
conversion increased. The increased steric hindrance around the Co center can explain this trend,

as it would create greater lability of the organo-cobalt bond and increase propagating radical
concentrations. Greater deviations between theoretical and experimental values were obtained
with the bulkier cycloheptyl substituent (complex 2c), which Đs were broader, indicating that
this polymerization was not well controlled and suggesting that irreversible termination reactions
were prevalent. Based on this, it is possible to infer that the difference in the reactivity of the
complexes studied is directly related to the steric characteristics of the Schiff base ligands, which
are modulated by their substituents (cycloalkyl groups).
A computational investigation was carried out to investigate the polymerization
mechanism by mediators 2a-2c. The theoretical simulations show that only one of the faces of
the complexes is active to formation of R-Co(III) from Co(II) species. The high electron density
at the side positioned by the oxygen atoms makes it not suitable for a coupling of the vinyl
radical with the Co(II) center (figure 4a). The regions with intense red are those with the highest
electronic density, which is in agreement with the distribution of Mulliken's charges (figure S5,
table S3). On the other hand, the high spin density on the Co(II) at the opposite side, in addition
to the lower electron density on the nitrogen atoms, tends to effectively direct the coupling
(figure 4b and figure S6). In fact, all attempts done in the calculations to make the coupling
between the vinyl radical and the Co(II) via the face with higher electronic density were
unsuccessful. The estimated bond order of the Co(III)–C bond for these complexes varies
between 0.53 and 0.54, values typical of single bonds, which characterizes the coordination
between the monomer and the metal. For a DT mechanism, a second coupling of another
propagating vinyl radical with a Co(III) center must occur for the formation of an intermediate
six-coordinate complex R-Co(IV)-R. In all cases, the ΔG values are positive (ΔG = 1.4965,
23.3013 and 65.9027 kJ mol-1 for 2a, 2b and 2c, respectively), but only for 2c, both ΔG and ΔH
values are positive (ΔH = 55.2326, 47.0910 and 3.6862 kJ mol-1 for 2a, 2b and 2c,
respectively), suggesting that formation of this intermediate complex is a rather disadvantageous
process. Thus, the thermodynamic parameters explain the RT mechanism exerted by 2c, whereas
the polymerization mediated by 2a and 2b is associated with a DT mechanism.

4. Conclusion
The Schiff bases ligands 1a-1c and their respective Co(II) complexes 2a-2c were successfully
synthesized. The Schiff base-Co(II) complexes 2a-2c were characterized by FTIR, UV-Vis
spectroscopy, elemental analysis, cyclic voltammetry, computational methods, and two of the
complexes (2a and 2b) were further studied by single crystal X-ray crystallography. The
preliminary VAc polymerizations mediated by 2a-2c as a function of time ([VAc]/[AIBN]/[Co]
= 542/3.25/1 at 55 °C) were first order in monomer, as assessed by the linear dependence of
ln([M]0/[M]t) versus time. The VAc polymerization mediated by 2a-2c suggests that the level of
control can be slightly tuned by the substitution of the cycloalkyl group on the Schiff base
ligand. The difference in the reactivity of the complexes studied was related to the steric
characteristics of the Schiff base ligands, which are modulated by their substituents (cycloalkyl
groups). The thermodynamic parameters explain the RT mechanism exerted by 2c, while the
polymerization mediated by 2a and 2b is associated with a DT mechanism.
Supplementary data
CCDC 1817953 contain the supplementary crystallographic data for 2a. These 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.
Acknowledgements
The authors are indebted to the financial support from FAPESP (Proc. 2013/11883-0 and
2013/10002-0), FAPEMIG (APQ 00583-13 and 03017-16) and CNPq (307443/2015-9). The
authors also acknowledge the Instituto de Química de São Carlos/USP-Brazil for the NMR
spectra acquisition.

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Figure 1. Thermal ellipsoid representation of 2a. Only one of the two molecules in the
asymmetric unit is shown. The cyclopentyl group of one of the ligands is disordered over two
different sites (components labeled a and b). Hydrogens were omitted for clarity.

Figure 2. Dependence of conversion and ln([VAc]0/[VAc]) on the reaction time for CMRP of
VAc with 2a (▲), 2b (▼) and 2c (●); [VAc]/[AIBN]/[Co] = 542/3.25/1 with 20 µmol of
complex in bulk at 55 C.

Figure 3. Dependence of Mn and Mw/Mn on the reaction time for CMRP of VAc with 2a (▲), 2b
(▼) and 2c (●); [VAc]/[AIBN]/[Co] = 542/3.25/1 with 20 µmol of complex in bulk at 55 C;
Mn(theor) (dashed line).

Figure 4. Electron density from total SCF density, mapped with the ESP: a) left - face susceptible
to suffer coupling with the vinyl radical and b) right - face not susceptible to coupling.

