Pentacoordinated copper-sparteine complexes with chelating nitrite or nitrate ligand: Synthesis and catalytic aspects

Article abstract of DOI:10.1016/j.ica.2011.03.027

Copper(II)-sparteine complexes, [Cu^II{(-)Sp}(NO₂)Cl] (1) and [Cu^II{(-)Sp}(NO₃)Cl] (2) (Sp = sparteine) with chelating nitrite and nitrate ligands, respectively, have been synthesized and structurally characterized. The penta-coordinated 1 or 2 exhibits distorted square pyramidal geometry and shows characteristic EPR spectra with g _||: 2.27 and g_⊥: 2.06. 1 and 2 behave similarly towards the catalytic epoxidation of alkenes as well as oxidation of alcohols. Though the epoxidation of cyclohexene using 1 or 2 as a catalyst and tertiarybutyl hydroperoxide (TBHP) as an oxidant at 298 K in acetonitrile results in 100% cyclohexene oxide product, under identical reaction conditions styrene selectively transforms to benzaldehyde (～90%) instead of any styrene oxide. However, at higher temperature (353 K) the selectivity of cyclohexene to corresponding epoxide formation decreases appreciably and additional products, cyclohexanol and cyclohexanone are formed. Furthermore, 1 or 2 effectively catalyzes the oxidation of benzyl alcohol to benzoic acid and cyclohexanol to cyclohexanone in presence of molecular oxygen (O₂) as an oxidant at 353 K in acetonitrile.

Full text of DOI:10.1016/j.ica.2011.03.027

Inorganica Chimica Acta 374 (2011) 415–421
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Pentacoordinated copper–sparteine complexes with chelating nitrite or nitrate
ligand: Synthesis and catalytic aspects
a,
Arindam Indra a, Shaikh M. Mobin a, Sumit Bhaduri b, , Goutam Kumar Lahiri
⇑
⇑
a Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India
b Department of Chemistry, Northwestern University, Evanston, IL 60208, USA
a r t i c l e i n f o
a b s t r a c t
Copper(II)–sparteine complexes, [CuII{(-)Sp}(NO2)Cl] (1) and [CuII{(ꢀ)Sp}(NO3)Cl] (2) (Sp = sparteine)
with chelating nitrite and nitrate ligands, respectively, have been synthesized and structurally character-
ized. The penta-coordinated 1 or 2 exhibits distorted square pyramidal geometry and shows character-
istic EPR spectra with g||: 2.27 and g\: 2.06. 1 and 2 behave similarly towards the catalytic epoxidation
of alkenes as well as oxidation of alcohols. Though the epoxidation of cyclohexene using 1 or 2 as a cat-
alyst and tertiarybutyl hydroperoxide (TBHP) as an oxidant at 298 K in acetonitrile results in 100% cyclo-
hexene oxide product, under identical reaction conditions styrene selectively transforms to benzaldehyde
(ꢁ90%) instead of any styrene oxide. However, at higher temperature (353 K) the selectivity of cyclohex-
ene to corresponding epoxide formation decreases appreciably and additional products, cyclohexanol and
cyclohexanone are formed. Furthermore, 1 or 2 effectively catalyzes the oxidation of benzyl alcohol to
benzoic acid and cyclohexanol to cyclohexanone in presence of molecular oxygen (O2) as an oxidant at
