Synthesis, characterization, and mechanism studies of bis(imino)pyridine ligands and their Cr(III) compounds

Article abstract of DOI:10.1134/S1070328415020050

Two new bis(imino)pyridine ligands L¹, L² (L¹ = 2,6-bis[1-(4-bromophenylimino)]pyridine, L² = 2,6-bis[1-(4-methylphenylimino)]pyridine) have been prepared. Their compounds [CrCl₃L¹] (I) and [Cr(Cl)(μ-Cl)(L²′)]₂ (II) (L²′ = 6-bis(6-methylquinoline)pyridine) for olefin oligomerization were obtained by hydrothermal methods, during which L²′ was appeared from L², and characterized by elemental analysis, single-crystal X-ray analysis (CIF files CCDC nos. 1025448 (I),1025447 (II)), IR spectra and melting point test. Compound I is a mononuclear compound, while compound II, bridged with the chlorine atoms, is a binuclear compound. The transformation mechanism between L² and L²′ is also studied, and the possible reason is due to the presence of the electron-donating groups as substituents on L². The subsequent studies on that will be carried on.

Full text of DOI:10.1134/S1070328415020050

ISSN 1070ꢀ3284, Russian Journal of Coordination Chemistry, 2015, Vol. 41, No. 2, pp. 129–134. © Pleiades Publishing, Ltd., 2015.
Synthesis, Characterization, and Mechanism Studies
1
of Bis(imino)pyridine Ligands and Their Cr(III) Compounds
a
a
a
a,
b
S. W. Li , Y. F. Wang , X. P. Li , J. S. Zhao *, and J. L. Wang
a
Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, Shaanxi Key Laboratory
of PhysicoꢀInorganic Chemistry, College of Chemistry & Material Science, Northwest University,
Xi’an, Shaanxi, 710069 P.R. China
b
Composites Research Institute, Weinan Normal University, Weinan, 714000 P.R. China
*
eꢀmail: jszhao@nwu.edu.cn
Received July 8, 2015
1
2
1
Abstract—Two new bis(imino)pyridine ligands L , L (L = 2,6ꢀbis[1ꢀ(4ꢀbromophenylimino)]pyridine,
2
1
L = 2,6ꢀbis[1ꢀ(4ꢀmethylphenylimino)]pyridine) have been prepared. Their compounds [CrCl L ] (
I) and
3
2
2
[
Cr(Cl)(
by hydrothermal methods, during which L was appeared from L , and characterized by elemental analysis,
singleꢀcrystal Xꢀray analysis (CIF files CCDC nos. 1025448 ( ),1025447 (II)), IR spectra and melting point
test. Compound
binuclear compound. The transformation mechanism between L and L ' is also studied, and the possible reaꢀ
son is due to the presence of the electronꢀdonating groups as substituents on L . The subsequent studies on
µ
ꢀCl)(L ')] (II) (L ' = 6ꢀbis(6ꢀmethylquinoline)pyridine) for olefin oligomerization were obtained
2
2'
2
I
I
is a mononuclear compound, while compound II, bridged with the chlorine atoms, is a
2
2
2
that will be carried on.
DOI: 10.1134/S1070328415020050
1
INTRODUCTION
reactivity and enantioselectivity of the catalytic reacꢀ
tion.
Recently, there has been considerable interest in
the use of metal compounds for olefin oligomerizaꢀ
Steric effects and the electronic factor of ligands
tion, which are important reactants used in the prepaꢀ affect the catalytic behavior of ethylene polyꢀ or oligoꢀ
ration of plasticizers, lubricants, detergents, oil field
chemicals, and monomers for copolymerization. In
the past decade, late transition metal compounds are
the most attractive candidates toward ethylene activaꢀ
tion both in academic and industrial research [1–5].
