Synthesis, characterization and catalytic study of Schiff base copper(I) complexes for the amination of aryl halide

Article abstract of DOI:10.1016/j.inoche.2009.05.010

A series of Schiff base copper(I) complexes [Cu(L₁)(PPh₃)₂]X (1a-d) and [Cu(L₂)(PPh₃)₂]X (2a-d) [where L₁ - (3-trifluoromethyl-phenyl)-pyridine-2-ylmethylene-amine and L₂

Full text of DOI:10.1016/j.inoche.2009.05.010

Inorganic Chemistry Communications 12 (2009) 632–635
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis, characterization and catalytic study of Schiff base copper(I) complexes
for the amination of aryl halide
*
S.K. Sawant, G.A. Gaikwad, V.A. Sawant, B.A. Yamgar, S.S. Chavan
Department of Chemistry, Shivaji University, Kolhapur (MS) 416004, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of Schiff base copper(I) complexes [Cu(L1)(PPh3)2]X (1a–d) and [Cu(L2)(PPh3)2]X (2a–d) [where L1
– (3-trifluoromethyl-phenyl)-pyridine-2-ylmethylene-amine and L2 – 2-chloro-(5-trifluoromethyl-phe-
nyl)-pyridine-2-ylmethylene-amine); X ¼ Clꢀ; CNꢀ; ClO4ꢀ and BFꢀ4 ] have been synthesized by the reaction
of 3-aminobenzotrifluoride and 3-amino-4-chlorobenzotrifluoride with 2-pyridinecarboxaldehyde fol-
lowed by the reaction with [Cu(MeCN)2(PPh3)2]Cl, [Cu(MeCN)2(PPh3)2]CN, [Cu(MeCN)2(PPh3)2]ClO4 and
[Cu(MeCN)2(PPh3)2]BF4. The Schiff base ligands L1, L2 and complexes 1a–d and 2a–d were then charac-
terized by elemental analysis, IR, UV–visible and 1H NMR spectral studies. The catalytic activity of these
complexes was tested and it was found that all the complexes worked as active catalyst for the amination
of iodobenzene at low temperature.
Received 22 January 2009
Accepted 13 May 2009
Available online 21 May 2009
Keywords:
Schiff base
copper(I) complexes
Amination of aryl halide
Ó 2009 Elsevier B.V. All rights reserved.
The amination of aryl halide by transition metal catalyzed cou-
pling methodology has been the subject of significant interest dur-
ing past few decades [1–5]. This method involves production of
arylamines by coupling of aryl halides and amines with stoichiom-
etric amount of transition metal ions. Arylamines are attractive tar-
gets for chemical synthesis because of their wide utility in fine
chemicals, dyes and polymers. These are important components
in many biologically active natural products, medicinally impor-
tant compounds as well as in materials with useful electrical and
mechanical properties [6–10]. Traditionally, the most common
way to access these compounds is Pd-catalyzed C–N coupling of
amines and aryl halides [11–14]. However, the cost of reagents, re-
moval of trace palladium from late stage synthetic intermediate,
and difficulties in coupling of electron-rich or ortho-substituted
aryl halides are the major drawbacks of this method. Copper med-
iated N-arylation is another choice of reaction for the production of
these compounds due to cheap price and environmental friendly
nature. Therefore, copper mediated Ullmann condensation is pow-
erful method for coupling of aryl halide and amines in spite of
necessity of using highly polar solvents, high temperature and
large amount of copper reagents [15,16]. Many organic compounds
such as 1,10-Phenanthroline [17], trans-1,2-cyclohexanediamine
[18–20], diols [21,22], amino acid [23–25] and other nitrogen-oxy-
gen containing compounds [26–29] together with copper reagents
have also been studied extensively. However, only few papers have
been contributed to the use of copper complexes as catalysts for
this carbon-nitrogen bond forming process [30,31].
In this paper, we report synthesis of two different Schiff base li-
gands (3-trifluoromethyl-phenyl)-pyridine-2-ylmethylene-amine
(L1) and 2-chloro-(5-trifluoromethyl-phenyl)-pyridine-2-ylmeth-
ylene-amine (L2). Subsequently they were coordinated to copper(I)
by reaction with [Cu(MeCN)2(PPh3)2]Cl, [Cu(MeCN)2(PPh3)2]CN,
[Cu(MeCN)2(PPh3)2]ClO4 and [Cu(MeCN)2(PPh3)2]BF4 to form eight
copper(I) mixed ligand complexes 1a–d and 2a–d (Scheme 1). The
complexes prepared were characterized on the basis of elemental
analysis, IR, UV–vis and 1H NMR spectral studies. The catalytic per-
formance of the Schiff base copper(I) complexes in the amination
of iodobenzene in solvent toluene at 90 °C (Scheme 2) have also
been discussed.
