Olefin aziridination by copper(II) complexes: Effect of redox potential on catalytic activity

Article abstract of DOI:10.1016/j.ica.2010.08.051

A series of new copper(II) complexes of four sterically hindering linear tridentate 3N ligands N′-ethyl-N′-(pyrid-2-ylmethyl)-N,N- dimethylethylenediamine (L1), N′-benzyl-N′-(pyrid-2-ylmethyl)-N,N- dimethylethylenediamine (L2), N′-benzyl-N′-(6-methylpyrid

Full text of DOI:10.1016/j.ica.2010.08.051

Inorganica Chimica Acta 365 (2011) 143–151
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Inorganica Chimica Acta
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Olefin aziridination by copper(II) complexes: Effect of redox potential
on catalytic activity
Thirumanasekaran Dhanalakshmi a, Eringathodi Suresh b, Mallayan Palaniandavar a,
⇑
a Centre for Bioinorganic Chemistry, School of Chemistry, Bharathidasan University, Tiruchirappallli 620024, India
b Analytical Science Discipline, Central Salt and Marine Chemicals Research Institute, Bhavnagar 364002, India
a r t i c l e i n f o
a b s t r a c t
A series of new copper(II) complexes of four sterically hindering linear tridentate 3N ligands N0-ethyl-N0-
(pyrid-2-ylmethyl)-N,N-dimethylethylenediamine (L1), N0-benzyl-N0-(pyrid-2-ylmethyl)-N,N-dimethyl-
ethylenediamine (L2), N0-benzyl-N0-(6-methylpyrid-2-yl-methyl)-N,N-dimethylethylenediamine (L3)
and N0-benzyl-N0-(quinol-2-ylmethyl)-N,N-dimethylethylenediamine (L4) have been isolated and exam-
ined as catalysts for olefin aziridination. The complexes [Cu(L1)Cl2]ꢀCH3OH 1, [Cu(L2)Cl2]ꢀCH3OH 2,
[Cu(L3)Cl2]ꢀ0.5 H2O 3 and [Cu(L4)Cl2] 4 have been structurally characterized by X-ray crystallography.
In all of them copper(II) adopts a slightly distorted square pyramidal geometry as inferred from the val-
Article history:
Received 19 June 2010
Received in revised form 19 August 2010
Accepted 31 August 2010
Available online 7 September 2010
Keywords:
Catalytic aziridination
PhINTs
ues of trigonality index (s) for them (s: 1, 0.02; 2, 0.01; 3, 0.07; 4, 0.01). Electronic and EPR spectral stud-
ies reveal that the complexes retain square-based geometry in solution also. The complexes undergo
quasireversible Cu(II)/Cu(I) redox behavior (E1/2, ꢁ0.272 ꢁ ꢁ0.454 V) in acetonitrile solution. The ability
of the complexes to mediate nitrene transfer from PhINTs and chloramine-T trihydrate to olefins to form
N-tosylaziridines has been studied. The complexes 3 and 4 catalyze the aziridination of styrene very
slowly yielding above 80% of the desired product. They also catalyze the aziridination of the less reactive
olefins like cyclooctene and n-hexene but with lower yields (30–50%). In contrast to these two complexes,
1 and 2 fail to catalyze the aziridination of olefins in the presence of both the nitrene sources. All these
observations have been rationalized based on the Cu(II)/Cu(I) redox potentials of the catalysts.
Ó 2010 Published by Elsevier B.V.
Chloramine-T trihydrate
Copper(II) complexes
Tridentate 3N ligands
Cu(II)/Cu(I) redox potential
1. Introduction
of the copper(II) complexes is significantly enhanced for aziridin-
ation of styrene when the ligand denticity is lowered from tetra-
The chemistry of aziridines, the smallest heterocycles similar to
epoxides, has been attracting the interest of synthetic organic
chemists due to their greater potential as the building blocks of
biologically significant nitrogen containing compounds that exist
in numerous natural products [1]. Some natural products that con-
tain aziridine rings are shown to exhibit anti-cancer and anti-tu-
mor activities also [2–8]. Aziridines can be obtained by reacting
olefins with suitable nitrene sources like (tosylimino)phenyliod-
inane (PhINTs) [9–21], iodobenzene diacetate (PhI(OAc)2) [22] to-
syl azide (TsN3) [23], chloramine-T (TsNClNa) [24,25] etc. in the
presence of catalysts (Scheme 1). A number of catalysts mostly
based on copper [26] have been reported; however, structurally
characterized copper catalysts are rare. The complex [Cu(i-Pr3TAC-
N)(O2CCF3)2] with the tridentate 3N ligand was found to effect the
quantitative aziridination of styrene derivatives [27]. Halfen et al.
utilized the copper(II) complexes of both tetradentate and triden-
tate nitrogen containing ligands and showed that the reactivity
dentate to tridentate [28,29]. They also suggested that the axial
coordination site(s) either vacant or occupied by a readily dis-
placed solvent or counterion facilitates the aziridination with the
nitrene source PhINTs. Vadernikov et al. also reported maximum
yield of aziridines with copper(II) complexes of bidentate ligands
and suggested that coordinative unsaturation at copper is respon-
sible for the very high catalytic activity [30].
The most employed and the best reagent till date for the olefin
aziridination reaction by copper catalysts is PhINTs. However, it
suffers from certain drawbacks. The synthesis of PhINTs involves
two steps [31] and iodobenzene is obtained as the byproduct dur-
ing aziridination and so a search for better reagents other than
PhINTs has resulted. An obvious alternative is chloramine-T trihy-
drate (TsNClNaꢀ(H2O)3), which gives NaCl as the byproduct, and it
is cheaper and readily available [27]. Taylor et al. reported [31] a
yield of about 76% for the aziridination of olefins using a Cu(I) cat-
alyst of the ligand N-(2-pyridinylmethylene)-1-pentanamine. In
this procedure, chloramine-T trihydrate is used as such without
dehydrating it as reported by Komatsu [33]. Bromamine-T has
been also used as a nitrene precursor [34,35]. Hutchings et al.