However, until the middle of 20th century, only few
reports introduced these compounds as catalysts,
mechanism, and characteristics of the polymer prodꢀ
uct. Furthermore the changing of substituents on the
ligands only causes the transfer of iron catalyst from
oligomerize ethylene to linear
αꢀolefins with high
activity [12–14]. Halogens are a family of interesting
substituents, which mainly lead to the electronic
which major due to
β
ꢀhydrogen elimination occurring effects. Compared to the small difference in electronic
in competition with ethylene insertion and chain effect origination from changing the alkyl groups, the
propagation of the polymer [6–8]. Based on the prinꢀ
ciple, which the use of bulky substituents on the aryl
groups of the diimine ligands blocks associative olefin
exchange, so retarding the chain transfer reactions and
leading to high molecular weight polymers. Brookhart
et al. and Gibson et al. reported a new class of iron and
cobalt compounds bearing bis(imino)pyridine ligands
electronic effect from varying halogens is much more
significant due to the large difference in their electronꢀ
gativities. The halogens are generally electronꢀattractꢀ
ing, while the alkyls are electronꢀdonating.
In this paper, we have synthesized two new 2,6ꢀ
1
2
bis(imino)pyridine ligands L and L , and their comꢀ
[
9–11].
As an important class of N–N ligands, pyridineꢀ Cl)(L ')] (II) under hydrothermal conditions [15–
1
pounds, namely [CrCl L ] (
I
) and [Cr(Cl)( ꢀ
μ
3
2
2
Schiff based imines have drawn increasing attention 18]. The two new ligands and their compounds were
recently due to their flexible coordination behavior
associated with steric and electronic properties. The
presence of a pyridine Nꢀdonor could significantly
affect the way a ligand compound to the central metal,
which in turn would lead to dramatic changes in the
well confirmed by elemental analysis, while the singleꢀ
crystal Xꢀray analysis was only applied to the comꢀ
pounds. In addition, the transformation mechanism
between L and L ' has been studied, which possibly
due to the presence of the electronꢀdonating groups as
substituents.
2
2
1
The article is published in the original.
129

130
LI et al.
EXPERIMENTAL
Materials and measurements. All manipulations of
airꢀ and/or moistureꢀsensitive compounds were perꢀ
formed under a nitrogen atmosphere using standard
Schlenk techniques. Starting chemicals and solvents
were of reagent grade and were used as received. Eleꢀ
perature, the precipitate was filtered and washed sucꢀ
cessively with ꢀhexane. The product was dried under
vacuum, and obtained light yellow powder (the yield
n
was 0.814 g (76.1%); m.p. = 203.7–204.4
°
C).
For C H N Br
19 13
3
2
mental analyses were performed by the VARIOEL III. anal. calcd., %: C, 51.50;
H, 2.96;
H, 2.97;
N, 9.48.
1
12
H and C NMR were recorded on INOVAꢀ400 MHz
Found, %:
C, 50.37;
N, 9.16.
spectrometers. The FT–IR spectra were obtained
from KBr pellets on a Bruker Vectorm 22 spectrometer
–1
1
IR (KBr;
400 MHz; CDCl3;
ν
, cm ): 1621
, ppm): 8.28 (d.,
= 7.6 Hz, 1H), 7.75(s., 2H), 7.55(d.,
ν
(C=N). H NMR
–1
in the 400–4000 cm region.
(
δ
J = 7.7 Hz, 2H),
Synthesis of the 2,6ꢀpyridineꢀdimethylformate.
A
8
.19 (t.,
J
J
=
solution of 2,6ꢀpyridinedicarboxylic acid (15.0 g),
8.4 Hz, 4H), 7.31 (d., 4H).
SOCl and
N,Nꢀdimethylformamide was refluxed for
2
Synthesis of the L2 was carried out by the same synꢀ
1
0 h, cooled and then added 100 mL absolute methaꢀ
1
thetic procedure as that for L , except that
pꢀtoluidine
nol carefully under stirring in ice baths. The reaction
mixture was refluxed for 1 h and then diluted with
methanol. The white crystal was collected, washed
with cold methanol and dried in vacuum (the yield was
was used instead of 4ꢀbromaniline. A light yellow acicꢀ
ular crystal was obtained (the yield was 0.814 g
(
69.7%); m.p. = 178.8–179.3 C).
