Mixed-ligand complexes of copper(I) with Schiff base and triphenylphosphine: Effective catalysts for the amination of aryl halide

Article abstract of DOI:10.1016/j.inoche.2011.05.025

Some copper(I) complexes of the type [Cu(L)(PPh₃)₂]X (1-4) [where L = 2-chloro-(5-trifluoromethyl-phenyl)-pyridine-2-ylethylene- amine; X = Cl^-, CN^-, ClO₄^- and BF₄^-] have been prepared by the condensation of 3-amino-4-chloro-benzotrifluoride with 2-acetylpyridine followed by the reaction with [Cu(MeCN)₂(PPh₃)₂]Cl, [Cu(MeCN) ₂(PPh₃)₂]CN, [Cu(MeCN)₂(PPh ₃)₂]ClO₄ and [Cu(MeCN)₂(PPh ₃)₂]BF₄. All the complexes 1-4 were characterized on the basis of elemental analysis, IR, UV-visible and ¹H NMR spectral studies. The representative complex of the compound 3 has been characterized by single crystal X-ray diffraction which reveals that the central copper(I) ion assumes highly distorted tetrahedral geometry. All the complexes worked as effective catalyst for the amination of aryl halide.

Full text of DOI:10.1016/j.inoche.2011.05.025

Inorganic Chemistry Communications 14 (2011) 1373–1376
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Mixed-ligand complexes of copper(I) with Schiff base and triphenylphosphine:
Effective catalysts for the amination of aryl halide
a
a
b
S.S. Chavan a, , S.K. Sawant , V.A. Sawant , G.K. Lahiri
⁎
a
Department of Chemistry, Shivaji University, Kolhapur (MS), 416004, India
Department of Chemistry, Indian Institute of Technology, Bombay, 400076, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Some copper(I) complexes of the type [Cu(L)(PPh3)2]X (1–4) [where L=2-chloro-(5-trifluoromethyl-phenyl)-
pyridine-2-ylethylene-amine; X=Cl−, CN−, ClO4− and BF4−] have been prepared by the condensation of 3-amino-4-
chloro-benzotrifluoride with 2-acetylpyridine followed 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. All the complexes 1–4 were characterized on the
basis of elemental analysis, IR, UV–visible and 1H NMR spectral studies. The representative complex of the compound
3 has been characterized by single crystal X-ray diffraction which reveals that the central copper(I) ion assumes
highly distorted tetrahedral geometry. All the complexes worked as effective catalyst for the amination of aryl halide.
© 2011 Elsevier B.V. All rights reserved.
Received 27 March 2011
Accepted 25 May 2011
Available online 1 June 2011
Keywords:
Mixed ligand
Copper(I) complexes
Single crystal X-ray diffraction
Amination of aryl halides
Aryl amines are attractive targets for chemical synthesis because of
their prevalence and wide utility in fine chemicals, dyes and polymers
[1–3]. These are important components in many biological active
natural products, medicinally important compounds as well as in
materials with useful electrical and mechanical properties [4–6]. High
purity triarylamines find applications in xerographic photoreceptors
as constituents of non linear optical chromophores useful in the
design of integrated electro-optic switches and modulators, and as
hole-transport materials for organic electroluminescent (EL) display
devices [7]. They are found in biologically active compounds such as
pharmaceuticals [8] and agrochemicals [9]. Aryl amines have also
been employed as ligands for transition metals, and in design of
conducting polymers and other electrochemically interesting mate-
rials [7]. This wide spread importance of arylamines has led to the
development of many synthetic methodologies for the production of
arylamines. Although the Pd-catalyzed amination of aryl halide has
recently become the most important method for laboratory scale
synthesis of arylamines; copper stands out as a convenient alternative
to palladium catalysts, because copper salts are often cheap and
environmentally benign. Many ligand systems such as amino acid
[10], diamines [11], di-imines [12], aminoarenethiolate [13], phos-
phine [14,15] and other nitrogen, oxygen containing ligands [16–19]
together with copper(I) reagents have been applied for the produc-
tion of arylamines under mild condition.
copper(I) complexes 1–4 by the reaction of Schiff base ligand 2-
chloro-(5-trifluoromethyl-phenyl)-pyridine-2-ylethylene-amine (L)
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. All the compounds were
characterized on the basis of elemental analysis, IR, UV–vis, 1H NMR
spectral studies and compound 3 also by X-ray crystallography. The
catalytic performance of all the copper(I) complexes in the amination
of bromobenzene has also been discussed.
The elemental analysis and spectral data of the ligand L[21] and its
copper(I) complexes [23] are listed in Table 1. The CHN and spectral
data presented in Table 1 confirm the assigned composition of the
complexes. The IR spectrum of ligand L shows a strong band at
1622 cm−1, which is a characteristic band of azomethine (NC_N)
group. The shifting of this band towards the lower frequency region by
20–30 cm−1 in complexes indicates involvement of azomethine
(NC_N) nitrogen in coordination with metal ion [24]. Another
characteristic band observed at 997 cm−1 in L is associated with
pyridine ring breathing mode of vibration. On complexation, this band
observed to higher energy at around 1024–1028 cm− 1 in the
complexes indicates copper–nitrogen bond formation. This view is
further supported by the appearance of a band corresponding to the
metal–nitrogen ν(Cu\N) stretching vibration at ~482 cm−1 in the
complexes [25]. The spectra of all the copper(I) complexes show
strong phosphine bands at around 1480, 1434, 669, 517 cm−1 as
expected. The medium intensity band observed at 2110 cm−1 in 2
corresponds to CN− anion. A strong band observed at 1095 cm−1 in
complex 3 indicates existence of ClO4− anion. However, a broad band at
1094 cm−1 in 4 corresponds to presence of BF4− anion in the complex.
The electronic spectral data of copper(I) complexes (1–4) were
recorded in CH2Cl2 (10−4 M). In the spectra of complexes, no d–d
As a continuation of our research on copper(I) catalyzed amination
of aryl halide [20], we report herein synthesis of four mixed ligand
⁎
Corresponding author. Tel.: +91 231 2609164; fax: +91 231 2691533.
E-mail address: sanjaycha2@rediffmail.com (S.S. Chavan).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.05.025