353 K in acetonitrile.
Article history:
Available online 12 March 2011
Dedicated to Prof. W. Kaim
Keywords:
Copper
Sparteine
EPR spectra
Crystal structure
Catalysis
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
nitrite complex where the crystal structure reveals the presence of
chelated, non-chelated as well as bridging modes of the nitrite ion
Metal complexes with sparteine (Sp) as a bidentate N,N donor
ligand exhibit important physico-chemical, magnetic and struc-
tural properties [1–5]. The largely strained structural feature of
the alkaloid sparteine and the distance between the two coordinat-
ing nitrogen atoms make it an interesting ligand. Consequently a
large number of copper–sparteine complexes along with different
monodentate anionic ancillary ligands have been reported [1,2,6–
14]. However, only one example of Cu(II)–Sp complex with chelat-
ing ancillary ligand, maleonitriledithiolato-S,S0 has been reported
by Choi and co-workers [15]. In all the cases the four coordinated
copper complexes exhibit pseudo tetrahedral structure. A five
coordinated Cu(II)–Sp complex has also been reported by Masuda
et al. but it has a dicopper core with bridging peroxo and carboxyl-
ate groups [16].
in the three independent molecules in the unit cell [21]. Using
tris(pyrazolyl)borate as the co-ligand chelating –NO2 group with
O,O donors has been reported by Tolman et al. as nitrite reductase
model complex [22].
In view of these reports the synthesis of monomeric copper–Sp
complex with nitrite or nitrate as a chelating ligand, is syntheti-
cally challenging and structurally important as the steric demands
ꢀ
for the two ligands (Sp and NO2 or NO3ꢀ) are different. Here we
report the synthses and characterizations monomeric copper–
sparteine complexes [Cu{(ꢀ)Sp}(NO2)Cl] (1) and [Cu{(ꢀ)Sp}(NO3)
Cl] (2) with –NO2 and –NO3 as chelating ligands. The single-crystal
X-ray structures of both 1 and 2 have been determined and the
catalytic potentials of 1 and 2 towards epoxidation of alkenes
and oxidation of alcohols have been investigated.
Monodentate or bridging nitrite and nitrate ligands attached to
Cu(I) or Cu(II) center along with ancillary tripodal ligands were re-
ported in the literature [17–19]. However, copper complexes with
chelating nitrite or nitrate ligand are rather rare. Hsu reported
monomeric and dimeric Cu(I) complexes with nitrite as chelating
and bridging ligand [20]. Hubberstey et al. reported one copper(II)
2. Experimental section
2.1. Chemicals
(ꢀ)Sparteine, cyclohexene, cyclohexene oxide, styrene, styrene
oxides, benzaldehyde, benzyl alcohol, 3-methylbenzylalcohol,
3-methoxybenzylalcohol, cyclohexanol, cyclohexanone, tertia-
rybutyl hydroperoxide (TBHP) (5–6 M in decane), ( )-1-phenyleth-
anol, acetophenone have been obtained from Aldrich. Other
⇑
Corresponding authors.
E-mail addresses: s-bhaduri@northwestern.edu (S. Bhaduri), lahiri@chem.iitb.
ac.in (G.K. Lahiri).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.03.027

416
A. Indra et al. / Inorganica Chimica Acta 374 (2011) 415–421
chemicals and solvents were reagent grade and used as received.
The precursor complex [Cu{(ꢀ)Sparteine}Cl2] was prepared
according to the literature reported procedure [23].
respectively. Magna 550 from Nicolet Instruments, USA was used
for recording IR spectra as KBr disk. Elemental analyses were per-
formed by using a Perkin–Elmer 240C elemental analyzer. UV–Vis
spectra were taken in a Perkin–Elmer 950 spectrophotometer. The
EPR measurements were made with a Varian model 109C E-line
X-band spectrometer fitted with a quartz dewar for measurements
at 77 K. Solution electrical conductivity was measured in Systronic
conductivity bridge-305. Cyclic voltammetric measurements were
2.2. Instrumentation
Bruker AV III 400 MHz FT-NMR spectrometer, Micromass Q-ToF
mass spectrometer were used for NMR and mass measurements,
N
N
AgClO4 ,
N
AgNO3
N
II
Cu
N
N
II
Cu
II
Cu
NaNO2
CH2Cl2 : CH3CN (1:1)
O
O
O
CH2Cl2 : CH3CN (1:1)
Cl
Cl
O
N
O
Cl
N
Cl
1
2
Scheme 1. Syntheses of the complexes 1 and 2.