⇑
Corresponding author.
E-mail address: palanim51@yahoo.com (M. Palaniandavar).
0020-1693/$ - see front matter Ó 2010 Published by Elsevier B.V.
doi:10.1016/j.ica.2010.08.051

144
T. Dhanalakshmi et al. / Inorganica Chimica Acta 365 (2011) 143–151
tate (Merck, Germany), were used as received. Styrene was distilled
NTs
NTs
PhINTs
PhI
+
+
from KOH pellets at 25 °C under vacuum (0.3 Torr) and was stored at
ꢁ20 °C. Anhydrous acetonitrile was used in catalytic aziridinations
and was handled and stored under N2. The iodinane PhINTs was pre-
pared by a modified literature procedure and was stored in the dark
[31]. tetra-n-Butylammonium perchlorate (TBAP) was prepared by
the addition of perchloric acid to a aqueous solution of tetra-n-butyl-
ammonium bromide. The product was recrystallised from aqueous
ethanol and was tested for the absence of bromide. Elemental anal-
yses were performed on a Perkin–Elmer Series II CHNS/O analyzer
2400. 1H NMR spectra were obtained using a Bruker 200 MHz spec-
trometer at room temperature. 1H NMR chemical shifts are reported
versus TMS and are referenced to residual solvent peaks. Electronic
absorption spectra were acquired using a Agilent Diode Array Spec-
trophotometer (200–1100 nm). FTIRspectra(4000–400 cmꢁ1 range,
KBr pellets) were recorded on a Jasco FT-IR spectrometer. Electron
paramagnetic resonance (EPR) spectra were recorded on a Bruker
EMX 6/1 spectrometer, the field being calibrated with diph-
enylpicrylhydrazyl (dpph). The g0 and A0 values were estimated at
ambient temperature and g|| and A|| at 77 K. The values of g\ and
A\ were computed as ½(3g0 ꢁ g||) and ½(3A0 ꢁ A||), respectively.
Electrochemical experiments were conducted using a EG & G PAR
273 Potentiostat/Galvanostat with EG & G M270 software, using a
platinum sphere working electrode, an Ag/AgNO3 reference elec-
trode, and a platinum plate auxiliary electrode. Cyclic voltammo-
grams were obtained in acetonitrile using 0.1 M TBAP as
supporting electrolyte. Conductivity measurements on acetonitrile
solution of the complexes were made using Elico conductivity
bridge.
R
R
Catalyst
R
+
NaCl
Chloramine-T
trihydrate
Scheme 1. Olefin aziridination using Cu(II) complexes.
examined the potential of both chloramine-T and bromamine-T as
a nitrene source in the asymmetric aziridination with copper-ex-
changed zeolite and found that PhINTs is far superior [36].
In our laboratory we have already investigated [37] the aziridin-
ation of olefins using Cu(II) complexes of cyclic tridentate 3N li-
gands as catalysts and the strongly oxidizing PhINTs as nitrene
source. All the complexes were found to be fast and efficient cata-
lysts when a high (10–5) olefin:PhINTs ratio was used. Also we
have shown that ligand steric hindrance plays a vital part in accel-
erating the reaction rate and providing higher yields of aziridines.
In continuation of the above study to determine whether electronic
effects or steric factors are responsible for the increased product
yield and minimum reaction time, the electron rich and sterically
hindering –NMe2, 6-methylpyridyl and quinolyl moieties have
been incorporated in the open chain tridentate 3N ligands
(Scheme 2) chosen for the present study. The coordinative unsatu-
ration as well as steric crowding around copper(II) in the present
complexes is expected to lead to improved selectivity, reaction rate
and broad substrate tolerance. Since copper halides are till now the
best catalysts reported, we have examined and compared the nit-
rene sources PhINTs and chloramine-T trihydrate in the presence
of Cu(II) chloride complexes of the 3N ligands. The X-ray crystal
structures of the complexes have been determined to establish
the degree of coordinative unsaturation and distortion in the com-
plexes. The solution structures of the complexes have been as-
serted by employing spectroscopic and electrochemical methods
to understand the effect of ligand architecture on copper(II) coor-
dination geometry. The more sterically hindered complexes 3
and 4 are found to catalyze the aziridination of olefins more effi-
ciently than 1 and 2 and this observation correlates with the higher
ability of the former complexes to stabilize Cu(I) or destabilize the
Cu(II) oxidation state in solution.
2.2. Synthesis of ligands
2.2.1. N0-ethyl-N0-pyrid-2-ylmethyl-N,N-dimethylethylenediamine
(L1)
Pyridine-2-carboxaldehyde (0.535 g, 5 mmol) and N0-ethyl-N,N-
dimethylethylenediamine (0.580 g, 5 mmol) were mixed in 1,2-
dichloroethane (20 mL). Sodium triacetoxy borohydride (1.49 g)
was added to the above solution and the reaction mixture was stir-
red at room temperature for 42 h. The reaction was quenched with
saturated aqueous sodium bicarbonate and the product was ex-
tracted with ethyl acetate. The ethyl acetate extract was dried
and the solvent was evaporated to give yellow oil. The product ob-
tained was used as such for the complex preparation. Yield, 0.67 g,
65%. Anal. Calc. for C12H21N3 (207.17). dH (200 MHz; CDCl3) 8.53–
7.18 (m, 4H), 3.76 (s, 2H), 2.63 (q, 4H), 2.43 (m, 4H), 2.21 (s, 6H),
1.06 (t, 3H) ppm.