The elemental analysis and spectral data of the ligands L1, L2
[32] and their copper(I) complexes [34] are listed in Table 1.
The C, H, N and selected spectroscopic data presented in Table 1
confirm the assigned composition of the ligands and complexes. A
strong band observed at 1620 cmꢀ1 in L1 and 1624 cmꢀ1 in L2 is a
characteristic band of azomethine (HC@N) group. The shifting of
this band towards lower frequency region by 25–30 cmꢀ1 in com-
plexes indicates involvement of azomethine nitrogen in coordina-
tion with metal ion [35,36]. The characteristic band observed at
around 997 cmꢀ1 in ligands L1 and L2 is associated with the pyri-
dine ring breathing mode of vibrations. On complexation this band
observed to higher energy at around 1020–1026 cmꢀ1 in the com-
plexes indicating copper–nitrogen bond formation [37]. This view
was further supported by appearance of
a band at around
480 cmꢀ1 in the complexes due to
m(Cu–N) stretching mode of
vibrations. The IR spectra of all the copper(I) complexes shows
strong phosphine bands at around 1434, 742, 692, 506 cmꢀ1 as ex-
pected. The medium intensity band observed at around 2108 cmꢀ1
in 1b and 2b corresponds to CNꢀ anion. A strong band observed at
* Corresponding author. Tel.: +91 231 2609164; fax: +91 231 2691533.
E-mail address: sanjaycha2@rediffmail.com (S.S. Chavan).
1387-7003/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2009.05.010

S.K. Sawant et al. / Inorganic Chemistry Communications 12 (2009) 632–635
633
azomethine (HC@N) proton. The downfield shift of this resonance
in Cu(I) complexes relative to the free ligands can be attributed
to deshielding effect resulting from coordination of the ligands
[39]. The proton resonance of coordinated ligands is commonly ob-
served in complexes 1a–d and 2a–d. The resonances of aromatic
protons of the coordinated Ph3P ligands overlap to some extent
with those of phenyl hydrogen atoms of L1 and L2 in complexes.
However, the ring proton peaks observed at d 6.80–8.50 ppm in
L1 and d 6.32–8.67 ppm in L2 shows slight downfield shift in their
respective complexes.
N
R
R
N
NH2
ethanol
reflux
+
OHC
N
CF3
CF3
L1or L2
Cu[(CH3CN)2(PPh3)2]X
The electronic absorption spectra of the ligands and corre-
sponding complexes were recorded in dichloromethane. In absorp-
+
tion spectra of free ligands,
a band observed at 329 nm
= 7.6 ꢁ 103 Mꢀ1 cmꢀ1
)
in L1 and 326 nm
(e
= 7.50 ꢁ 103
N
R
(e
Mꢀ1cmꢀ1) in L2 are due to
p–p
transition of heterocyclic and its
*
N
Cu
PPh3
PPh3
conjugate system. In the spectra of complexes, no d–d transitions
are expected for d10 complexes; the UV–visible band observed at
CF3
X-
ꢂ398 nm assigned to metal to ligand charge transfer (MLCT) or li-
*
gand centered
p
–p
transition [40].
The scope of copper(I) complex catalysts (1a–d, 2a–d) for the
amination of aryl halide was explored by coupling reaction of iodo-
benzene with primary amines using KOt–Bu as base in solvent tol-
uene at 900C. Under this experimental condition [41], the purified
product obtained was characterized by elemental analysis, IR, 1H
NMR and mass spectral studies (Table 2). When no catalyst was
added, the blank reaction with solvent toluene and KOt–Bu as base
exhibited extremely low reactivity towards the yield of triaryl-
amine with many byproducts from aryl halide substrate. The effect
of complex catalysts such as 1a–d and 2a–d on the amination of
iodobenzene was investigated and it was found that the copper(I)
complex catalysts significantly enhance the conversion of iodoben-
zene into desired amination product. The catalytic efficiency was
compared with the previously reported [42] similar copper(I) com-
plexes. It has been found that all reactions were smoothly carried
out at relatively low temperature and reached the amination yield
up to 45–69% (Table 3). No further increase in the yield of the ami-
nation product was observed even when the reaction time was in-
creased for a longer time. With p-bromoaniline containing electron
withdrawing group at para position, the amination yield was ob-
served up to 45–62% (Table 3, entries 9–12 and 21–24). However,
with p-anisidine containing electron donating group at para posi-
tion, the amination reaction proceeds with considerable increase
-
-
R = H, Cl; X = Cl- ,CN-, ClO4 , BF4
Scheme. 1. Synthetic route of Schiff base ligands and Cu(I) complexes.
