Manganese(ii) Schiff-base-mediated reversible deactivation controlled radical polymerization of vinyl acetate

Article abstract of DOI:10.1039/d1nj00493j

A series of new manganese(ii) complexes were prepared with three bidentate Schiff-base ligands derived by condensation of salicylaldehyde and different cycloalkylamines (cycloalkyl = cyclopentyl, cyclohexyl, and cycloheptyl): [Mn(CyPen-Salicyl)2] (1), [Mn(CyHex-Salicyl)2] (2), and [Mn(CyHep-Salicyl)2] (3). The complexes 1-3 were characterized by FTIR, UV-vis and EPR spectroscopy, elemental analysis, cyclic voltammetry, and computational methods. The manganese(ii) complexes (1-3) were used as controlling agents for the polymerization of vinyl acetate (VAc) initiated by AIBN according to an organometallic-mediated radical polymerization (OMRP) mechanism. The VAc polymerization mediated by the Mn complexes suggests that the control level can be slightly tuned through substitution of the cycloalkyl group on the Schiff-base ligand. Kinetics studies and computational investigations support an RT mechanism and a tailorable Mn complex reactivity mainly altered by steric factors of the Schiff-base ligands. The calculated thermodynamic parameters agree with the mediating ability of these complexes, since the degree of polymerization control decreases with increasing steric hindrance of the Schiff-base ligands, as well as with increasing ΔG values for the formation of dormant species.

Full text of DOI:10.1039/d1nj00493j

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Manganese(II) Schiff-base-mediated reversible
deactivation controlled radical polymerization
of vinyl acetate†
Cite this: New J. Chem., 2021,
45, 10109
a
Patricia K. Hashimoto,a Larissa F. Oliveira,a Beatriz A. Riga-Rocha,
Antonio E. H. Machado,b Vinicius T. Santana,c Otaciro R. Nascimento,d
a
a
Valdemiro P. Carvalho-Jr
and Beatriz E. Goi
*
A series of new manganese(II) complexes were prepared with three bidentate Schiff-base ligands derived by
condensation of salicylaldehyde and different cycloalkylamines (cycloalkyl = cyclopentyl, cyclohexyl, and
cycloheptyl): [Mn(CyPen-Salicyl)2] (1), [Mn(CyHex-Salicyl)2] (2), and [Mn(CyHep-Salicyl)2] (3). The complexes 1–3
were characterized by FTIR, UV-vis and EPR spectroscopy, elemental analysis, cyclic voltammetry, and
computational methods. The manganese(II) complexes (1–3) were used as controlling agents for the polymeri-
zation of vinyl acetate (VAc) initiated by AIBN according to an organometallic-mediated radical polymerization
(OMRP) mechanism. The VAc polymerization mediated by the Mn complexes suggests that the control level
can be slightly tuned through substitution of the cycloalkyl group on the Schiff-base ligand. Kinetics studies and
computational investigations support an RT mechanism and a tailorable Mn complex reactivity mainly altered by
steric factors of the Schiff-base ligands. The calculated thermodynamic parameters agree with the mediating
ability of these complexes, since the degree of polymerization control decreases with increasing steric
hindrance of the Schiff-base ligands, as well as with increasing DG values for the formation of dormant species.
Received 29th January 2021,
Accepted 29th April 2021
DOI: 10.1039/d1nj00493j
rsc.li/njc
should be reduced to prevent side reactions that compete with
the propagation process. This fine adjustment, that lowers the
1. Introduction
As an advanced radical polymerization (RP) technique, possibility of occurrence of termination processes, is mediated
reversible-deactivation radical polymerization (RDRP) has been by a controlling agent, whose specific role is to mediate a
extensively studied and applied in the fields of materials dynamic equilibrium between the propagating radicals and
science and biological engineering. Such an innovative techni- the dormant species.6 The search for better controlling agents
que has revolutionized the synthesis of polymers as it enables and reaction conditions has given rise to several different RDRP
obtainment of materials with well-defined compositions, methodologies, each of them with a specific type of controlling
dimensions, uniformity, and architectures.1–5 RDRP is defined agent and activation/deactivation mechanism. Some of these
as a radical polymerization (RP) in which the growing chains mechanisms are transition metal-assisted,7 as in the case of
are deactivated reversibly, allowing an active–dormant equili- organometallic-mediated radical polymerization (OMRP).