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S.S. Chavan et al. / Inorganic Chemistry Communications 14 (2011) 1373–1376
Table 1
Microanalytical and spectral data of Schiff base and its copper(I) complexes.
C, H, N found (calculated)
IR (cm−1
)
UV–vis λmax/nm
1H NMR (δ ppm)
(ε×103/M−1cm−1
)
C
H
N
L
1
2
3
4
56.51 (56.29)
3.39 (3.37)
9.45 (9.38)
1622, υ(C_N)
261 (9.1), 290 (9.4), 329 (7.1)
2.24 (s, CH3), 6.32–8.80 (m, Ar-H),
8.98(s, HC_N)
2.50 (s, CH3), 6.38–8.85 (m, Ar-H),
9.18(s, HC_N)
2.52 (s, CH3), 6.40–8.87 (m, Ar-H),
9.18(s, HC_N)
2.50 (s, CH3), 6.36–8.90 (m, Ar-H),
9.17(s, HC_N)
65.44 (65.12)
67.53 (67.10)
61.25 (60.89)
61.92 (61.68)
4.36 (4.37)
4.39 (4.42)
4.55 (4.09)
4.16 (4.14)
3.08 (3.04)
4.62 (4.60)
2.72 (2.84)
2.88 (2.87)
1590, υ(C_N); 1480, 1434, 669, 516,
υ(PPh3)
1594, υ(C_N); 1480, 1434, 669, 517,
υ(PPh3); 2110, υ(C`N)
1595, υ(C_N); 1480, 1435, 669, 517,
υ(PPh3); 1095, υ(ClO4)
1603, υ(C_N); 1480, 1434, 669, 517,
υ(PPh3); 1094, υ(BF4)
264 (15.66), 249 (9.70), 326 (0.83),
400 (0.53)
262 (16.02), 288 (10.65), 319 (0.99),
402 (0.64)
264 (15.66), 294 (9.73), 326 (0.83),
400 (0.53)
269 (16.45), 283 (9.32), 315 (1.13),
404 (0.84)
2.50 (s, CH3), 6.38–8.84 (m, Ar-H),
9.18(s, HC_N)
Table 2
azomethine group appears as a singlet at δ 2.50–2.52 ppm in all the
complexes. The downfield shift of imine protons relative to free ligand L
can be assigned to deshielding effect resulting from the coordination of
azomethine nitrogen [27].
The structure of the complex 3 was determined by X-ray single
crystal analyses at room temperature [28]. The selected bond lengths
and bond angles are given in Table 2. However, more details
including those of crystal data and structural refinements are
available on-line as supplementary material. The complex 3 crystal-
lizes in triclinic system with two molecules per asymmetric unit
probably due to some small conformational differences and is shown
in Fig. 1.
The structure can be described as formed by a cationic complex
molecule with a perchlorate as a counter ion and the copper(I) ion
coordinates to two triphenylphosphine and a bidentate Schiff base
ligand L in its neutral form. In the monomeric complex, the copper(I)
exhibits highly distorted tetrahedral geometry arising from the intra-
ligand chelate angles N(1)\Cu(1)\N(2) in I and N(3)\Cu(2)\N(4)
in II, are much less than 109.4°, being 78.5(2) and 78.4(2)°,
respectively. However, the P(1)\Cu(2)\P(2) angle 115.37(7)° in
I and the P(3)\Cu(2)\P(4) angle 118.90(8)° in II, have opened up
due to the steric effects from the bulky PPh3 ligands. The average
Cu\N (2.052 Å) and Cu\P (2.251 Å) bond distances are compara-
ble to those reported for [Cu(A)(PPh3)2]ClO4 (2.098 and 2.251 Å)
[30].
Selected bond lengths (Ǻ) and bond angles (°) for [Cu(L)(PPh3)2]ClO4 (3).
Bond lengths (Ǻ)
Bond angles (°)
Cu(1)\N (1)
Cu(1)\N (2)
Cu(1)\P (1)
Cu(1)\P (2)
Cu(2)\N (3)
Cu(2)\N (4)
Cu(2)\P (3)
Cu(2)\P (4)
2.053(6)
2.106(5)
2.259(19)
2.269(19)
2.051(6)
2.090(5)
2.228(2)
2.252(18)
N(1)\Cu(1)\N(2)
N(1)\Cu(1)\P(1)
N(2)\Cu(1)\P(1)
N(1)\Cu(1)\P(2)
N(2)\Cu(1)\P(2)
P(1)\Cu(1)\P(2)
N(3)\Cu(2)\N(4)
N(3)\Cu(2)\P(3)
N(4)\Cu(2)\P(3)
N(3)\Cu(2)\P(4)
N(4)\Cu(2)\P(4)
P(3)\Cu(2)\P(4)
78.5(2)
111.54(16)
119.24(16)
120.75(17)
106.36(15)
115.37(7)
78.4(2)
113.79(16)
120.11(16)
108.17(15)
110.09(15)
118.90(8)
transitions are expected for d10 complexes, the UV–visible band
observed at 402 nm assigned to metal to ligand charge transfer
(MLCT) or ligand centered π–π* transition [26]. The absorption bands
observed below 400 nm in all the complexes are attributed to ligand-
centered π–π* and n–π* transitions.
The 1H NMR spectra of all the complexes (Table 1) indicate that the
resonances of aromatic protons of the coordinated PPh3 ligands overlap
to some extent with those of phenyl hydrogen atoms of L in the
complexes. However, the ring proton peaks observed at δ 6.32–8.80 ppm
in L show slight downfield shift in its complexes. The methyl proton of
Fig. 1. Single crystal X-ray diffraction structure of [Cu(L)(PPh3)2]ClO4 (3).