R
Cu(I) catalyst
N
N2 atm, KOt Bu
R
NH2
+
2 I
p-sub.-N-N-diphenylaniline
R= H, Br, OMe
Scheme. 2. Amination reaction catalyzed by Cu(I) complexes.
around 1093 cmꢀ1 in complexes 1c and 2c indicating existence of
ClOꢀ4 anion. However,
a broad band observed at around
1094 cmꢀ1 in 1d and 2d corresponds to BFꢀ4 anion in respective
complexes [38].
The 1H NMR spectra of the Schiff base ligands in CDCl3 exhibit
as a singlet at d 8.10 ppm in L1 and d 8.60 ppm in L2 assigned to
Table 1
Microanalytical and spectral data of Schiff bases and their Cu(I) complexes.
C, H, N found (calculated)
IR (cmꢀ1
)
UV–vis kmax nm (
e
ꢁ 103, Mꢀ1 cmꢀ1
)
1H NMR (d ppm)
C
H
N
L1
62.38 (62.40)
3.65 (3.63)
11.25 (11.20)
1620,
1590,
t
t
(HC@N)
284 (9.6), 294 (9.7), 329 (7.6)
290 (13.7), 298 (9.8), 398 (1.3)
286 (13.7), 308 (9.7), 402 (1.1)
290 (13.5), 300 (9.4), 394 (1.4)
288 (13.0), 310 (9.4), 392 (1.3)
286 (9.6), 298 (8.8), 326 (7.5)
284 (12.2), 296 (9.5), 396 (1.8)
280 (13.0), 304 (9.5), 404 (1.1)
294 (13.1), 304 (9.4), 398 (1.4)
284 (13.3), 308 (9.4), 394 (1.3)
6.80–8.50 (m, Ar–H), 8.10
(s, HC@N)
6.99–8.60 (m, Ar–H), 9.20
(s, HC@N)
7.00–8.60 (m, Ar–H), 9.20
(s, HC@N)
6.99–8.60 (m, Ar–H), 9.20
(s, HC@N)
7.00–8.62 (m, Ar–H), 9.20
(s, HC@N)
6.32–8.67 (m, Ar–H), 8.60
(s, HC@N)
7.19–8.67 (m, Ar–H), 9.26
(s, HC@N)
6.80–8.80 (m, Ar–H), 9.20
(s, HC@N)
1a
1b
1c
1d
L2
67.65 (67.35)
69.30 (69.48)
62.71 (62.76)
63.71 (63.61)
54.99 (54.85)
64.66 (64.80)
67.01 (66.82)
60.80 (60.53)
60.87 (61.33)
4.48 (4.50)
4.61 (4.55)
4.44 (4.19)
4.38 (4.25)
2.81 (2.83)
4.52 (4.22)
4.18 (4.26)
4.02 (3.94)
4.47 (3.99)
3.25 (3.21)
4.82 (4.86)
3.46 (2.98)
3.10 (3.03)
9.70 (9.84)
3.25 (3.08)
4.81 (4.68)
2.91 (2.88)
3.89 (2.92)
(HC@N); 1434, 743, 694, 516,
(HC@N); 1434, 742, 693, 506,
t
(PPh3)
1590,
t
t
(PPh3); 2108,
t
(C§N)
(HC@N); 1435, 743, 694, 516,
(ClO4)
(HC@N); 1434, 742, 694, 506,
(PPh3); 1094, t(BF4)
1590,
t
1590,
t
t
(PPh3); 1093, t
t
1624,
t
(HC@N)
2a
2b
2c
2d
1585,
t
(HC@N); 1435, 744, 694, 517,
t
(PPh3)
1586,
t
(HC@N); 1435, 744, 694, 517,
t
(PPh3); 2108,
t
(C§N)
(HC@N); 1435, 744, 694, 517,
(ClO4)
(HC@N); 1435, 744, 694, 517,
(PPh3); 1093, t(BF4)
1586,
t
1586,
t
t
6.80–8.80 (m, Ar–H), 9.30
(s, HC@N)
6.90–8.79 (m, Ar–H), 9.20
(s, HC@N)
(PPh3); 1094, t
t

634
S.K. Sawant et al. / Inorganic Chemistry Communications 12 (2009) 632–635
Table 2
Microanalytical and spectral data of amination product.