brium to be estabilished.6 The main aspect, which must be
The OMRP activation/deactivation process is based on the
considered in order to minimize side reactions and improve the reversible formation of a metal–alkyl covalent bond. The reversi-
reversible deactivation character of a RP, is the probability of bility of such bonds and the existence of the persistent radical effect
termination events that occur. In essence, this probability (PRE)8 ensure controlled chain growth while keeping the concen-
tration of the active chains to a minimum, effectively preventing
unwanted bimolecular radical coupling reactions.9,10 The OMRP
a
ˆ
Faculdade de Ciencias e Tecnologia, UNESP – Univ. Estadual Paulista,
propagation process, depending on the experimental conditions,
may occur by either reversible termination (RT) or degenerative
transfer (DT) mechanisms.1,11–14 Following the RT mode, the
transition metal complex, unsaturated and at low oxidation state,
reacts reversibly with a propagating chain and generates a one-
electron oxidized organometallic complex as the dormant species.
In fact, as stated by Debuigne et al.,11 chain size homogeneity relies
on this equilibrium between the active and dormant species, and
CEP 19060-900, Presidente Prudente, SP, Brazil. E-mail: beatriz.goi@unesp.br
b
´
ˆ
Instituto de Quımica, Universidade Federal de Uberlandia, P.O. Box 593,
ˆ
Uberlandia 38400-089, Minas Gerais, Brazil
c CEITEC – Central European Institute of Technology,
ˇ
Brno University of Technology, Purkynova 123, 61200 Brno, Czech Republic
d
´
˜
˜
Instituto de Fısica de Sao Carlos, USP Universidade de Sao Paulo,
˜
CEP 13563-120, Sao Carlos, SP, Brazil
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1nj00493j
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Scheme 1 Reversible-termination (RT) and degenerative transfer (DT) mechanisms of OMRP reaction.
must be dynamic enough to give only a few monomeric units to be these properties, the first compounds intentionally applied in
added to the main chain at each new activation, maintaining the OMRP were the organo-cobalt porphyrin [(por)Co-R] and related
polymerization under control. The presence of the metal complex, low spin organo-cobalt complexes.20,21 Later, the high-spin cobalt
at low oxidation state, during the combination-dissociation process acetylacetonate complexes were presented as equally efficient
allows the polymerization control, in an RT mechanism, to be mediators.22,23 Currently, there are d-transition metal complexes
governed by the PRE (Scheme 1).11 In contrast, when the radical of iron, nickel, titanium, molybdenum, vanadium, palladium, and
concentration in the reaction medium surpasses the amount of the chromium.7,19 The possibility to change both the metal center and
metal complex, the DT mode prevails over the RT mode. Differently the ligand provides a broader range of opportunities for tailoring
from the latter, in the DT mechanism the dormant chain attached the mediating behavior compared with other CRP methods.
to the metal complex changes places with a propagating chain via a
Aiming to contribute to the development of new controlling
concerted reaction known as associative exchange.10,11 It is inter- systems for OMRP, our research group has investigated
esting to note that, as the DT mechanism does not imply the the mediating activity of cobalt complexes bearing either
presence of the low-oxidized metal complex during propagation, it a-diimine24,25 or Schiff-base26 ligands. We were also the first
cannot be governed by the PRE. In this way, radical concentration group to report NiII complexes with Schiff-bases as mediators
and polymerization rate during DT resemble those of a conven- for OMRP of vinyl acetate (VAc).27 This study reports the
tional RP. Along with radical concentration, there are other char- synthesis and characterization of a series of bidentate MnII
acteristics that may influence whether a mechanism will prevail or Schiff-base complexes derived from different cycloalkylamines
not, e.g., coordination sphere and nature of the transition metal, as along with their assessment as controlling agents for OMRP of
well as the nature of the monomer.11 Tailoring these properties VAc. Moreover, to the best of our knowledge, this is the first
enables obtainment of a fine adjustment such that the bond example of a mediator for CRP of VAc using a Mn complex.
strength is adequate to favor reversibility of the metal–carbon
covalent bond and rapid radical exchange.10
2. Results and discussion
2.1. Synthesis and characterization
The MnII Schiff-base complexes were successfully synthesized
and presented good yields via reaction of the appropriate
phenoxy-imine ligand with manganese chloride in a 2 : 1 mole
Several studies have been conducted in the past few decades
with the aim of producing suitable transition metal complexes for
OMRP.7,15–18 These studies have shown that the controlling ability
of a complex can be enhanced by properly selecting the metal and
its oxidation state, as well as its ancillary ligands.19 Combining
Scheme 2 Synthesis of ON donor Schiff-bases and their MnII complexes (1–3).