°
8
5.1%, m.p. = 41.0–42.5
Synthesis of the 2,6ꢀbis(hydroxymethyl)pyridine.
solution of 2,6ꢀpyridinedicarboxylic acid (3.30 g) in
absolute ethyl alcohol (60 mL) was added NaBH4 Found, %:
4.01 g) dropwise under stirred in iceꢀsalt baths. After
h, refluxed for 10 h following stirred at the room
°
C).
For C H N
3
21
19
A
anal. calcd., %: C, 80.48;
H, 6.11;
H, 6.09;
N, 13.41.
N, 13.34.
C, 79.62;
(
2
–1
1
IR (KBr;
ν
, cm ): 1620
ν
(C = N). H NMR
temperature for 3 h, the reaction mixture was evapoꢀ
rated under reduced pressure. Then, acetone was
added (20 mL), and solution was refluxed and evapoꢀ
rated again. After cooling to room temperature, potasꢀ
sium carbonate solution (20 mL) and water were
added, and the reaction mixture was extracted with
(
400 MHz; CDCl3;
δ
, ppm): 8.68 (d.,
= 7.6 Hz, 1H), 7.75 (s., 2H), 7.55 (d.,
.4 Hz, 8H), 6.96 (d., = 7.4 Hz, 4H), 6.62 (d.,
J = 7.3 Hz, 2H),
8
8
7
.28 (t.,
J
J
J
=
=
J
.8 Hz, 4H), 2.42 (s., 6H).
Synthesis of compound I. CrCl3 6H O (0.0139 g)
was added to an ethanol solution, which contained L
⋅
2
1
CHCl . The collected organic layers were dried over
3
(
0.0226 g) and distilled water. Then mixture was put
into a stainlessꢀsteel reactor with Teflon liner, heated
to 180 C and kept at constant temperature for 72 h.
NaSO , filtered and evaporated under reduced presꢀ
4
sure to give a pale white acicular crystal (the yield was
°
1
.20 g (51.1%); m.p. = 115.6–116.7 C).
°
Dark green crystals of compound
cooling to room temperature.
I were obtained after
Synthesis of the 2,6ꢀbis(methanal)pyridine. The
newꢀmade MnO (10 g) was added into the stirred
2
solution of 2,6ꢀbis(hydroxymethyl)pyridine (1.39 g) in
For C H N Cl Br Cr
19
13
3
3
2
CHCl (90 mL). After cooling to room temperature,
3
anal. calcd., %: C, 37.93;
H, 2.8;
N, 6.99.
the reaction mixture was filtered, and MnO was
2
watered several times with CHCl . The combined Found, %:
C, 37.89;
H, 2.357;
N, 6.768.
3
organic layers were evaporated under reduced pressure
to give a pale acicular crystals (the yield was 0.706 g
Synthesis of compound II was carried out by the
same synthetic procedure as that for compound
except that L was used instead of L . Reddish brown
(
52.3%); m.p. = 114.0–115.1 C).
°
I,
2
1
For C H NO
2
crystals of compound II were obtained.
7
5
anal. calcd., %: C, 62.22;
Found, %: C, 62.39;
H, 3.73;
H, 3.19;
N, 10.37.
N, 10.61.
For C H N Cl Cr
2
5
0
38
6
4
anal. calcd., %: C, 62.12;
C, 62.05;
H, 3.95;
N, 8.68.
N, 8.54.
–1
1
IR (
400 MHz; CDCl3;
= 7.5 Hz, 2H), 8.22 (t.,
Synthesis of the L1. An ethanol solution of 4ꢀbroꢀ tion measurements were performed on Brucker Smart
maniline (0.6397 g, 5.0 mmol) and glacial acetic acid Apexccd diffractometer with a graphiteꢀmonochroꢀ
was added to an ethanol solution of 2,6ꢀbis(methaꢀ mated Mo radiation at 296(2) K. Absorption corꢀ
ν
, cm ): 3085, 2865, 1720, 620. H NMR Found, %:
H, 3.876;
(
J
δ
, ppm): 10.22 (s., 2H), 8.49(d.,
J
= 7.7 Hz, 1H).