Compound
C, H, N found (calculated)
IR (cmꢀ1
)
Mass
1H NMR (d ppm)
C
H
N
N, N–diphenyl-aniline
4-methoxy N, N-
diphenyl-aniline
4-bromo-N, N–
87.83 (88.13) 6.31 (6.16) 5.39 (5.71)
1586, 1493,1461 m/z 245 [(Ph)3N]+, 168 [(Ph)2N]+,77 [Ph]+
d 6.09–6.39, (m, Ar–H)
d 3.86, (s, 3H, OMe), d 6.79–
7.87, (m, Ar–H)
82.19 (82.88) 6.33 (6.22) 5.89 (5.09) 1586, 1490, 1461 m/z 275 [(Ph)3N–OCH3]+, 245 [(Ph)3N]+, 168
[(Ph)2N]+, 77 [Ph]+
67.05 (66.68) 4.18 (4.35) 4.51 (4.32) 1586, 1493, 1461 m/z 323 [(Ph)3NBr]+, 245 [(Ph)3N]+, 168
[(Ph)2N]+, 77 [Ph]+
d 6.91–7.32, (m, Ar–H)
diphenyl-aniline
difference in coordination ability of Clꢀ, CNꢀ, BF4ꢀ and ClO4ꢀ with
metal ion as well as difference in solubility of the complexes in sol-
vent during the reaction. However, the mechanism and correlation
between activity and structure of copper(I) complexes could not be
completely elucidated from these results; we are still in the pro-
cess of studying mechanism and future scope of this reaction in
our laboratory.
Table 3
Amination reaction catalyzed by Cu(I) complexes.
Entry
Aryl amine
Product
Complex
% yield
1
2
3
4
1a
1b
1c
1d
49
52
61
57
NH2
N
In conclusion, several Schiff base copper(I) complexes
have been synthesized by condensation of 3-aminobenzotrifluo-
ride and 3-amino-4-chlorobenzotrifluoride with 2-pyridinecarbox-
aldehyde followed by the reaction with [Cu(MeCN)2(PPh3)2]Cl,
[Cu(MeCN)2(PPh3)2]CN, [Cu(MeCN)2(PPh3)2]ClO4, [Cu(MeCN)2-
(PPh3)2]BF4. The structure of Schiff base ligands L1 and L2 and their
copper(I) complexes (1a–d; 2a–d) were confirmed by means of
elemental analysis, FTIR, UV–visible and 1H NMR spectroscopy.
The copper catalyzed C–N bond forming reaction of aryl halide
have been carried out and it was found that all the complexes
worked as active catalyst for the amination of aryl halide. Further,
the differences in the structure among these complexes signifi-
cantly influence the yield of the amination product.
5
6
7
8
1a
1b
1c
1d
60
62
67
64
MeO
MeO
NH2
N
Br
9
1a
1b
1c
1d
45
50
60
54
NH2
Br
10
11
12
N
Acknowledgement
N
13
14
15
16
2a
2b
2c
2d
56
57
65
59
NH2
We acknowledge Indian Institute of Science (IISc), Bangalore for
providing elemental analysis and 1H NMR facilities.
MeO
References
17
18
19
20
2a
2b
2c
2d
61
64
69
67
MeO
NH2
N
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shows greater activity than the complexes with Clꢀ, CNꢀ, BF4ꢀ
and reached the amination yield up to 60–69% (Table 3, entries
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>
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S.K. Sawant et al. / Inorganic Chemistry Communications 12 (2009) 632–635
635
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catalyst was added to 4 mmol of respective aryl amine, 8 mmol of
iodobenzene, 12 mmol of KOt–Bu in toluene and the reaction mixture was
stirred for 12 h at 90° under nitrogen atmosphere. The reaction mixture was
then cooled to room temperature and the solution was filtered to remove the
precipitated base. The product was isolated by column chromatography using
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trifluoromethyl-phenyl)-pyridine-2-ylmethylene-amine (L1) and 2-chloro-(5-
trifluoromethyl-phenyl)-pyridine-2-ylmethylene-amine (L2) were prepared
by adopting and modifying the method described in the literature [33]. A
solution of 2-pyridinecarboxaldehyde (1 mmol, 0.107 g) in 10 ml of ethanol
was added to a solution of 3-aminobenzotrifluoride (1 mmol, 0.160 g) or 3-
amino-4-chlorobenzotrifluoride (1 mmol, 0.195 g) dissolved in 10 ml of
absolute ethanol while stirring. The resulting mixture was refluxed at 80 °C
until the completion of reaction (checked by TLC). The resultant brown
coloured liquid was purified by using column chromatography (9:1; ether:
chloroform). The solvent was then removed by rotary evaporator to receive
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appropriate metal salt (1 mmol, 0.0989 g, CuCl, 0.0895 g, CuCN, 0.327 g,
[Cu(MeCN)4]ClO4
and
0.314 g,
[Cu(MeCN)4]BF4),
2
equivalent
triphenylphosphine (2 mmol, 0.522 g) were added and solution was stirred
for 30 min. The volume of the solvent was evaporated under vacuum at room
temperature. The crystalline product of [Cu(MeCN)2(PPh3)2]X (where
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X ¼ Clꢀ; CNꢀ; BF4ꢀ and ClOꢀ4
)
obtained was added to
a
stirring solution of
Schiff base ligand L1 (1 mmol, 0.250 g) or L2 (1 mmol, 0.284 g) in 10 ml