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ratio of ligand to metal precursor (Scheme 2). Spectroscopic and delocalization of the whole electronic system and results in
data confirmed the formation of the complexes as expected, the energy change of the p - p* and n - p* transition of the
whereas elemental analysis data were consistent with the conjugated chromophore.33,34 The results clearly indicate that
theoretical results.
the ligands coordinated to the MnII ions, which is in accor-
The FTIR spectra of the ligands were compared with those of dance with the other spectral data. Furthermore, the absorp-
the manganese(II) complexes in order to confirm the binding tion bands in the region around 380 nm were observed as low
mode of the Schiff-base ligands to the metallic ion in the intensity bands. These bands are considered to arise from the
complexes. Coordination of the ligands to the metal through forbidden d–d transition, which is generally not sufficiently
the nitrogen atom is expected to reduce the electron density in strong. The absorption spectra of complexes 1–3 are quite
the azomethine link and thus shift the n(CQN) absorption to a similar, showing the same amounts of bands at practically
lower frequency. In all complexes, the n(CQN) stretching the same absorption maxima. This similarity is a strong indica-
vibration of the azomethine group was shifted to a lower tion that the three complexes have the same geometric struc-
frequency range, 1590–1610 cmꢀ1. This decrease in bond ture with similar qualitative configuration of the molecular
strength was due to the transfer of double bond electron orbital.
density towards the metal and a decrease in the double bond
The optimized structures of complexes 1–3 are presented in
character of CQN, hence decreasing bond strength that, in Fig. 1 and their selected bond lengths and angles are listed in
turn, confirms coordination of the metal with azomethine- Table 1. As suggested by the theoretical data, these complexes
N.28,29 On complexation, the band due to phenolic C–O were stabilized in a distorted square planar coordination with
stretching was shifted to a higher frequency, 1440–1465 cmꢀ1
,
the two N- and O-atoms in trans configuration, with the
showing that the coordination occurred through the phenolic diagonal angles O–Mn–O (150.344–156.347) and N–Mn–N
oxygen atom. This was further supported by the disappearance (141.352–154.216) around the manganese atom. The bond
of the broad band around 3000 cmꢀ1, assigned to O–H lengths in the complexes were quite similar, with discrepancies
stretching, in the complexes.30 Coordination between the azo- of about 0.01–0.05 Å. The metal–ligand bond lengths were in
methine nitrogen and the phenolic oxygen was further sup- reasonable agreement with similar bond distances reported in
ported by the appearance of two bands around 525–545 the literature,29 with the Mn–O lengths slightly shorter than the
and 460–495 cmꢀ1 assigned to n(Mn–O)29 and n(Mn–N),29 Mn–N bond lengths, as expected. The Mn–O and Mn–N bond
respectively.
lengths estimated theoretically varied between 1.8645 and
The absorption spectra of the MnII complexes were recorded 1.8846 Å for Mn–O1, between 1.8627 and 1.8791 Å for
in the 200–800 nm range in CH2Cl2 (Fig. S1, ESI†), and their Mn–O2, between 1.9842 and 2.0006 Å for Mn–N1, and between
corresponding data are shown in the Experimental section. In 1.9539 and 1.9914 Å for Mn–N2 (Table 1; Fig. 1). Both phenolic
the absorption spectra of complexes 1–3, the wide range bands hydroxyl groups in the ligand were deprotonated upon
were due to either the p - p* and n - p* transition of the complexation.