2. Experimental section
2.1. Materials and methods
CuCl2ꢀ2H2O (Merck, India), pyridine-2-carboxaldehyde, quino-
2.2.2. N0-benzyl-N0-pyrid-2-ylmethyl-N,N-dimethylethylenediamine
(L2)
line-2-carboxaldehyde,
6-methyl-pyridine-2-carboxaldehyde,
N0-ethyl-N,N-dimethylethylenediamine, N0-benzyl-N,N-dimethyl-
ethylenediamine, chloramine-T trihydrate, sodium triacetoxyborohy-
dride, styrene, cis-cyclooctene, n-hexene, tetra-n-butylammonium
bromide (Aldrich), p-toluene sulfonamide and iodosobenzenediace-
The ligand L2 was prepared using the same procedure em-
ployed for L1, except that the amine used here was N0-benzyl-
N,N-dimethylethylenediamine (0.89 g, 5 mmol). Yield, 1.08 g, 80%.
Anal. Calc. for C17H23N3 (269.19). dH (200 MHz; CDCl3) 8.64–7.29
N
N
N
N
N
N
N
N
N
N
N
N
L1
L2
L3
L4
Scheme 2. Ligands employed for the present study.

T. Dhanalakshmi et al. / Inorganica Chimica Acta 365 (2011) 143–151
145
= 3502(b), 2925(s), 2362(w), 1515(s), 1344(w), 904(s) cmꢁ1. X-
ꢀ
(m, 4H), 7.06–7.14 (m, 6H), 3.95 (s, 2H), 2.46 (m, 4H), 3.62 (s, 3H),
2.27 (br s, 6H) ppm.
m
ray diffraction quality crystals of 2 were obtained by the vapor dif-
fusion of diethylether into the complex dissolved in methanol.
2.2.3. N0-benzyl-N0-(6-methylpyrid-2-ylmethyl)-N,N-
dimethylethylenediamine (L3)
The ligand L3 was prepared by the same procedure used for L1,
except that the aldehyde used here was 6-methyl-pyridine-2-car-
boxaldehyde (0.63 g, 5 mmol). Yield, 0.84 g, 60%. Anal. Calc. for
2.3.3. [Cu(L3)Cl2]ꢀ0.5ꢀH2O (3)
A procedure analogous to that used to prepare 1 was followed,
using CuCl2ꢀ2H2O (0.17 g, 1 mmol) and L3 (0.28 g, 1 mmol) instead
of L1. The solution was stirred which results in the formation of
green colored precipitate after one hour and it was filtered. Yield,
0.21 g, 50%. Anal. Calc. for C18H26N3Cl2CuO0.5 (426.87): C 50.65; H
ꢀ
C18H25N3 (285.41). dH (200 MHz; CDCl3) 7.81–7.18 (m, 3H), 7.07–
7.14 (m, 6H), 3.95 (s, 2H), 2.46 (m, 4H), 3.62 (s, 2H), 2.55 (s, 3H),
2.27 (br s, 6H) ppm.
6.14; N 9.84. Found: C 50.60; H 6.13; N 9.90%. IR:
m = 3315(b),
2925(s), 1735(s), 1446(s), 939(s), 634 (s) cmꢁ1. X-ray diffraction
quality crystals of 3 were obtained by the vapor diffusion of dieth-
ylether into the complex dissolved in acetonitrile.
2.2.4. N0-benzyl-N0-quinol-2-ylmethyl-N,N-dimethylethylenediamine
(L4)
The ligand L4 was prepared using the same procedure em-
ployed for L1, except that the aldehyde used here was 2-quinoline
carboxaldehyde (0.79 g, 5 mmol). Yield, 0.64 g, 40%. Anal. Calc. for
2.3.4. [Cu(L4)Cl2] (4)
C21H25N3 (319.44). dH (200 MHz; CDCl3) 9.09–7.29 (m, 6H), 7.07–
This was prepared by the addition of L4 (0.32 g, 1 mmol) in
methanol (15 mL) to a methanolic solution of CuCl2ꢀ2H2O (0.17 g,
1 mmol) with stirring. After 15 min of stirring, green colored pre-
cipitate was obtained. It was filtered and washed with diethyl
ether. Yield, 0.26 g, 65%. Anal. Calc. for C21H25N3Cl2Cu (453.9): C
55.57; H 5.55; N 9.26. Found: C 55.62; H 5.38; N 9.38%. IR:
ꢀ
7.14 (m, 6H), 3.95 (s, 2H), 2.46 (m, 4H), 3.62 (s, 2H), 2.27 (br s,
6H) ppm.
2.3. Preparation of copper(II) complexes
= 3546(b), 2925(w), 1467(s), 1031(s), 773(s) cmꢁ1. Suitable single
2.3.1. [Cu(L1)Cl2]ꢀCH3OH (1)
m
A solution of L1 (0.21 g, 1 mmol) in methanol (15 mL) was trea-
ted with CuCl2ꢀ2H2O (0.17 g, 1 mmol) to obtain a deep blue color
solution. After 15 min of stirring, diethyl ether was allowed to dif-
fuse into the solution. Blue crystals of the product were deposited
after standing for a period of several days. Yield, 0.32 g, 56%. Anal.
Calc. for C13H25N3Cl2CuO (373.81): C 41.77; H 6.74; N 11.24.