S.S. Chavan et al. / Inorganic Chemistry Communications 14 (2011) 1373–1376
1375
Scheme 1. Amination reaction catalyzed by copper(I) complexes.
The copper(I) complexes (1–4) have been found promising for
catalyzing the amination of bromobenzene [31] by coupling reaction
with different primary amines in toluene at 90 °C in presence of
KOt\Bu as base (Scheme 1). The effect of complex catalysts (1–4) on
the amination of bromobenzene was investigated and it was found
that copper(I) complexes significantly enhance the conversion of
bromobenzene into desired amination product in moderate to good
yield. The catalytic efficiency was compared with the previously
reported [20] similar copper(I) complexes and 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 product was observed even when the
reaction time was increased for a longer time.
complexes with Cl−, CN−, and BF4− as anions and reached the
amination yield up to 65–75% (Table 3, entries 3, 7 and 11). It was also
found that the catalytic activity of these complexes decreases in the
order of ClO4− NBF4− NCN− NCl− as their counter anions. These results
could be attributed to difference in coordination ability of Cl−, CN−
,
BF4− and ClO4− with metal ion as well as difference in solubility of
complexes in solvent during the reaction.
Acknowledgment
We acknowledge the Indian Institute of Science (IISc), Bangalore
for providing elemental analysis and 1H NMR facilities.
It was observed that the catalytic amination of bromobenzene
with aniline reached the amination yield up to 52–71% (Table 3,
entries 1–4). With p-bromoaniline containing electron withdrawing
group at para position, the amination reaction yield was observed up
to 48–65% (Table 3, entries 5–8). However, with p-anisidine contain-
ing electron donating group at para-position, the amination reaction
proceeds with considerable increase in the yield up to 60–75%
(Table 3, entries 9–12). These results confirm that the variety of
functional group such as bromo and methoxy tolerated on arylamine
component under the reaction condition. A marginal increase in the
yield was observed for substituted aryl amine containing electron
donating \OCH3 group at para-position.
Appendix A. Supplementary material
CCDC 753005 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. Supplementary data to this article can be found
online at doi:10.1016/j.inoche.2011.05.025.
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Entry
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bromobenzene catalyzed by copper(I) complexes was carried out according to
the procedure: 0.05 mmol of copper(I) catalyst was added to 4 mmol of
respective aryl amine, 8 mmol of bromobenzene, 12 mmol of KOt-Bu in toluene
and the reaction mixture was stirred for 12 h at 90 °C under nitrogen. The
reaction mixture was then cooled to room temperature and the solution was
filtered to remove the precipitated base. The filtrate was concentrated and crude
product was purified by column chromatography using ether:chloroform (9:1).
0.121 g) in 10 ml of ethanol was added to
a solution of 3-amino-4-chloro-
benzotrifluoride (1 mmol, 0.195 g) dissolved in 10 ml of ethanol while stirring.