CQN chromophore or charge transfer transition arising from p
Fig. 2 shows the EPR spectra of the frozen CH2Cl2 solution of
electron interactions between the metal and the ligand, which the MnII complexes (1–3) measured at X band frequency and at
involves either a metal-to-ligand or ligand-to-metal electron 4.7 Kelvin. These spectra present only one broad line in the
transfer.31,32 The absorption bands of the free ligands attrib- range of 10 to 810 mT characteristic of a zero field line, crossing
uted to the p - p* and n - p* transitions in the benzene ring the zero intensity around 40 mT. Using the approximate spin
or azomethine (–CQN) groups (316–318 nm) were shifted to Hamiltonian hn = D ꢁ gmB (D 4 4 gmB), where D is the zero
higher energy and presented different intensities in the spectra field splitting, m is the Bohr magneton, and B is the applied
of the metal complexes. The absorption shift and intensity magnetic field, it is possible to estimate the value of D as
change in the spectra of the metal complexes most likely 0.33 cmꢀ1, using g = 2. The spectrum of complex 1 shows
originate from the metalation that increases the conjugation hyperfine lines of the nuclear spins of Mn (I = 5/2) and two
Fig. 1 View of the optimized structures of complexes 1–3.
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Table 1 Theoretical data of relevant geometrical parameters of the
studied complexes. As reference for the atom numbering, see Fig. 5
Geometrical parameter
Complex 1
Complex 2
Complex 3
Mn–N(1)
Mn–N(4)
1.9907
1.9914
1.9842
1.9840
2.0006
1.9539
Mn–O(2)
Mn–O(3)
1.8645
1.8641
1.8792
1.8791
1.8846
1.8627
O(3)–Mn–O(2)
N(4)–Mn–N(1)
153.084
141.352
156.347
154.216
150.344
144.902
Fig. 3 Cyclic voltammograms of 1–3 in 1.0 ꢂ 10ꢀ3 M DMF solutions with
0.1 M n-Bu4NPF6 vs Ag/AgCl, obtained at 100 mV sꢀ1 at 25 1C.
Table 2 Cyclic voltammetry results for complexes 1–3
a
b
Complex
Epa (V)
Epc (V)
E1/2 (V)
DEp (V)
Ipa/Ipc
1
2
3
ꢀ0.326
ꢀ0.226
ꢀ0.321
0.527
ꢀ0.741
ꢀ0.762
—
ꢀ0.534
ꢀ0.494
—
0.415
0.536
—
1.481
1.430
—
Ferrocene
0.418
0.471
0.109
1.081
a
b
E1/2 = (Epa + Epc)/2. DEp = Epa ꢀ Epc
.
Fig. 2 EPR spectra of the frozen CH2Cl2 solution of MnII complexes 1, 2
and 3 measured at X band frequency and at 4.7 Kelvin. The inset shows the
hyperfine lines from the nuclear spins of Mn (I = 5/2) and two equivalent N
was calculated for complexes 1 and 2 at different scan rates,
and the Ipa/Ipc values were higher than that expected for a
reversible process (Table S1, ESI†), as observed for the ferrocene
(Ipa/Ipc = 1.081).
(I = 1) in complex 1. The EPR parameters are estimated as D B 0.33 cmꢀ1
AMn = 9.38 mT, AN> = 3.00 mT, and AN8 = 5.50 mT.
,
2.2. Polymerization of VAc with Mn complexes
equivalent N (I = 1) with values AMn = 9.38 mT, AN> = 3 00 mT,
and AN8 = 5.50 mT. The line width of the EPR signal was Studies addressing the controlling ability of VAc polymerization
strongly dependent on temperature, and no signal was by complexes 1–3 were performed with AIBN as the initiator
observed at room temperature. The presence of hyperfine under a [VAc]/[AIBN]/[Mn] molar ratio of 542/3.25/1 at 55 1C for
splitting due to two equivalent N (I = 1) in complex 1 corrobo- 240 min. The polymerization rate of VAc mediated by com-
rates the proposed structures of these molecules.