Fig. 1. Crystal structures of complexes (a) [Cu(Sp)(NO2)Cl] (1) and (b) [Cu(Sp)(NO3)Cl] (2). Ellipsoids are drawn at 50% probability. Hydrogen atoms are omitted for clarity.

A. Indra et al. / Inorganica Chimica Acta 374 (2011) 415–421
417
carried out using a PAR model 273A electrochemistry system. A
platinum wire working electrode, a platinum wire auxiliary elec-
trode and a saturated calomel reference electrode (SCE) were used
in a standard three-electrode configuration. Tetraethylammonium
perchlorate (TEAP) was used as the supporting electrolyte and the
solution concentration was ca. 10ꢀ3 M; the scan rate used was
100 mV sꢀ1. Electrochemical experiments were carried out under
dinitrogen atmosphere. % conversion and the product distribution
ratio for the catalytic experiments were established by gas chro-
matographic technique with the FID detector (Shimadzu GC-2014
gas chromatograph) and using a capillary column (Sigma Aldrich,
Supelco, Astec, Chiraldex B-DM; length 50 m, inner diameter
0.25 mm, thickness 0.12
l
m).
15
12
9
15
12
9
6
Table 1
Crystallographic data and structural refinements for [Cu(Sp)(NO2)Cl] (1) and
[Cu(Sp)(NO3)Cl] (2).
6
3
0
400 600 800 1000 1200 1400 1600
λ/nm
Molecule
[Cu(Sp)(NO2)Cl]
(1)
[Cu(Sp)(NO3)Cl]
(2)
3
Molecular formula
Formula weight
T (K)
C
15H26ClCuN3O2
C15H26ClCuN3O3
395.38
150(2)
0
379.38
150(2)
400
600 800 1000 1200 1400 1600
Crystal symmetry
Space group
a (Å)
b (Å)
c (Å)
orthorhombic
P212121
7.8797(3)
12.4862(5)
17.1936(8)
90
orthorhombic
P21212
16.370(4)
13.334(2)
7.6803(9)
90
λ/nm
Fig. 2. UV–Vis spectrum of [Cu(Sp)(NO2)Cl] (1) in CH3CN. Inset shows the spectrum
of [Cu(Sp)(NO3)Cl] (2) in CH3CN.
a
(°)
b (°)
90
90
c
(°)
90
90
1676.4(5)
4
1.567
1.480
V (Å3)
1691.64(12)
4
1.490
Z
Dcalc (g/cm3)
l
(mmꢀ1
)
1.459
F(0 0 0)
796
828
2h Range (°)
Data/restraints/parameters
Goodness-of-fit (GOF)
6.56–49.96
2978/0/209
0.945
6.60–50.00
2942/0/208
0.794
R1, wR2 [I > 2
R1, wR2 (all data)
r
(I)]
0.0416, 0.1004
0.0589, 0.1044
0.700 and ꢀ0.252
0.0524, 0.0879
0.1061, 0.0972
0.570 and ꢀ0.430
Largest difference in peak/hole
(e Åꢀ3
)
Table 2
Selected bond distances and bond angles for [Cu(Sp)(NO2)Cl] (1) and [Cu(Sp)(NO3)Cl]
(2).