Table 1. Crystal data and structure refinement for 2a and 2b.
2a
2b
Empirical formula
Formula weight
C24H28CoN2O2
435.41
C26H32CoN2O2
463.46
Temperature (K)
Wavelength (Å)
296(2)
296(2)
0.71073
0.71073
Crystal system
Monoclinic
Cc*
Monoclinic
P21/c
Space group
a (Å)
12.3519(3)
44.7869(12)
8.9883(2)
21.8053(13)
10.0973(5)
11.4065(6)
105.16
b (Å)
c (Å)
β (°)
V (Å)3
118.1300(10)
4385.02(19)
8
2424.0(2)
Z
4
Dcalc (g cm-3)
1.319
1.270
Absorption coefficient (mm-1)
θ range for data collection (°)
Crystal size (mm3)
Number of reflections collected
Number of independent reflections/Rint
Data / restraints / no. of parameters
Final R indices [I>2sigma(I)]
R indices (all data)
GOF
0.804
0.732
1.819 to 26.452
0.500  0.140  0.070
29970
1.935 to 26.465
0.220  0.110  0.060
27486
7308 / 0.0288
7308 / 22 / 523
R1 = 0.0359, wR2 = 0.0894
R1 = 0.0425, wR2 = 0.0933
1.032
4366 / 0.0592
4366 / 0 / 280
R1 = 0.0351, wR2 = 0.0578
R1 = 0.0691, wR2 = 0.0631
0.825
Largest difference peak and hole (eÅ-3)
0.397 and -0.392
0.148 and -0.209
*Flack parameter: 0.011(17)

Table 2. Experimental and calculated selected bond lengths (Å) and angles (°) for 2a and 2b.
Complex 2a
Complex 2b
Experimentala
Theoretical
(UPBE0)b
Discrepancy
between the
theoretical
parameter for the
isolated molecule
and the
Experimental
Theoretical
(UPBE0)b
Discrepancy
between the
theoretical
parameter for
the isolated
molecule and
the
Geometric
parameter
experimental
data (%)
experimental
data (%)
N(1)-Co
N(2)-Co
1.996(3), 1.996(4)
1.999(4), 1.988(3)
1.907(3), 1.896(3)
1.909(4), 1.904(3)
1.286(6), 1.285(6)
1.283(6), 1.286(6)
1.297(6), 1.302(6)
1.307(6), 1.308(5)
2.052 (2.057)
2.052 (2.057)
1.922 (1.943)
1.922 (1.943)
1.283 (1.282)
1.283 (1.282)
1.299 (1.300)
1.299 (1.300)
+2.8, +2.8
+2.6, +3.2
+0.8, +1.4
+0.7, +0.9
-0.3, -0.2
1.9924(15)
2.0018(15)
1.8951(14)
1.9182(14)
1.289(2)
2.044 (2.055)
2.044 (2.055)
1.926 (1.944)
1.926 (1.944)
1.285 (1.284)
1.285 (1.284)
1.298 (1.299)
1.298 (1.299)
+2.6
+2.1
+1.6
+0.4
-0.3
O(1)-Co
O(2)-Co
N(1)C(1)
N(2)C(21)
O(1)C(7)
O(2)C(27)
N(1)CoN(2)
0.0, -0.3
1.281(2)
+0.3
-0.8
+0.1, -0.3
-0.7, -0.7
1.309(2)
1.313(2)
-1.1
116.62(16),
123.08(17)
157.62
(162.91)
+35.2, +28.1
118.15(6)
156.04
(160.64)
+32.1
111.66(17),
116.60(16)
137.32
(128.04)
+23.0, +17.8
120.93(7)
137.96
(129.99)
+14.1
O(1)CoO(2)
O(1)-Co-N(1)
O(2)-Co-N(1)
95.87(15), 96.70(15)
92.38 (92.46)
95.73 (95.01)
-3.6, -4.5
96.17(6)
92.28 (92.09)
96.27 (96.07)
-4.0
119.00(15),
114.76(15)
-19.6, -16.6
113.53(7)
-15.2
O(1)-Co-N(2)
118.65(17),
110.72(16)
95.73 (95.00)
92.37 (92.46)
-19.3, -13.5
113.50(6)
96.27 (96.06)
92.28 (92.09)
-15.2
O(2)-Co-N(2)
C(7)-O(1)-Co
96.53(16), 96.36(14)
125.1(3), 125.9(3)
-4.3, -4.1
+3, +2.3
96.18(6)
-4.1
128.87
125.15(13)
128.90
+3.5
(128.46)
(128.62)
C(27)-O(2)-Co
C(1)-N(1)-Co
C(21)-N(2)-Co
125.6(3), 125.7(3)
119.9(3), 119.7(3)
120.5(3), 119.8(3)
128.87
(128.45)
+2.6, +2.5
+2.6, +2.8
-0.6, +2.7
124.58(13)
120.11(14)
120.41(13)
128.89
(128.62)
+3.5
+2.6
+2.4
123.09
(123.13)
123.27
(123.41)
123.09
123.27
(123.13)
(123.41)
a Values for two crystallographically independent molecules; bisolated molecule (in parenthesis, values for the
solvated compound)

Graphical Abstract