plexes 1–3 obeyed the following order: 3 4 2 4 1, with 2 and 3
Cyclic voltammetry measurements for complexes 1–3 were proceeding with higher activity compared to 1. All cases
recorded in DMF in the potential range ꢀ1.5 to 1.0 V at a scan followed a linear correlation of ln([VAc]0/[VAc]t) as a function
rate of 100 mV sꢀ1 using tetrabutylammonium perchlorate as of time (Fig. 4), with a pseudo first-order rate constant (kobs
)
an electrolyte. The cyclic voltammograms are presented in equal to 4.16 ꢂ 10ꢀ3, 8.71 ꢂ 10ꢀ3, and 9.67 ꢂ 10ꢀ3 sꢀ1 for
Fig. 3, whereas additional data are summarized in Table 2. A complexes 1, 2, and 3, respectively. Complexes 2 and 3 resemble
pair of cathodic/anodic peak potentials, assigned to the each other in terms of kinetic behavior. As presented in Fig. 4,
MnII/MnIII couple, was observed for complexes 1 and 2 with their conversion values and even their rate of polymerization
E1/2 values around ꢀ0.50 V, while complex 3 showed an are very similar, differentiating them from complex 1. A closer
irreversible peak at Epa = ꢀ0.32 V. The DEp values of these look at the order of magnitude of the rate constants reveals
complexes were higher than 0.059 V, indicating that this redox that, despite the conversion differences, all complexes seem to
system is not Nernstian, but rather, quasi-reversible. By varying mediate the VAc polymerization through the same mechanism.
the scan rate, the recorded voltammograms showed a contin- Thus, the lower conversions obtained with complex 1, when
uous increase for both the anodic and cathodic peak currents compared with 2 and 3 at the same polymerization time, seem
(Ipa, Ipc) followed by a shift in the anodic and cathodic peak to be a consequence of its rate of polymerization, which is
potentials to positive and negative values, respectively, increas- approximately half of the rate of polymerization for the other
ing the peak-to-peak difference DEp (Table S1, ESI†). Another two complexes. The order of activity of complexes 1–3 can be
criterion for reversibility is an anode/cathode peak current ratio explained by the increased steric hindrance with the increasing
close to 1 regardless of the scan rate. This reversibility criterion cycloalkyl ring of the Schiff-base ligand, favoring the lability of
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Fig. 5 Dependence of Mn with conversion and Ð on the reaction for
OMRP of VAc with 1 (&), 2 (J), and 3 (D); [VAc]/[AIBN]/[Mn] = 542/3.25/1
Fig. 4 Dependence of conversion and ln([VAc]0/[VAc]t) on the reaction
time for OMRP of VAc with 1 (&), 2 (J), and 3 (D); [VAc]/[AIBN]/
[Mn] = 542/3.25/1 with the complex in the bulk at 55 1C.
with the complex in the bulk at 55 1C. The dashed line is Mn(theo)
.
complexes presented rate constants within the order of magni-
tude of the typical OMRP polymerization proceeding via the
reversible-termination mechanism.37 In line with this, compu-
tational studies confirmed that the coupling of the propagating
radical with the MnII center occurred on only one face of the
molecule to form MnIII-VAc, whereas the double coupling of the
radical did not occur on both faces of the Mn center (VAc-MnIV-
VAc) to satisfy a DT mechanism. Formation of the organome-
tallic complexes MnIII-VAc with 1, 2, and 3 was not thermo-
dynamically favorable in any of the cases (Fig. 6). The DG values
increased with increasing number of CH2 groups in the
cycloalkyl ring (DG = 73.10, 81.50 and 87.05 kJ molꢀ1 for
complexes 1, 2, and 3, respectively), although the thermal
energy of the reaction mixture was sufficient to overcome these
potential energy barriers. The calculated thermodynamic para-
meters agree with the mediating ability of these complexes,
since the degree of polymerization control decreased with
increasing steric hindrance of the Schiff-base ligands, as well
as with increasing DG values for the formation of dormant
species.
the organo-manganese bond and thus displacing the OMRP
equilibrium towards the radical production. The linear profile
indicates that a constant concentration of propagating radicals
([Rꢃ]) was reached during the reaction and the rate of polymer-
ization depends only on the concentration of the radicals
entering the reaction. These affirmations are supported by
the mathematical definition of the kinetic plot slope, which
corresponds to kp[Rꢃ] in a controlled polymerization. Further-
more, the concentration of radicals in each different mecha-
nism can be described either by (ki/(2 ꢂ kt) ꢂ [initiator])1/2 in a
DT mechanism or by [MnIII-R]/([MnII] ꢂ keq) in a RT mecha-
nism. The experimental observations, combined with mathe-
matical interpretation, indicate that the VAc polymerization
mediated by complexes 1–3 is proceeding via one of the two
OMRP controlling mechanisms.