ꢀ
crystals of 4 for X-ray diffraction were obtained by diffusion of
diethyl ether into the methanolic solution of the complex.
2.4. Catalytic aziridination
Found: C 41.70; H 6.77; N 11.28%. IR:
m = 3525(b), 2925(s),
Aziridination of the reactive as well as the non-reactive olefins
like styrene, cyclooctene, 1-hexene were performed using the nit-
rene sources, PhINTs and chloramine-T trihydrate by the proce-
dures reported previously [28,32]. Aziridinations were performed
by stirring mixtures of PhINTs or chloramine-T trihydrate (0.3–
0.4 mmol), olefins (5 equiv versus nitrene source), and the copper
catalyst (5 mol% versus nitrene source) in 2 mL of anhydrous
CH3CN under a dry N2 atmosphere. All the products obtained were
characterized by 1H NMR spectroscopy.
2360(w), 1590(s), 908(s) cmꢁ1
.
2.3.2. [Cu(L2)Cl2]ꢀCH3OH (2)
A procedure analogous to that used to prepare 1 was followed,
using CuCl2ꢀ2H2O (0.17 g, 1 mmol) and L2 (0.27 g, 1 mmol) instead
of L1. The pure product was isolated as pale green precipitate.
Yield, 0.19 g, 47%. Anal. Calc. for C18H27N3Cl2CuO (435.88): C
49.60; H 6.24; N 9.64. Found C 49.62; H 6.28; N 9.58%. IR:
Table 1
Crystal data and structure refinement details for the complexes 1–4.
1
2
3
4
Empirical formula
Formula weight
Crystal system
Crystal size (mm)
Space group
a (Å)
C
13H25Cl2CuN3O
C18H27Cl2CuN3O
435.87
Monoclinic
0.28 ꢂ 0.14 ꢂ 0.08
P21/c
9.035(5)
24.322(13)
9.561(5)
90.000
99.093(10)
90
C36H52Cl4Cu2N6O
853.72
Triclinic
C21H25Cl2CuN3
453.90
Monoclinic
0.09 ꢂ 0.12 ꢂ 0.32
C2/c
32.363(3)
11.4343(12)
13.0705(14)
90
112.928(2)
90
373.81
Monoclinic
0.07 ꢂ 0.09 ꢂ 0.17
P21/c
7.3916(8))
12.5856(15)
17.679(2
90
91.575(2)
90
1644.0(3)
4
0.41 ꢂ 0.26 ꢂ 0.21 mm
ꢀ
P1
10.8374(11)
12.3977(13)
16.0820(16)
107.351(2)
101.618(2)
102.710(2)
1927.4(3)
2
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
2074.6(19)
4
4454.6(8)
8
Z
k (Å)
Mo K
a
, 0.71073
Mo K
a, 0.71073
Mo K
a, 0.71073
Mo Ka, 0.71073
Dcalc g cmꢁ3
1.510
1.18
8124
2893
0
1.395
1.063
7688
2695
0
1.471
1.074
16709
8745
0
1.354
0.96
13044
5162
0
Goodness-of-fit on F2
Number of reflections measured
Number of reflections used
Number of L.S. restraints
Number of refined parameters
186
230
448
243
Final R indices [I > 2
r(I)]
R1a
0.0595
0.1451
0.1061
0.2272
0.0398
0.0914
0.0739
0.1698
wR2b
a
R1 = [
wR2 = {[
R
(||Fo| ꢁ |Fc||)/
R|Fo|].
b
2
R
(w(Fo ꢁ Fc2)}^{2})/
R
(wFo^{4})]1/2}.

146
T. Dhanalakshmi et al. / Inorganica Chimica Acta 365 (2011) 143–151
2.5. X-ray crystallography
the ligands occupy three meridional coordination positions and
the two chloride ions in the remaining positions. Also, conductivity
studies reveal that the complexes behave as 1:1 electrolytes
X-ray single-crystal diffraction data for all the copper(II) com-
plexes were collected on a Bruker SMART Apex diffractometer
equipped with CCD area detector at 273 K for 1, 2 and 3 and at
(130
X
ꢁ1 cm2 Mꢁ1) in acetonitrile solution corresponding to the
displacement of the axial chloride to give [Cu(L)Cl]+ ions. The long-
er equatorial chloride can dissociate in solution in the presence of
the substrates and facilitate aziridination reaction.
LN temperature (100 K) for
4 with Mo Ka radiation (k,
0.71073 Å). A crystal of suitable size was immersed in paraffin oil
and then mounted on the tip of a glass fiber and cemented using
epoxy resin. The crystallographic data were collected in Table 1.
For all the complexes, the SMART [50–52] program was used for col-
lecting frames of data, indexing the reflections, and determination
of lattice parameters; SAINT [50–52] program for integration of the
intensity of reflections and scaling; SADABS [50–52] program for
absorption correction, and the SHELXTL [53] program for space group
and structure determination, and least-squares refinements on F2.
The structure was solved by heavy atom method. Other non-
hydrogen atoms were located in successive difference Fourier
syntheses. The final refinement was performed by full-matrix
least-squares analysis. Hydrogen atoms attached to the ligand moi-
ety were located from the difference Fourier map and refined
isotropically.