Bond distance (Å)
[Cu(Sp)(NO2)Cl] (1)
Bond angle (°)
Cu(1)–Cl(1A)
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–O(1)
Cu(1)–O(2)
O(1)–N(3)
O(2)–N(3)
1.998(10)
2.019(3)
2.019(4)
2.033(4)
2.358(5)
1.269(7)
1.249(8)
Cl(1A)–Cu(1)–N(1)
Cl(1A)–Cu(1)–N(2)
N(1)–Cu(1)–N(2)
Cl(1A)–Cu(1)–O(1)
N(1)–Cu(1)–O(1)
N(2)–Cu(1)–O(1)
Cl(1A)–Cu(1)–O(2)
N(1)–Cu(1)–O(2)
N(2)–Cu(1)–O(2)
O(1)–Cu(1)–O(2)
N(3)–O(1)–Cu(1)
N(3)–O(2)–Cu(1)
O(2)–N(3)–O(1)
167.1(3)
95.6(4)
90.56(18)
84.8(4)
94.94(19)
152.40(18)
77.0(3)
113.55(17)
96.64(18)
56.45(18)
102.8(4)
87.7(4)
112.7(5)
[Cu(Sp)(NO3)Cl] (2)
Cu(1)–Cl(1)
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–O(1)
Cu(1)–O(2)
O(1)–N(3)
O(2)–N(3)
O(3)–N(3)
2.238(2)
Cl(1)–Cu(1)–N(1)
Cl(1)–Cu(1)–N(2)
N(1)–Cu(1)–N(2)
Cl(1)–Cu(1)–O(1)
N(1)–Cu(1)–O(1)
N(2)–Cu(1)–O(1)
Cl(1)–Cu(1)–O(2)
N(1)–Cu(1)–O(2)
N(2)–Cu(1)–O(2)
O(1)–Cu(1)–O(2)
N(3)–O(1)–Cu(1)
N(3)–O(2)–Cu(1)
O(2)–N(3)–O(1)
135.17(19)
105.26(16)
89.7(2)
96.06(18)
92.5(2)
147.2(2)
101.20(15)
119.8(3)
94.5(2)
2.005(6)
1.998(5)
2.051(6)
2.420(5)
1.260(7)
1.232(8)
1.240(7)
56.5(2)
100.8(5)
84.1(4)
Fig. 3. X-band EPR spectra of complexes (a) [Cu(Sp)(NO2)Cl] (1) and (b)
[Cu(Sp)(NO3)Cl] (2) in 1:1 dichloromethane–toluene glass.
118.4(7)

418
A. Indra et al. / Inorganica Chimica Acta 374 (2011) 415–421
2.3. Crystallography
conductivity [KM (X
ꢀ1 cm2 Mꢀ1)] in acetonitrile: 30. ESI(+) Mass
spectrum in CH3CN: 359.82 m/z, corresponding to [2ꢀCl]+ (calcd.
Single crystals of [Cu{(ꢀ)Sp}(NO2)Cl] (1) and [Cu{(ꢀ)Sp}-
(NO3)Cl] (2) were grown by slow evaporations of the correspond-
ing acetonitrile:dichloromethane (1:1) solutions. X-ray data were
collected using an OXFORD XCALIBUR-S CCD single crystal X-ray
diffractometer. The structures were solved and refined by full-ma-
trix least-squares techniques on F2 using the SHELX-97 program [24].
The absorption corrections were done by the multi-scan technique.
All data were corrected for Lorentz and polarization effects, and the
non-hydrogen atoms were refined anisotropically. Hydrogen
atoms were included in the refinement process as per the riding
model.
359.13).
3. Result and discussion
3.1. Syntheses of the complexes 1 and 2
The synthetic procedures of the complexes 1 and 2 are shown in
Scheme 1. Using [Cu{(ꢀ)Sp}Cl2] as the starting material the
complexes are synthesized in single step reactions. The precursor
complex [Cu{(ꢀ)Sp}Cl2] has been synthesized according to the
literature procedure by the reaction of (ꢀ)Sp with CuCl2ꢂ2H2O [23].
The electrically neutral complexes (1 and 2) exhibit satisfactory
microanalytical and mass spectral data (see Experimental section).
2.4. Catalytic experiment
The catalytic runs for the epoxidation reaction were carried out
in glass vials with the specified ratios of substrates and catalyst in
acetonitrile under magnetic stirring condition (stirring rate
P900 rpm). Calculated amount of TBHP was added at regular time
intervals in small quantities over the whole span of the reaction
time. At the end of the catalytic run the reaction mixture was
subjected to GC and the extent of conversion was calculated on
the basis of the ratio of the areas of the starting material and the
product(s) by using meta-xylene as an external standard. All prod-
ucts were initially identified by using authentic commercial
samples.