The linear increase in molecular weight with conversion
(Fig. 5) in conjunction with moderate polydispersity (Ðs) values
illustrates a certain level of control on the polymerization when
using complexes 1–3 as mediators. Molecular weight distribu-
tion curves (Fig S2, ESI†) were monomodal and shifted towards
lower elution times throughout the polymerization. However,
molecular weights were somewhat higher than the theoretical
values, which is a consequence of the number of growing
radical chains being lower than expected. Once the theoretical
3. Experimental
3.1. General remarks
molecular weight is calculated based on the [VAc]/[MnII] ratio All reagents were purchased from Aldrich Chemical Co. All
(see the Experimental section), it is presumed that the complete reactions and manipulations were performed under a nitrogen
conversion of MnII species to organo-MnIII complexes leads to atmosphere by using standard Schlenk techniques. Vinyl acet-
one growing chain per organo-MnIII unit. In this regard, ate (VAc) was washed with 5% NaOH solution, dried over
experimental molecular weights higher than the theoretical anhydrous MgSO4, degassed by several freeze-thawing cycles
ones indicate that the system is not achieving complete con- before being distilled from CaH2 and stored at ꢀ18 1C under
version of MnII to MnIII, implying that only a small fraction of nitrogen. MnCl2 was dried in a reaction flask carefully under
the manganese centers is operating as a chain growth reduced pressure (0.5 torr) by heating with a hair dryer for 2 h
mediator.35,36
immediately before use. Cyclopentylamine, cyclohexylamine,
Although polymerization occurred in the presence of excess cycloheptylamine, salicylaldehyde, and 2,20-Azobis(2-methyl-
radicals, in which a DT mechanism could be favored, the propionitrile) solution (AIBN) (0.2 M in toluene) were used as
results suggest that a RT mechanism was observed in the acquired. The Schiff-base ligands were prepared following the
kinetics studies using 1, 2, or 3 as OMRP mediators. All literature38 and their purity was checked by satisfactory
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Fig. 6 Proposed mechanism for OMRP of VAc mediated by complexes 1, 2, and 3.
elemental analysis and spectroscopic examination and the weight distribution of the polymers were determined by gel
results are reported in the ESI.†
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 1C and a
3.2. Analyses
Elemental analyses were performed using a PerkinElmer CHN RID-10A detector, equipped with two Shimadzu columns (GPC-
2400 at Elemental Analysis Laboratory at Institute of Chemistry – 805 : 30 cm, Ø = 8.0 mm). Theoretical molecular weights (Mn,th
)
USP. The FTIR spectra of KBr pellets were obtained on a Shi- were calculated from the [VAc]/[Mn] ratio and polymerization
madzu IRAffinity-1 FT-IR spectrometer. The 1H and 13C{1H}NMR conversion, using the relation Mn,th = ([VAc]0/[Mn]0) ꢂ MMVAc
ꢂ
spectra were obtained in CDCl3 at 298 K on an Agilent MR 400 Conversion. The retention time was calibrated using standard
Ultrashield spectrometer operating at 400.13 and 100.61 MHz, monodispersed polystyrene using HPLC-grade THF as an
respectively. The obtained chemical shifts were reported in ppm eluent at 40 1C with a flow rate of 1.0 mL minꢀ1. UV-vis
relative to the high frequency of TMS. Conversion was determined measurements were performed on a Cary 400 UV-vis spectro-
from the concentration of the residual monomer measured by gas photometer (Varian) using 1 cm path length quartz cells. Toluene
chromatography (GC) using a Shimadzu GC-2010 gas chromato- solutions of the complexes of 0.2 mM concentrations were used
graph equipped with a flame ionization detector and a 30 m for these measurements. Electrochemical measurements were
(0.53 mm I.D., 0.5 mm film thickness) SPB-1 Supelco fused silica performed using an Autolab PGSTAT204 potentiostat with a
capillary column. Anisole was added in the system as an internal stationary platinum disk and a wire as working and auxiliary
standard. Analysis conditions: injector and detector temperature, electrodes, respectively. The reference electrode was Ag/AgCl. The
250 1C; temperature program, 40 1C (4 min), 20 1C minꢀ1 until measurements were performed at 25 1C ꢁ 0.1 in DMF with 0.1 M
200 1C, 200 1C (2 min). The molecular weights and the molecular n-Bu4NPF6.