3.2. Structures of the copper(II) complexes
The ORTEP representation of the structures of [Cu(L1)Cl2]ꢀCH3OH
1, [Cu(L2)Cl2]ꢀCH3OH 2, [Cu(L3)Cl2]ꢀ0.5 H2O 3 and [Cu(L4)Cl2] 4 are
shown in Figs. 1–4, respectively along with atom numbering
scheme. The crystallographic data were collected in Table 1 and
the relevant bond lengths and bond angles for the complexes are
In the case of 4, there was no evidence of crystal decay during
data collection, even though the crystal was very unstable on expo-
sure to atmosphere. After locating the complex moiety, a large
number of diffused scattered peaks with residual electron density
ranging from 4.0 Åꢁ3 to 2.0 Åꢁ3 was observed in the difference Fou-
rier map, which can be attributed to disordered solvent molecule
(diethylether/methanol) used for crystallization, present in the
crystal lattice. Attempts were made to model this, but were unsuc-
cessful since the residual electron density peaks obtained was dif-
fused and there were no obvious major site occupations for the
solvent molecules. PLATON/SQUEEZE [54,55] was used to correct the
data for the presence of the disordered solvent. A potential solvent
accessible volume of 629.8 Å [9–13] was found. A total of 141 elec-
trons per unit cell worth of scattering were located in the void. This
electron counts correspond to approximately three molecules of
diethylether and one molecule of methanol present in the unit cell.
Thus, the structure of the compound revealed that in addition to
the refined copper complex, tentatively three disordered diethyl-
ether and one methanol molecules as solvent of crystallization
may be present in the unit cell corresponding to 144 electrons/
per unit cell, whose contribution was removed from the modified
reflection data by SQUEEZ program. For this compound, full-matrix
least-squares refinement of all non-hydrogen atoms with aniso-
tropic temperature factors were carried out till the convergence
reached. All the H atoms were positioned geometrically and trea-
ted as riding atoms. With the modified data set final cycles of
least-squares refinements improved both the R-values and Good-
ness of Fit significantly.
Fig. 1. ORTEP drawing of [Cu(L1)Cl2] 1 showing the atom numbering scheme and the
thermal motion ellipsoids (50% probability level).
3. Results and discussion
3.1. Synthesis of ligands and complexes
The tridentate ligands L1–L4 were prepared as oils in good
yields by the reductive amination of the corresponding aldehydes
like pyridine-2-carboxaldehyde, 6-methyl-pyridine-2-carboxalde-
hyde and quinoline-2-carboxaldehyde with the trisubstituted
ethylenediamines in the presence of sodium triacetoxyborohy-
dride. The copper(II) complexes of the ligands were readily pre-
pared in good yields by the addition of CuCl2ꢀH2O in methanol to
methanolic solutions of the ligands. All of them are soluble in com-
mon organic solvents like methanol and acetonitrile. They were
characterized by analytical and spectroscopic methods and studied
as catalysts for olefin aziridination. In the X-ray crystal structures
Fig. 2. ORTEP drawing of [Cu(L2)Cl2] 2 showing the atom numbering scheme and the
thermal motion ellipsoids (50% probability level).

T. Dhanalakshmi et al. / Inorganica Chimica Acta 365 (2011) 143–151
147
Table 2
Selected bond lengths [Å] and angles [°] for 1, 2, 3 and 4.
1
2
3
4
Bond lengths
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–N(3)
Cu(1)–Cl(1)
Cu(1)–Cl(2)
Cu(2)–N(4)
Cu(2)–N(5)
Cu(2)–N(6)
Cu(2)–Cl(3)
Cu(2)–Cl(4)
2.016(4)
2.086(4)
2.064(4)
2.2759(14)
2.5377(14)
2.028(11)
2.113(9)
2.053(12)
2.297(4)
2.542(4)
2.046(2)
2.1004(19)
2.067(2)
2.2793(6)
2.4914(7)
2.048(2)
2.102(2)
2.058(2)
2.2762(6)
2.4643(7)
2.069(4)
2.088(4)
2.069(4)
2.3073(15)
2.4139(13)
Bond angles
N(1)–Cu(1)–N(2)
N(1)–Cu(1)–N(3)
N(2)–Cu(1)–N(3)
Cl(1)–Cu(1)–N(1)
Cl(1)–Cu(1)–N(2)
Cl(1)–Cu(1)–N(3)
Cl(2)–Cu(1)–N(1)
Cl(2)–Cu(1)–N(2)
Cl(2)–Cu(1)–N(3)
Cl(1)–Cu(1)–Cl(2)
N(4)–Cu(2)–N(5)
N(4)–Cu(2)–N(6)
N(5)–Cu(2)–N(6)
Cl(3)–Cu(2)–N(4)
Cl(3)–Cu(2)–N(5)
Cl(3)–Cu(2)–N(6)
Cl(4)–Cu(2)–N(4)
Cl(4)–Cu(2)–N(5)
Cl(4)–Cu(2)–N(6)
Cl(3)–Cu(2)–Cl(4)
80.52(16)
161.06(17)
85.67(16)
94.36(12)
159.67(11)
94.14(12)
93.06(12)
103.37(11)
102.74(11)
96.51(5)
81.0(4)
158.7(4)
84.7(4)
79.92(8)
156.84(8)
84.43(8)
100.99(6)
101.39(6)
98.76(6)
95.04(6)
152.75(6)
91.08(6)
105.86(2)
79.13(8)
154.50(8)
83.67(8)
93.51(5)
151.37(6)
92.64(6)
105.99(6)
102.24(6)
95.92(6)
106.39(2)
79.40(15)
153.58(14)
83.89(15)
94.22(12)
154.13(11)
91.96(11)
104.11(11)
102.92(11)
99.46(11)
102.94(5)
96.9(3)
100.7(3)
101.2(3)
93.4(3)
159.5(3)
94.5(3)
Fig. 3. ORTEP drawing of [Cu(L3)Cl2] 3 showing the atom numbering scheme and the
thermal motion ellipsoids (50% probability level).