Oxidation reactions with dioxygen were carried out in a two-
necked round bottom flask fitted with a gas inlet and a condenser.
The mixture of substrate, catalyst, TEMPO [(2,2,6,6-tetramethylpi-
peridin-1-yl)oxyl] in fixed volume of acetonitrile in the round bot-
tom flask was placed in an oil bath previously heated at 353 K and
dioxygen was bubbled through the mixture at ten bubbles per
minute. Product was identified by 1H NMR and gas chromato-
graphic techniques.
3.2. X-ray crystal structure
Crystal structures of 1 and 2 are shown in Fig. 1. The selected
crystallographic parameters and bond distances and angles are
set in Tables 1 and 2, respectively. The CuN2O2Cl chromophore in
1 or 2 exhibits a distorted square pyramidal geometry. The nitrite
(–NO2) and nitrate (–NO3) groups in 1 and 2, respectively, are
bound to the copper ion through two oxygen donors forming four
member chelate rings. The bulky sparteine ligand binds to the
copper ion through the nitrogen donors forming a six-membered
chelate ring in both 1 and 2. The N1 and N2 donors of sparteine
2.5. Synthesis of [Cu{(ꢀ)Sp}(NO2)Cl] (1)
The precursor complex [Cu{(ꢀ)Sparteine}Cl2] (36.7 mg,
0.1 mmol) was dissolved in 10 mL of (1:1) CH2Cl2:CH3CN. AgClO4
(20.5 mg, 0.1 mmol) was dissolved in 5 mL of ethanol and added
drop wise to the above copper solution with vigorous stirring.
Immediate precipitation of AgCl was observed. 11 mg (0.15 mmol)
NaNO2 was added to the above mixture at a time in the solid form
and stirred magnetically for 6 h. The solution was filtered and the
solvent was removed under reduced pressure to get the green so-
lid. The solid mass thus obtained was redissolved in (1:1)
CH2Cl2:CH3CN solvent mixture to get the single-crystals by slow
evaporation technique at 253 K. Yield: 32 mg (84%). Anal. Calc.
for C15H26ClCuN3O2 (Mol. wt. 379.38): C, 47.49; H, 6.91; N, 11.08.
Found: C, 47.26; H, 6.86; N, 11.12%. Molar conductivity
[
KM (X
ꢀ1 cm2 Mꢀ1)] in acetonitrile: 26. ESI(+) Mass spectrum in
CH3CN: m/z = 343.22 corresponding to [1ꢀCl]+ (calcd. 343.13).
2.6. Synthesis of [Cu{(ꢀ)Sp}(NO3)Cl] (2)
The precursor complex [Cu{(ꢀ)Sparteine}Cl2] (36.7 mg,
0.1 mmol) was dissolved in 10 mL of (1:1) CH2Cl2:CH3CN. AgNO3
(17 mg, 0.1 mmol) was dissolved in 5 mL of ethanol and added
drop wise slowly to the previous solution with vigorous stirring.
Immediate precipitation of AgCl was observed. The solution was
filtered and the solvent was removed to get the bluish-green solid.
The solid was redissolved in (1:1) CH2Cl2:CH3CN solvent mixture
to get the crystal by slow evaporation at 253 K. Yield: 30 mg
(76%). Anal. Calc. for C15H26ClCuN3O3 (Mol. wt. 395.38): C, 45.47;
H, 6.63; N, 10.63. Found: C, 45.38; H, 6.49; N, 10.68%. Molar
Fig. 4. 1H NMR spectra of complexes (a) [Cu(Sp)(NO2)Cl] (1) and (b) [Cu(Sp)(-
NO3)Cl] (2) in CDCl3.

