B.A. Riga, Y.F. Silva, O.R. Nascimento et al.
Polyhedron 192 (2020) 114870
line, successfully improved ethylene polymerization by developing
V(III) complexes that also bear a-diimine ligands.
kac
[CoIII
Preformed
]
Pn
[CoII]
kt
+
Pn
kdeact
A recent branch in the organometallic catalysis field is the
mediation of free-radical polymerization reactions using
organometallic compounds. Organometallic mediated radical poly-
merization (OMRP) is an attractive area where the organometallic
compounds are planned to achieve robustness and efficiency in
order to provide fewer lateral reactions and greater definition to
the polymer chains. The general OMRP mechanism illustrating
the role played by cobalt complexes in this reaction is presented
in Scheme 1.
As suitable ligands capable of tailoring the behavior of coordi-
nation compounds, the literature addressing a-diimines bound to
different metals is vast [19-27]. Although there are many reports
on the use of early metal transition complexes for OMRP mediators
kp
or generateded in situ
Monomer
Scheme 1. General mechanism of cobalt-mediated radical polymerization.
2.2.2. Ligand Hex-DAB (1b)
Yield: 75%; (a) UV–Vis: kmax(n) (nm), emax(n) [Mꢀ1 cmꢀ1]: kmax(1)
(230),
e
max(1) [95960]; kmax(2) (268),
emax(2) [8040]; (b) IR (KBr): mx
(cmꢀ1):
m
C@N (1642); (c) 1H NMR (CDCl3, d): 7.93 (2H, –C@N), 3.15
(2H, 1-CH), 1.82–1.20 (8H, 2,6-CH2), 1.82–1.20 (4H, 4-CH2),
1.82–1.20 (8H, 3,5-CH2); (d) 13C NMR (CDCl3, d): 160.07 (–C@N),
69.38 (1-CH), 33.93 (2,6-CH2), 25.47 (4-CH2), 24.55 (3,5-CH2).
[12,28-37], studies on the behavior of Co(II)-a-diimine as mediator
for vinylic monomers are incipient. Our research group has taken
2.2.3. Ligand Hept-DAB (1c)
the first step into this challenge. In our previous report [38], we
Yield: 39%; (a) UV–Vis: kmax(n) (nm), emax(n) [Mꢀ1 cmꢀ1]: kmax(1)
(239), emax(1) [155410]; kmax(2) (281), emax(2) [20340]; (b) IR (KBr):
mx (cmꢀ1): mC=N (1622); (c) 1H NMR (CDCl3, d): 7.87 (2H, –C@N),
3.32 (2H, 1-CH), 1.51–1.49 (8H, 2,7-CH2), 1.61–1.59 (8H,
4,5-CH2), 1.73–1.71 (8H, 3,6-CH2); (d) 13C NMR (CDCl3, d): 159.31
(-C@N), 71.98 (1-CH), 35.96 (2,7-CH2), 28.52 (4,5-CH2), 24.53
(3,6-CH2).
evaluated how the a-diimine ligand steric hindrance could change
the mediator control capability. Herein, we report our observations
on how cycloalkyl substituents can modify the efficiency for a set
of cobalt(II)-a-diimine complexes in the mediation of vinyl acetate
(VAc) radical polymerization (Fig. 1). To this end, we systematically
varied the number of carbon atoms in the cycloalkyl substituent
and assessed their effect by measuring the main properties of the
formed polymer.
2.2.4. Ligand Oct-DAB (1d)
Yield: 65%; (a) UV–Vis: kmax(n) (nm), emax(n) [Mꢀ1 cmꢀ1]: kmax(1)
2. Experimental
(230),
e
max(1) [97290]; kmax(2) (276),
m
emax(2) [14070]; (b) IR (KBr): mx
(cmꢀ1):
C@N (1624); (c) 1H NMR (CDCl3, d): 7.89 (2H, –C@N), 3.35
2.1. General remarks
(2H, 1-CH), 1.80–1.54 (8H, 2,8-CH2), 1.80–1.54 (8H, 4,6-CH2),
1.80–1.54 (4H, 5-CH2), 1.80–1.54 (8H, 3,7-CH2); (d) 13C NMR
(CDCl3, d): 159.31 (–C@N), 71.17 (1-CH), 33.52 (2,8-CH2), 27.28
(4,6-CH2), 25.65 (5-CH2), 23.92 (3,7-CH2).