99.57(14)
The complex [Cu(L2)Cl2]ꢀCH3OH 2 crystallises with one metha-
nol molecule of crystallization. In this complex, the coordination
geometry around copper(II) is best described as trigonal bipyrami-
dal distorted square-based pyramidal (TBDSBP) with the corners of
the square plane being occupied by the three nitrogen atoms (N1,
N2, N3) and one chloride ion (Cl2), and the apical position by the
other chloride ion (Cl1). It is very much similar to that of complex
1, but has a lower trigonal constraint at Cu(II) as evident from the
lower value of the structural index
s (0.01) in 2. When the N-ethyl
substituent in [Cu(L1)Cl2] 1 is replaced by the bulkier N-benzyl
substituent as in 2, the Cu–N2 bond (2.113(9) Å) in 2 becomes
longer than in 1, as expected [43], and consequently the Cu–N3
bond (2.053(12) Å) in 2 becomes shorter than that in 1. As ob-
served in 1, the Cu–Npy bond (2.028(11) Å) in 2 is shorter than that
(2.0308(13) Å) in 5. The weaker coordination of ethyl and benzyl
substituted N2 in 1 and 2, respectively results in the stronger coor-
dination of pyridine nitrogen atom to copper.
The unit cell of the triclinic crystals of [Cu(L3)Cl2]ꢀ0.5 H2O 3
contains two crystallographically independent molecules along
with one water molecule. In both the molecules copper(II) pos-
sesses a trigonal bipyramidal distorted square-based pyramidal
(TBDSBP) coordination geometry, which is very similar to that of
Fig. 4. ORTEP drawing of [Cu(L4)Cl2] 4 showing the atom numbering scheme and the
thermal motion ellipsoids (50% probability level).
given in Table 2. The complex [Cu(L1)Cl2]ꢀCH3OH 1 crystallises in
the space group of P21/c with one methanol molecule of crystalli-
zation. In 1 copper(II) is coordinated by two nitrogen atoms (N2,
N3) of the ethylenediamine moiety and the pyridyl nitrogen (N1)
of ligand L1, and two chloride ions (Cl1, Cl2). The value of the struc-
tural index [38] (s, 0.02) reveals that the coordination geometry
all the above complexes. The
s values for both the molecules
around copper(II) is best described as trigonal bipyramidal dis-
torted square-based pyramidal (TBDSBP) [39–41] with the corners
of the square plane being occupied by the three nitrogen atoms
and a chloride ion (Cl1), and the apical position by the other
chloride ion (Cl2). The coordination of N2 and N3 nitrogens of
the ethylenediamine moiety is sterically hindered by the presence
of N-alkyl substituents. On incorporating an ethyl substituent on
N2 in [Cu(L5)Cl2] 5, where L5 is N,N-dimethyl-N0-(pyrid-2-
ylmethyl)ethylenediamine [42], to give 1, the lone pair orbital on
it becomes improperly oriented towards Cu(II) and consequently
the Cu–N2 bond (2.0319(14) Å) in 5 is elongated to 2.086(4) Å in 1.
(0.07 and 0.05) are slightly higher than those for 1 and 2 clearly
indicating that the steric bulk of the 6-methyl group on the pyridyl
moiety confers enhanced constraints on copper(II). The coordina-
tion of pyridine nitrogen to copper is hindered by 6-methyl group
in 3 leading to a Cu–Npy bond distance (2.046(2), 2.048(2) Å) long-
er than that (2, 2.016 Å) in the analogous complex 2. The incorpo-
ration of the benzyl substituent on N2 in [Cu(L6)Cl2] 6, where L6 is
N,N-dimethyl-N0((6-methyl)pyrid-2-ylmethyl)ethylenediamine
[32], to give 3 leads to weakening of the Cu–N2 bond (3,
2.1004(19); 6, 2.006(3) Å) obviously due to the improper orienta-
tion of the lone pair orbital on N2.

148
T. Dhanalakshmi et al. / Inorganica Chimica Acta 365 (2011) 143–151
In 4 copper(II) is coordinated by three nitrogen atoms, two from
the ethylenediamine moiety (N2, N3) and one (N1) from the quino-
lyl moiety of the ligand L4, and two chloride ions (Cl1, Cl2). The va-
3
1
1
0.8
0.6
0.4
0.2
0
2
4
lue of
s (0.01) reveals that the coordination geometry around
copper(II) is almost square-based pyramidal with the corners of
the square plane being occupied by the three nitrogen atoms and
one chloride ion (Cl1), and the apical position by the other chloride
ion (Cl2). The Cu–Nqui (Cu–N1; 2.069(4) Å) and the Cu–NMe2 (Cu1–
N3; 2.069(4) Å) bond distances are equal and are shorter than the
Cu–Namine distance (Cu–N2; 2.088(4) Å). On replacing the pyridyl
moiety in 2 by the sterically hindering quinolyl moiety to obtain
4, the Cu–Nqui bond becomes longer suggesting the effect of steric
hindrance from the benzene ring fused with the pyridyl ring. It is
interesting to note that the Cu(II) coordination geometry in
[Cu(L7)Cl2] 7, where L7 is 4-methyl-1-(quinol-2-ylmethyl)-1,4-
diazacycloheptane [37], is strongly distorted from square planar
300
400
500
600
700
800
900 1000 1100
Wavelength (nm)
Fig. 5. Electronic absorption spectra of all the complexes in acetonitrile solution.
Concentration of the complexes: 4 ꢂ 10ꢁ3 M.
geometry towards trigonal bipyramidal geometry (s, 0.48) obvi-
ously due to the presence of sterically constrained diazepane back-
bone connecting the N2 and N3 nitrogen donor atoms in L7.