Xꢀray crystallography. Singleꢀcrystal Xꢀray diffracꢀ
K
α
nol)pyridine (0.3313 g, 2.5 mmol). The mixture soluꢀ rections were applied to the date using SADABS proꢀ
tion was refluxed for 4 h. After cooling to room temꢀ gram. The hydrogen atoms were assigned with comꢀ
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 41
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SYNTHESIS, CHARACTERIZATION, AND MECHANISM STUDIES
131
Table 1. Crystallographic data and refinement parameters for complexes
I
and II
Value
Parameter
I
II
M
601.49
Block
Green
966.07
Block
Brown
Description
Color
Crystal size, mm
Crystal system
0.20
Monoclinic
2 /
×
0.13
×
0.09
0.30
×
0.19
×
0.09
Triclinic
Space group
P
n
1
P1
a
, Å
, Å
c, Å
, deg
, deg
, deg
, Å3
13.722(3)
18.905(2)
9.3530(3)
90
10.646(16)
b
11.114(16)
12.160(18)
α
91.039(19)
β
124.702(12)
90
110.219(18)
γ
99.676(19)
V
Z
ρ
F
θ
2156.4(7)
4
1326(3)
2
, g cm–3
1.853
1.456
calcd
(000)
1172
594
Range, deg
2.18–25.94
11189
1.79–25.10
Number of data collected
Number of unique data
Number of parameters
Goodnessꢀofꢀfit on F2
6515
4177
4625
317
254
0.967
1.045
R
R
indices (
indices (all data)
Å–3
I
> 2
σ
(
I
))
R1 = 0.0466, wR2 = 0.1147
R1 = 0.1043, wR2 = 0.1534
–0.413/0.3614
R1 = 0.1274, wR2 = 0.3312
R1 = 0.2310, wR2 = 0.3960
–0.998/0.928
Δρmin/Δρmax,
e
mon isotropic displacement factors and included in
the final refinement by use of geometrical restrains,
while the nonꢀhydrogen atoms were treated with comꢀ
mon anisotropic displacement factors and included in
the final refinement with geometrical restrains.
RESULTS AND DISCUSSION
The structure of is shown in Fig. 1a. The unit conꢀ
I
3+
1
–
tains one Cr ion, one L ligand, and three Cl ions.
3+
The coordination geometry of the Cr ion is a disꢀ
torted octahedron, coordinated by three nitrogen
The structures were solved by direct methods and
1
–
2
atoms from L and three Cl ions. The lengths of two
bonds (Cr(1)–N(1) 2.170(5) Å and Cr(1)–N(3)
refined on
F
by fullꢀmatrix leastꢀsquares using
the SHELXꢀ97 program. The details of the crystal
parameters, data collections and refinements for comꢀ
2
.105(5) Å) are slightly longer than that of Cr(1)
bond (1.997(4) Å). The bond lengths of N(1)–C(7)
1.283(7) Å) and N(3)–C(13) (1.277(7) Å) show the
⎯N(2)
pounds
I and II are summarized in Table 1, and the
(
selected bond lengths and angles are listed in Table 2.
typical double bond properties, while the bond angles
N(2)Cr(1)N(1), N(2)Cr(1)N(3), and
, 76.68(18) , 153.84(18)
), 1025447 (II)); deposit@ccdc. indicate that the Cr ion occupies at a serious disꢀ
cam.ac.uk or http://www.ccdc.cam.ac.uk). torted octahedral coordination environment.
Supplementary material has been deposited with of
the Cambridge Crystallographic Data Centre N(3)Cr(1)N(1) (77.86(11)
°
°
°
)
3+
(
nos. 1025448 (I
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132
LI et al.