A. Indra et al. / Inorganica Chimica Acta 374 (2011) 415–421
419
ligand attached to the copper ion in endo and exo fashions, respec-
tively, with regard to the pendant cyclohexane ring. The bite
angles, N1–Cu–N2 involving the sparteine ligand in 1 and 2 are
90.56° and 89.66°, respectively, which are much higher than that
reported for the pseudo-tetrahedral four coordinated copper-Sp
complexes, 50–60°. This difference in bite angle develops primarily
due to the five coodinated square pyramidal geometry in 1 or 2
which essentially widens the angle between the Cu and the two
nitrogen donors of the sparteine ligand. The much shorter bite
angles involving the nitrite or nitrate ligand in 1 or 2 of 56.45° or
56.48°, respectively, arises due to the steric constrains originated
through the four-membered chelate ring.
The UV–Vis spectrum of 2 shows the corresponding transitions at
321 nm
: 1685 Mꢀ1 cmꢀ1) and at 744 nm ( : 92 Mꢀ1 cmꢀ1).
e
e
The IR spectra of 1 and 2 show the characteristic vibrations of
N–O of coordinated –NO2 and –NO3 groups at 1465, 1383 and
1302 cmꢀ1 and 1481, 1384 and 1302 cmꢀ1, respectively [17,25].
The X-band EPR spectra of 1 and 2 in the solid state at 300 K ex-
hibit one broad signal with center field g values of 2.12 and 2.14,
respectively. The better picture is obtained at 77 K in 1:1 dichloro-
methane:toluene mixture. The complexes exhibit typical four lines
EPR spectra for 63Cu with I = 3/2 showing g|| > g\ > 2.0023 (Fig. 3).
This indicates a distorted square pyramidal or trigonal bipyramidal
geometry with dx2
as the ground state. The g|| values calculated
ꢀy2
to be 2.28 and 2.26 with A|| values of 120 ꢃ 10ꢀ4 and 132 ꢃ
10ꢀ4 cmꢀ1 and g\ to be 2.07 and 2.06 for complex 1 and 2, respec-
tively. For square planar CuN4 chromophore parallel hyperfine
splitting is reported always greater than 140 ꢃ 10ꢀ4 cmꢀ1 [1]. For
a five coordinated species the two possible geometries, trigonal
bipyramid (tbp) and square pyramid can easily be recognized from
the EPR spectral features. For tbp structure parallel hyperfine split-
ting (A||) value lies between 60 ꢃ 10ꢀ4 and 100 ꢃ 10ꢀ4 cmꢀ1 whereas
the same A|| value for the square pyramidal structure varies in
3.3. Spectral and redox properties
The complexes, 1 and 2 have been characterized by UV–Vis, IR,
NMR, EPR spectroscopic and cyclic voltammetric studies.
The UV–Vis spectrum of 1 exhibits one sharp charge-transfer
transition in the UV-region at 299 nm with the molar extinction
coefficient (e
) value 1625 Mꢀ1 cmꢀ1 and a broad d–d transition in
the visible region at 720 nm having
e
value 80 Mꢀ1 cmꢀ1 (Fig. 2).
Table 3
Product selectivity in the conversion of cyclohexene and styrene with 1 and 2 and [Cu{(ꢀ)Sp}Cl2] in presence of TBHP.a
OH
O
CHO
O
O
O
and
Ph
d
a
e
b
f
c
Entry
1
Substrate
Temperature (K)
Time (h)
1
2
Cu{(ꢀ)Sp}Cl2
(% conversion) selectivity
(% conversion) selectivity
(% conversion) selectivity
300
353
300
278
353
24
12
24
24
12
(86)
a = 100
(82)
a = 100
(81)
a = 52
b = 4
c = 44
–
2
3
4
5
(100)
a = 61
b = 25
c = 14
(100)
e = 92
f = 8
(100)
a = 54
b = 28
c = 18
(100)
e = 96
f = 4
(56)
d = 20
e = 42
f = 38
–
(5)
(8)
d = 41
e = 59
d = 32
e = 64
f = 4
(100)
e = 100
(100)
e = 100
–
a
All the reactions were carried out with 1 mmol substrate 0.01 mmol of complex 1, 2 or Cu(ꢀ)SpCl2 in 3 mL of acetonitrile using 1.5 mmol of TBHP.