The Cu–Npy/Cu–Nme-py bond in the complexes 1–3 (1, 2.016(4);
2, 2.028(11); 3, 2.046(2) Å) is shorter than the Cu–Namine bond dis-
tance (1, 2.086(4); 2, 2.113(9); 3, 2.1004(19) Å), which is expected
of the sp2 and sp3 hybridisations respectively of the pyridine and
amine nitrogen atoms. Also, in all the above square-based pyrami-
dal complexes 1–4, the apical Cu–Cl bond is longer than the equa-
torial Cu–Cl bond obviously due to the presence of two unpaired
electrons in the dz2 orbital of Cu(II).
3.3. Spectral properties
The solid state reflectance spectra of all the complexes show a
broad ligand field feature (540–850 nm) in the visible region,
which is typical of Cu(II) located in a square-based environment
as evidenced from the X-ray crystal structures of the complexes
(Table 3). On dissolution in acetonitrile, only one ligand field fea-
ture is observed (above 700 nm, Fig. 5) for all the complexes. This
suggests a negligible change in coordination geometry on dissolu-
Table 3
Electronic and EPR spectral data for the copper(II) complexes.
Complexes
Electronic spectraa kmax/nm
EPR spectrab
(e
max/Mꢁ1 cmꢁ1
)
Solid
Solution
(in acetonitrile)
Solid
Frozen
solutionc
[Cu(L1)Cl2] 1
[Cu(L2)Cl2] 2
[Cu(L3)Cl2] 3
[Cu(L4)Cl2] 4
650–800
750 (215)
g|| 2.223
g\ 2.13
g|| 2.37
A|| 177
g\ 2.12
g||/A|| 133
461 (192)
270 (32634)d
650–800
750–900
750–900
765 (240)
g|| 2.23
g\ 2.09
g|| 2.36
A|| 187
g\ 2.07
g||/A|| 126
270 (28483)d
792 (274)
g|| 2.226
g\ 2.056
g|| 2.29
A|| 172
g\ 2.080
g||/A|| 133
272 (27290)d
780 (225)
g|| 2.25
g\ 2.10
g|| 2.262
A|| 158
g\ 2.103
g||/A|| 142
276 (65254)d
238 (49779)d
a
Concentration, 4 ꢂ 10ꢁ3 M for ligand field and 2 ꢂ 10ꢁ5 M for ligand based
transitions.
b
A|| in 10ꢁ4 cmꢁ1
.
c
Acetonitrile: acetone (4:1 v/v) glass at 77 K.
p–p* Transitions within the ligand.
Fig. 6. X band EPR spectra of complexes 2 and 4 at 77 K in acetonitrile/acetone
(4:1 v/v) glass.
d

T. Dhanalakshmi et al. / Inorganica Chimica Acta 365 (2011) 143–151
149
Table 4
Electrochemical dataa for copper(II) complexes at 25 0.2 °C in acetonitrile solution.
Complexes
Epc (V)
Epa (V)
E1/2 (V)
D
Ep (mV)
ipa/ipc
D (10ꢁ6 cm2 sꢁ1
)
CV
DPVb
[Cu(L1)Cl2] 1
[Cu(L2)Cl2] 2
[Cu(L3)Cl2] 3
[Cu(L4)Cl2] 4
ꢁ0.542
ꢁ0.472
ꢁ0.432
ꢁ0.378
ꢁ0.378
ꢁ0.348
ꢁ0.176
ꢁ0.190
ꢁ0.460
ꢁ0.410
ꢁ0.304
ꢁ0.284
ꢁ0.454
ꢁ0.404
ꢁ0.308
ꢁ0.272
164
124
256
188
0.9
0.8
0.6
0.8
5.1
6.1
3.6
3.7
a
+
Potential measured vs. non-aqueous Ag/AgNO3 reference electrode; add 0.544 V to convert to standard hydrogen electrode (SHE); Fc/Fc couple, E1/2, 0.038 V (CV),
D
Ep,
88 mV; scan rate 0.05 V sꢁ1; supporting electrolyte, tetra-N-butylammonium perchlorate (0.1 M); complex concentration, 1 ꢂ 10ꢁ3 M.
b
Differential pulse voltammetry (DPV), scan rate 0.005 V sꢁ1, pulse height 50 mV.
tion. Further, the ligand-field energy of the complexes 1–4 are low-
er, and the molar absorptivities are higher than those of 5 and 6
the presence of a square based CuN3Cl/CuN3O chromophore [42].
The values of g||/A|| quotient for all the complexes (1; 133, 2; 126,
3; 133, 4; 142 cm) are in the range for complexes with a perfect
square planar coordination geometry (g||/A||, 105–135 cm) [47],
which is consistent with the above X-ray crystal structures and
electronic spectral results.
[32], which is expected of the weaker
r-coordination of N2 nitro-
gen atom on account of the steric hindrance imposed by the N-
ethyl or N-benzyl substituents on it.
The polycrystalline EPR spectra of 1–4 are axial, which is consis-
tent with the square-based geometry found in the X-ray crystal
structures of the complexes. The frozen-solution spectra (Fig. 6)
of all the complexes are axial [g|| > g? > 2.0, G = (g|| ꢁ 2)/
(g? ꢁ 2) = 2.5 ꢁ 4.0] [44,45], which is usual for mononuclear
tetragonal copper(II) complexes with dx2ꢁy2 ground state [46].