Table 2. Selected bond distances (Å) and angles (deg) for complexes
I
and II
*
Bond
d
, Å
Bond
d, Å
I
C r ( 1 ) – N ( 2 )
C r ( 1 ) – N ( 1 )
C r ( 1 ) – C l ( 2 )
1.997(4)
2 . 1 7 0 ( 5 )
2 . 3 0 2 5 ( 1 9 )
Cr(1)–N(3)
C r ( 1 ) – C l ( 1 )
C r ( 1 ) – C l ( 3 )
2.105(5)
2 . 2 9 5 9 ( 1 7 )
2 . 3 0 4 2 ( 1 9 )
II
Cr(1)–Cl(1)#1
Cr(1)–N(2)
Cr(1)–N(1)
2.303(3)
1.994(9)
2.162(8)
Cr(1)–Cl(1)
Cr(1)–N(3)
Cr(1)–Cl(2)
2.2960(17)
2.130(9)
2.296(4)
Angle
ω
, deg
Angle
ω
, deg
I
N(2)Cr(1)N(3)
N(3)Cr(1)N(1)
N(3)Cr(1)Cl(1)
N(2)Cr(1)Cl(2)
N(1)Cr(1)Cl(2)
N(2)Cr(1)Cl(3)
N(1)Cr(1)Cl(3)
Cl(2)Cr(1)Cl(3)
76.68(18)
153.84(18)
97.97(13)
87.19(14)
91.55(14)
87.83(14)
89.31(14)
174.62(7)
N(2)Cr(1)N(1)
N(2)Cr(1)Cl(1)
N(1)Cr(1)Cl(1)
N(3)Cr(1)Cl(2)
Cl(1)Cr(1)Cl(2)
N(3)Cr(1)Cl(3)
Cl(1)Cr(1)Cl(3)
77.86(11)
174.56(14)
108.18(14)
87.05(14)
91.60(7)
89.83(14)
93.18(7)
II
#
1
#1
Cl(1) Cr(1)Cl(1)
Cl(1)Cr(1)N(2)
Cl(1)Cr(1)N(3)
77.6(3)
87.6(3)
92.3(3)
102.3(3)
77.9(3)
96.8(2)
98.0(3)
89.0(3)
Cl(1) Cr(1)N(2)
165.2(3)
104.1(3)
76.4(4)
93.1(3)
153.5(4)
174.4(2)
88.2(3)
#1
Cl(1) Cr(1)N(3)
N(2)Cr(1)N(3)
Cl(1)Cr(1)N(1)
N(3)Cr(1)N(1)
Cl(1)Cr(1)Cl(2)
N(3)Cr(1)Cl(2)
#1
Cl(1) Cr(1)N(1)
N(2)Cr(1)N(1)
#1
Cl(1) Cr(1)Cl(2)
N(2)Cr(1)Cl(2)
N(1)Cr(1)Cl(2)
*
Symmetry codes: #1
–x
+ 1, –
y
+ 1, –
z
; #2
–x
+ 1, –
y
+ 2, –
z
+ 1 (II).
Xꢀray diffraction analysis reveals that the basic cenꢀ ions. The equatorial sites are occupied by three nitroꢀ
3
+
–
trosymmetric unit of compound II contains two Cr
gen atoms (N(1), N(2), N(3)) and one Cl ion
–
ions, which have the same coordinated environment (Cl(1A)), while the axial sites are formed by two Cl
ions (Cl(1), Cl(2)).
and bridged with the chlorine atoms. The molecular
structure II is shown in Fig. 1b. The new C–C bonds
have formed under the oxidative coupling of ethanol
and L in hydrothermal method, which is 2,6ꢀbis(6ꢀ
methylquinolinꢀ2ꢀyl)pyridine. In compound II, each
The new C–C bonds have formed under the oxidaꢀ
tive coupling of ethanol and L in hydrothermal
method, which is different from L , this may due to the
2
2
1
electron withdrawing of bromine atoms on the phenyl
3+
Cr ion is coordinated by three nitrogen atoms, one groups [19–21]. The possible reaction mechanism of
of which from pyridine and two from quinolines, and synthesis of 2,6ꢀbis(6ꢀmethylquinolinꢀ2ꢀyl)pyridine
–
3+
–
three Cl ions. Two Cr ions are bridged by two Cl
are below:
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 41
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2015

SYNTHESIS, CHARACTERIZATION, AND MECHANISM STUDIES
133
+
+
+
CH CH OH + H
C H OH
CH CH + H O
3
2
2
5
2
3
2
2
H
H
H
H
N
+
N
N
+
N
+
N
+
CH CH
3
2
N
N
H
N
+
N
+
L2
1
2
H
N
+
–2H+
+
N
N
N
N
N
2
1
Losing one water molecule, the methylene carbon was obtained, which cannot react in L . And then
cations have been formed under the acidic condition, attack to the C=N bonds, the 2,6ꢀbis(6ꢀmethylquinoꢀ
attacked to the phenyl rings, and the intermediate 1 linꢀ2ꢀyl)pyridine has been produced followed by losꢀ
(a)
C(10)
C(11)
C(12)
C(9)
C(8)
N(2)
C(7)
C(13)
Cl(2)
N(3)
C(5)
C(6)
C(15)
C(16)
N(1)
Cr(1)
C(14)
Cl(3)
C(19)
C(4)
C(17)
C(1)
Cl(1)
C(3)
C(18)
C(2)
Br(2)
Br(1)
(
b)
Cl(2A)
N(2A)
N(1A)
N(3A)
Cr(1
A
)
A)
Cl(1)
Cl(1
Cr(1)
N(3)
N(1)
N(2)
Cl(2)
Fig. 1. Coordination environment of compounds
I
(a) and II (b).