Table 4
Distribution of the products with time in the epoxidation of cyclohexene at 353 K with 1 and 2.a
Entry
Time (h)
Complex 1
Complex 2
% conversion
% conversion
OH
O
OH
O
O
O
1
2
3
4
5
6
2
4
6
8
10
12
24
43
62
76
89
86
82
74
68
64
61
12
14
20
23
27
25
2
4
6
9
9
27
38
56
70
86
88
76
70
62
56
54
12
19
21
26
28
28
0
5
9
12
16
18
100
14
100
a
All the reactions were carried out at 353 K with 1 mmol of cyclohexene 0.01 mmol of complex 1 or 2 in 3 mL of acetonitrile using 1.5 mmol of TBHP.

420
A. Indra et al. / Inorganica Chimica Acta 374 (2011) 415–421
the range of 120–150 ꢃ 10ꢀ4 cmꢀ1 [26]. The EPR spectra of 1 and 2
therefore establish the distorted square pyramidal geometry
around the copper ion.
based processes of [(Sp)CuII(L)] (1/2) ? [(Sp)CuIII(L)]+ (1+/2+) and
[(Sp)CuII(L)] (1/2) ? [(Sp)CuI(L)] (1ꢀ/2ꢀ), respectively. The poten-
tials vary appreciably based on the nitrite and nitrate ligands.
1H NMR spectrum of free (ꢀ)sparteine in CDCl3 shows the pro-
ton resonances within the chemical shift range of 1.0–2.8 ppm.
3.4. Catalytic study
However, the one-electron paramagnetic complexes (l: 1.91 B.M.
(1) and 1.95 B.M. (2) in the solid state at 298 K) display 1H NMR
resonances over a wide range of chemical shift values, ꢀ20 to
+12 ppm in CDCl3 (Fig. 4) due to paramagnetic contact shift effect
[27,28].
3.4.1. Epoxidation
With complexes 1 and 2 as catalysts epoxidation of the alkenes
are first carried out using TBHP as the oxidant. At room tempera-
ture cyclohexene is selectively converted to the corresponding
epoxide (Table 3, Entry 1). In contrast, styrene is selectively oxi-
dized to benzaldehyde at room temperature. The catalytic activity
of reported pseudo tetrahedral complex [Cu{(ꢀ)Sp}Cl2] has also
been studied in the similar condition for the comparison purpose.
With [Cu{(ꢀ)Sp}Cl2] as the catalyst both cyclohexene and styrene
produce a mixture of products at room temperature (Table 3, Entry
1 and 3). The effect of temperature on the reaction has been stud-
ied at three different temperatures. At 353 K the product is ob-
served to be benzaldehyde with full selectivity (Table 3, Entry 5).
At low temperature (278 K) the same reaction is found to produce
low amount of styrene oxide along with benzaldehyde, though the
conversion is only 5% and 8% with 1 and 2, respectively (Table 3,
Entry 4).
The selective formation of benzaldehyde from styrene is consid-
ered to be a challenging catalytic process and only a few reports
are known so far. Palladium chloride catalyzed oxidation of styrene
to acetophenone in homogeneous medium using molecular oxygen
has recently been reported by Gusevskaya and co-workers [29].
Depending on the reaction conditions, formation of varying
amounts of benzaldehyde along with acetophenone is mentioned.