All reagents were purchased from Aldrich Chemical Co. All reac-
tions and manipulations were performed under nitrogen atmo-
sphere using standard Schlenk techniques. Vinyl acetate (VAc)
(>99%) was washed with 5% NaOH solution, dried over anhydrous
Mg2SO4, degassed by several freeze-thawing cycles before being
distilled from CaH2 and stored at ꢀ 8 °C under nitrogen. Glyoxal
(40% aqueous solution) was stored at + 4 °C. CoCl2, cyclopenty-
lamine, cyclohexylamine, cycloheptylamine, cyclooctylamine,
2,2,6,6-tetramethyl-1-piperidinoxyl (TEMPO) and 2,20-Azobis(2-
methylpropionitrile) solution (AIBN) (0.2 M in toluene) were used
as acquired.
2.3. Synthesis of [CoCl2(R-DAB)]
A solution of anhydrous CoCl2 (1 mmol) in degassed acetone
(50 mL) was treated by adding the respective ligand (R-N@CH-
CH@N-R) (1 mmol) in a mixture of acetone and dichloromethane
1:1 (25 mL). The reaction mixture was stirred for 30 min at
25 °C. Then the volume was partially reduced and the precipitate
formed was isolated by filtration. The excess of ligand was
removed by washing the precipitate with small portions of cold
n-pentane (3 ꢁ 15 mL). This procedure was adapted from those
described by Barral [40] and Avilés [41].
2.2. Synthesis of ligands R-DAB
Adapting the procedure described by Delaude et al. [39], a mix-
ture of glyoxal (0,25 mol), propanol (100 mL) and water (50 mL)
was added dropwise to a solution of the respective cycloalky-
lamine (0,5 mol, cyclopentylamine-1a, cyclohexylamine-1b, cyclo-
heptylamine-1c and cyclooctylamine-1d) in propanol (300 mL).
The reaction mixture was stirred for 2 h at 25 °C, then the
orange-colored resulting suspension was filtered with suction
and the compound was rinsed with cold propanol (2 ꢁ 100 mL).
Further purification was performed by recrystallization from ace-
tonitrile, yielding 1a-d as white crystalline solids.
2.3.1. Complex [CoCl2(Pent-DAB)] (2a)
Yield: 90%; (a) UV–Vis: kmax(n) (nm), emax(n) [Mꢀ1 cmꢀ1]: kmax(1)
(232), emax(1) [93210]; kmax(2) (296), emax(2) [39250]; kmax(3) (349),
emax(3) [30920], kmax(4) (627), emax(4) [2960], kmax(5) (664), emax(5)
[2990], kmax(6) (701), emax(6) [2240]; (b) IR (CsI): mx (cmꢀ1): mC@N
(1644), masCo–Cl (312), msCo–Cl (286), mCo–N (262); (c) Anal. calculated
for C12H20Cl2CoN2: C, 44.74; H, 6.26; N, 8.70, found: C, 44.97; H,
6.01; N, 8.81.
2.3.2. Complex [CoCl2(Hex-DAB)] (2b)
2.2.1. Ligand Pent-DAB (1a)
Yield: 79%; (a) UV–Vis: kmax(n) (nm), emax(n) [Mꢀ1 cmꢀ1]: kmax(1)
(230), emax(1) [90390]; kmax(2) (256), emax(2) [99440]; kmax(3) (419),
emax(3) [4190] , kmax(4) (598), emax(4) [2700], kmax(5) (650), emax(5)
[4060], kmax(6) (698), emax(6) [6250]; (b) IR (CsI): mx (cmꢀ1): mC@N
(1624), masCo–Cl (350), msCo–Cl (334), mCo–N (287); (c) Anal. calculated
for C14H24Cl2CoN2: C, 48.02; H, 6.91; N, 8.00, found: C, 48.12; H,
Yield: 33%; (a) UV–Vis: kmax(n) (nm), emax(n) [Mꢀ1 cmꢀ1]: kmax(1)
(234),
emax(1) [99105]; kmax(2) (278), emax(2) [13590]; (b) IR (KBr): mx
(cmꢀ1):
m
C@N (1615); (c) 1H NMR (CDCl3, d): 7.92 (2H, –C@N), 3.73
(2H, 1-CH), 1.88 (8H, 2,5-CH2), 1.70 (8H, 3,4-CH2); (d) 13C NMR
(CDCl3, d): 160.03 (–C@N), 71.41 (1-CH), 34.38 (2,5-CH2), 24.75
(3,4-CH2).
6.89; N, 8.13.
2