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 41 No. 2
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134
LI et al.
ing two water molecules. The bromine atoms have
more withdrawing electron ability than methyl groups,
so that there are no more electrons on the phenyl
groups to attack to the methylene carbon cations,
which is different from the methyl groups as substituꢀ
4. Natta, G., Pino, P., Mazzanti, G., et al., J. Am. Chem.
Soc., 1957, vol. 79, no. 11, p. 2975.
5. Kaminsky, W., Miri, M., Sinn, H., et al., Chem. Rapid.
Commun., 1983, vol. 4, no. 6, p. 417.
6. Johnson, L.K., Mecking, S., Brookhart, M., et al.,
1
J. Am. Chem. Soc., 1996, vol. 118, no. 1, p. 267.
ents on the L . Therefore, the intermediate 1 will not
been formed and nor as to the 2,6ꢀbis(6ꢀmethylquinoꢀ
linꢀ2ꢀyl)pyridine under the presence of the methyl
groups.
In summary, two new late metal compounds for
olefin oligomerization have been synthesized by using
the bis(imino)pyridine ligands. These results show
7. Ittel, S.D., Johnson, L.K., Brookhart, M., et al., Chem.
Rev., 2000, vol. 100, no. 4, p. 1169.
8. Johnson, L.K., Killian, C.M., Brookhart, M., et al.,
J. Am. Chem. Soc., 1995, vol. 117, p. 6414.
9
. Britovsek, G.J.P., Gibson, V.C., Spitzmesser, S.K.,
et al., Dalton Trans., 2002, p. 1159.
1
1
1
1
1
1
0. Britovsek, G.J.P., Gibson, V.C., Kimberley, B.S., et al.,
that compound
I is a mononuclear compound, in
3+
Dalton Trans., 2001, vol. 10, p. 1639.
1. Bianchini, C., Mantovani, G., Meli, A., et al., Organoꢀ
metallics, 2003, vol. 22, no. 13, p. 545.
which the Cr ion is sixꢀcoordinated with distorted
octahedral coordination geometry. Meanwhile, in
compound II, the Cr3 ions are bridged by two Cl
+
–
2. Sun, W.H., Zhang, S., and Zuo, W.W., C. R. Chimie
2007, p. 1.
,
ions to form a binuclear compound. The diverse strucꢀ
2
2'
tures between L and L may be caused by the electron
withdrawing of bromine atoms on the phenyl groups
3. Bianchini, C., Giambastiani, G., Rios, G.I., et al.,
2
Coord. Chem. Rev., 2006, vol. 250, nos. 11–12, p. 1391.
4. Gibson, V.C., Redshaw, C., and Solan, G.A., Chem.
Rev., 2007, vol. 107, no. 5, p. 1745.
5. Osman, D., Dogan, F., Ismet, K., et al., Synth. React.
Inorg., Met.ꢀOrg., NanoꢀMet. Chem., 2010, vol. 40,
no. 5, p. 337.
in L . And the subsequent studies will be focused on
the influence of the halogen atoms as substituents on
the ligands.
ACKNOWLEDGMENTS
1
1
1
1
2
6. Dotterl, M., Schmidt, R., Englmann, T., et al., WO
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Science Foundation of China (nos. 21171139 and
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