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3.3. General procedure for the preparation of Schiff-base
metal complexes
symmetry constraints. As the MnII complex under study is a high-
spin pseudotetrahedral compound (S = 3/2), all calculations were
done considering a multiplicity equal to 4, unless when the
product of the coupling between the complex and the vinyl acetate
radical was studied, when the multiplicity was defined as being
one. In order to reproduce the experimental conditions, the
simulations involving these compounds were also done under
the solvation condition. For this, the previous structure was
reoptimized using the IEFPCM method42 to build a dielectric
continuum with the characteristics of dichloromethane, in a self-
consistent reaction field procedure. The structure of the product
of coupling between the MnII complexes and the vinyl acetate
radical was optimized only under the solvation condition. The
thermodynamic parameters DH, DG, and TDS were estimated for
the formation of some of the studied complexes, being calculated
from thermochemical data generated from the calculation of the
vibrational frequencies.
Synthesis of the MnII complexes was accomplished according to
the following procedure: 0.42 g (2 mmol) of the appropriate
Schiff-base was dissolved in 20 mL of absolute methanol, and
the solution was transferred to a 50 mL three-necked flask
fitted with a reflux condenser. Afterwards, a methanolic
solution (10 mL) of 0.13 g (1 mmol) of MnCl2 was added
dropwise to the solution of the ligand, and the stirred mixture
was refluxed for 12 h. After being cooled to room temperature,
the solution was reduced to half its original volume and was
stored in a refrigerator overnight. Finally, the red precipitate
was collected by filtration, washed with methanol, and dried
under reduced pressure for several hours to afford the desired
complexes in 76–87% yield before being characterized.
Complex 1 (R = pentyl): Yield: 76%; m.p. = 222 1C (a) UV–Vis
(CH2Cl2): lmax(n) (nm), emax(n) [Mꢀ1 cmꢀ1]: lmax(1) (227), emax(1)
[7.7 ꢂ 104]; lmax(2) (255), emax(2) [5.3 ꢂ 104]; lmax(3) (316), emax(3)
[2.1 ꢂ 104]; (b) IR (KBr, cmꢀ1): nCQN (1603), nC–O (1444),
nMn–O (529). nMn–N (494); (c) Anal. calculated for
3.6. OMRP procedure
The Schiff-base metal complex (100 mg, 0.02 mmol) and AIBN (10
mg, 0.065 mmol) 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 mixture was stirred and heated at 55 1C 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 the addition of TEMPO to neutralize the radicals.
C
24H28MnN2O2: C, 66.81; H, 6.54; N, 6.49; found: C, 66.79; H,
6.36; N, 6.22.
Complex 2 (R = hexyl): Yield: 87%; m.p. = 225 1C (a) UV–Vis
(CH2Cl2): lmax(n) (nm), emax(n) [Mꢀ1 cmꢀ1]: lmax(1) (227), emax(1)
[7.2 ꢂ 104]; lmax(2) (255), emax(2) [4.8 ꢂ 104]; lmax(3) (316), emax(3)
[1.9 ꢂ 104]; (b) IR (KBr, cmꢀ1): nCQN (1606), nC–O (1448),
nMn–O (542). nMn–N (463); (c) Anal. calculated for
C
26H32MnN2O2: C, 67.96; H, 7.02; N, 6.10; found: C, 67.74; H,
6.86; N, 6.28.