A water soluble palladium complex has been used by the Hou
group to produce benzaldehyde as the major product from styrene
The irreversible oxidation and reduction processes of 1 and 2 in
CH3CN appear at 1.2, ꢀ0.58 V and 0.76, ꢀ0.70 V (vs. SCE/Pt/[Et4N]-
[ClO4]), respectively which are tentatively assigned to copper
100
80
60
40
Conversion with 1
20
0
Conversion with 2
Epoxide selectivity with 1
Epoxide selectivity with 2
0
2
4
6
8
10
12
Time (h)
Fig. 5. % Conversion and selectivity with time during cyclohexene epoxidation with
1 and 2.
0.7
Cyclohexene oxide
cyclohexanol
cyclohexanone
Table 5
Oxidation of alcohols with 1 and 2 by molecular oxygen.a
0.6
Entry
1
Substrate
Product
% Conversion
0.5
0.4
0.3
0.2
0.1
0.0
1
2
100
100
OH
COOH
2
3
100
100
100
100
OH
OH
COOH
COOH
0
2
4
6
8
10
12
Time (h)
Cyclohexene oxide
cyclohexanol
cyclohexanone
0.6
0.5
0.4
0.3
0.2
0.1
0.0
OMe
OMe
4
5
100
7
100
11
OH
O
OH
0
2
4
6
8
10
12
Time (h)
Ph
a
1 mmol substrate, 5 mol% catalyst, 10 mol% TEMPO in acetonitrile for 24 h at
353 K in bubbling oxygen.
Fig. 6. Change in the concentration of the products with time during the
epoxidation of cyclohexene by (a) 1 and (b) 2.

A. Indra et al. / Inorganica Chimica Acta 374 (2011) 415–421
421
in the presence of molecular oxygen [30]. More relevant in the
present context is the report by Koner et al. of a copper Schiff base
complex, where during styrene oxidation with TBHP, formation of
both benzaldehyde and styrene oxide was observed [31]. Using
heterogeneous catalytic system or polyoxometallate there are re-
ports of selective formation of benzaldehyde from styrene [32–
35] but in homogeneous medium the reaction is not very common
[36].
When the epoxidation of cyclohexene is carried out at 353 K,
the selectivity towards the epoxide decreases and a mixture of
cyclohene oxide, cyclohexanol and cyclohexanone are obtained
(Table 4, Entry 1). A time monitored profile of the product forma-
tion for the cyclohexene at 353 K is shown in Fig. 5 and Table 4. At
high temperature even under low conversions all the three prod-
ucts, cyclohexene oxide, cyclohexanol, and cyclohexanone are
found to be present. With increasing time selectivity towards
cyclohexene oxide decreases and the amounts of cyclohexanol
and cyclohexanone increase. Time monitored concentration profile
of the products reveals that with time concentrations of all the
three products increase till full conversion is reached (Fig. 6).
During the reaction formation of the other oxidized species like
2-cyclohexene-1-ol or 2-cyclohexene-1-one or cyclohexane-
1,2-diol was not observed.
structural studies were carried out at the National Single Crystal
Diffractometer Facility, Indian Institute of Technology, Bombay.
EPR experiments were carried out at the Sophisticated Analytical
Instrument Facility (SAIF), Indian Institute of Technology Bombay.
Appendix A. Supplementary material
CCDC 810122 and 810123 contain the supplementary crystallo-
graphic data for complexes 1 and 2, respectively. 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 associated with this article can be found, in the online version,
at doi:10.1016/j.ica.2011.03.027.
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Five coordinated copper–sparteine complexes with nitrite (1)
and nitrate (2) as chelating ligands have been synthesized. The
complexes have been characterized by X-ray crystallography and
spectroscopic techniques. The catalytic activities of the complexes
for the epoxidation and oxidation reactions have been explored. At
room temperature with TBHP as the oxidant cyclohexene oxide
with high selectivity is obtained from cyclohexene, whereas under
the same conditions oxidation of styrene selectively gives
benzaldehyde.
Acknowledgments
Financial assistance from Council of Scientific and Industrial
Research, New Delhi, India is gratefully acknowledged. X-ray