4. Conclusions
Complex 3 (R = heptyl): Yield: 82%; m.p. = 219 1C (a) UV–Vis
(CH2Cl2): lmax(n) (nm), emax(n) [Mꢀ1 cmꢀ1]: lmax(1) (227), emax(1)
[5.7 ꢂ 104]; lmax(2) (255), emax(2) [3.7 ꢂ 104]; lmax(3) (316), emax(3)
[1.5 ꢂ 104]; (b) IR (KBr, cmꢀ1): nCQN (1597), nC–O (1462),
nMn–O (543). nMn–N (481); (c) Anal. calculated for
The manganese(II) Schiff-base complexes (1–3) were successfully
synthesized and characterized by FTIR, UV-vis and EPR spectro-
scopy, elemental analysis, cyclic voltammetry, and computational
methods. The preliminary VAc polymerizations mediated by 1–3
as a function of time ([monomer]/[AIBN]/[complex] = 542/3.25/1)
at 55 1C were first order in monomer, as assessed by the linear
dependence of ln([M]0/[M]t) vs. time. In VAc polymerization, the
MnII complexes presented a reactional similarity with relatively
low Ð values. The VAc polymerization mediated by these com-
plexes suggests that the control level can be slightly tuned through
substitution of the cycloalkyl group on the Schiff-base ligand and
through the metal used in the mediating complexes. The differ-
ence in the reactivity between the complexes studied was asso-
ciated with the steric characteristics of the Schiff-base ligands,
which are modulated by their substituents (cycloalkyl groups) and
by the nature of the metal–carbon bond. The calculated thermo-
dynamic parameters agree with the mediating ability of these
complexes, since the degree of polymerization control decreases
with increasing steric hindrance of the Schiff-base ligands, as well
as with increasing DG values for the formation of dormant
species.
C
28H36MnN2O2: C, 68.98; H, 7.14; N, 5.75; found: C, 69.18; H,
7.12; N, 5.87.
3.4. EPR measurements
The MnII complexes in frozen CH2Cl2 solution forms were
placed into quartz tubes and introduced in a rectangular
microwave cavity in the Varian X-band equipment (Model E-
109) using an Oxford continuous helium flow cryostat to obtain
EPR spectra. The measurements were taken at a temperature of
4.7 K. EPR conditions were as follows: microwave frequency of
9.270 GHz, 100 kHz field modulation with an amplitude
of 1.0 mT, 5 mW of microwave power, gain of 1000, center
field of 410 mT and scan range of 800 mT, time constant of
0.128 s, and scan field at 4 min collecting 4096 points.
3.5. Theoretical calculations
The structure of the compounds under study was optimized using
the density functional theory (DFT) at the level of the hybrid
functional M06,39 implemented in Gaussian 09,40 in combination
with the triple-zeta basis set def2-TZVP.41 The optimizations and
Conflicts of interest
calculation of the vibrational frequencies were done without any There are no conflicts to declare.
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´ ˆ
23 A. Debuigne, J.-R. Caille and R. Jerome, Angew. Chem., Int.
Acknowledgements
Ed., 2005, 44, 1101–1104.
BEG and VPCJ are indebted to FAPESP (Proc. 2018/19395-9 and
Proc. 2018/06340-1 respectively) for financial support. ORN and
AEHM are also indebted to FAPEMIG (Proc. CEX – APQ-03017-
16) and CNPq (Proc. 307443/2015-9 and Proc. 305668/2014-5)
for financial support. This work was supported by CNPq
24 B. A. Riga, M. D. Neves, A. E. H. Machado, D. M. S. Arau´jo,
J. R. Souza, O. R. Nascimento, V. T. Santana,
C. C. S. Cavalheiro, V. P. Carvalho-Jr and B. E. Goi, Inorg.
Chim. Acta, 2018, 471, 620–629.
25 B. A. Riga, Y. F. Silva, O. R. Nascimento, A. E. H. Machado,
V. P. Carvalho-Jr and B. E. Goi, Polyhedron, 2020,
192, 114870.
26 Y. F. Silva, B. A. Riga, V. M. Deflon, J. R. Souza,
L. H. F. Silva, A. E. H. Machado, P. I. S. Maia,
V. P. Carvalho-Jr and B. E. Goi, J. Coord. Chem., 2018, 71,
3776–3789.
˜
´
and Coordenaçao de Aperfeiçoamento de Pessoal de Nıvel
Superior – Brasil (CAPES) – Finance Code 001. AEHM partici-
pated in this study during his stay as a Visiting Professor at the
´
˜
´
Federal University of Goias, Catalao, Goias, Brazil.
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