Copper(II) complexes with square-based CuN4 coordination sphere
3.4. Electrochemical properties
The electrochemical data obtained for the present complexes in
acetonitrile solution using TBAP as supporting electrolyte are col-
lected in Table 4. The cyclic (CV) and differential pulse voltammo-
grams (DPV) have been recorded using Pt as working electrode and
Ag/AgNO3 as reference electrode. For all the complexes the Cu(II) to
Cu(I) reduction (Epc, ꢁ0.378 ꢁ ꢁ0.542 V) is associated with a reox-
idation peak (Epa, ꢁ0.176 ꢁ ꢁ0.378 V) in the reverse scan (Fig. 7).
exhibit g|| and A|| values around 2.200 and 200 ꢂ 10ꢁ4 cmꢁ1
,
respectively and replacement of a coordinated nitrogen in this
chromophore by oxygen (solvent methanol) is expected to enhance
the g|| value and decrease the A|| value. Thus the g|| (2.262–2.237)
and A|| (187–158 ꢂ 10ꢁ4 cmꢁ1) values of 1–4 are suggestive of
The values of the limiting peak-to-peak separation (
D
Ep, 124–
+
256 mV) are higher than that for Fc/Fc couple (DEp, 88 mV) under
identical conditions. This suggests that the heterogeneous elec-
tron-transfer process in the present complexes is far from revers-
ible and that on electron transfer considerable stereochemical
reorganization of the coordination sphere occurs [37]. The ob-
served Cu(II)/Cu(I) redox potentials follow the trend 4 > 3 > 2 > 1
(Fig. 8). Upon replacement of the N-ethyl group in 1 by the N-ben-
zyl group in 2, the redox potential becomes slightly more positive.
With the replacement of the pyridyl group in 2 by 6-methylpyridyl
group as in 3 and by quinolyl moiety as in 4, there is an enormous
positive shift in the E1/2 value. This reveals the importance of ste-
rically hindering quinolyl and 6-methylpyridyl moieties in destabi-
lizing the Cu(II) forms of 3 and 4, thereby rendering the CuII ? CuI
electron transfer more facile.
50
40
2
1
3
4
30
20
10
0
-10
-20
0.1
0
-0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.8
E (V)
Fig. 7. Cyclic voltammograms of 1, 2, 3 and 4 in acetonitrile solution at 25 °C at
Table 5
0.05 V sꢁ1 scan rate. Complex concentration: 0.001 M.
Results of catalytic aziridination studies using copper(II) complexes 1–4.
Olefin
Entry
Catalysta
Olefin
equiv
Yield (%)b
PhINTs
Chloramine-T
trihydrate
2
50
Styrene
1
2
3
4
1
2
3
4
5
5
5
5
32
24
90
85
–
–
75
63
40
1
30
4
cis-Cyclooctene
n-Hexene
5
6
7
8
1
2
3
4
5
5
5
5
–
–
56
48
–
–
50
45
3
20
10
0
9
1
2
3
4
5
5
5
5
–
–
42
30
–
–
40
30
10
11
12
0.1
0
-0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.8
E (V)
a
5 mol% vs. PhINTs or chloramine-T trihydrate.
Isolated yields of N-tosylaziridine after purification with respect to 0.3 mM of
PhINTs or chloramine-T trihydrate used.
b
Fig. 8. Differential pulse voltammograms of 1, 2, 3 and 4 in acetonitrile solution at
25 °C at 0.005 V sꢁ1 scan rate. Complex concentration: 0.001 M.

150
T. Dhanalakshmi et al. / Inorganica Chimica Acta 365 (2011) 143–151
3.5. Olefin aziridination with PhINTs and chloramine-T trihydrate
crease in yield. Thus the complex 7 of the ligand containing the ste-
rically congested diazepane backbone imposes greater steric
constraints around copper(II) leading to the highest aziridination
rate and yield among the complexes investigated. On the other
hand, the incorporation of the bulky ligand moieties like quinolyl
and 6-methyl-pyridyl in the less sterically congested ethylenedia-
mine backbone fail to impose any significant steric constraints on
the copper(II) coordination geometry in 1–4 (see above). So, all the
present complexes would be expected to be less efficient catalysts
for aziridination and thus 1 and 2 fail to effect aziridination. Inter-
estingly, 3 and 4 catalyze the aziridination of olefins to give quan-
titative yields of aziridine, however, with decreased rates of
reaction. The wide difference in reactivities can be illustrated
based on the ability of the present ligands to destabilize Cu(II) oxi-
dation state in solution. Thus the E1/2 values of 3 and 4 (3,
ꢁ0.308 V; 4, ꢁ0.272 V) are more positive than those of 1 and 2
(1, ꢁ0.454 V; 2, ꢁ0.404 V) suggesting that the ligands in the former
complexes destabilize the Cu(II) oxidation state more than those in
the latter. This leads to enhanced ability of the former complexes
to facilitate the transfer of @NTs from the metallo-nitrene interme-
diate to olefins resulting in better yields of the aziridine. However,
their efficiency is much lower than the diazapane-based ligand in 7
the Cu(II)/Cu(I) redox potential (E1/2, ꢁ0.252 V) of which is more
positive than that of the present complex. This suggests that the
Cu(II)/Cu(I) redox potentials rather than the trigonal constraint
on the copper(II) coordination geometry may be used to illustrate
the ability of copper(II) complexes to catalyze the aziridination of
olefins.
Also, the catalytic activity of the present complexes can be ex-
plained on the basis of the ability of the 3N ligands to stabilize
the Cu(I) oxidation state in solution. Theoretical calculations [48]
have suggested that Cu(I) rather than Cu(II) as the active oxidation
state for aziridination. However, for the present complexes, the
green color of the reaction mixture persists even after dissolution
of the PhINTs indicating that the catalyst resting state is Cu(II)
but evidences [48] show a favorable path for conversion of Cu(II)
to Cu(I) when they interact with alkenes under the reaction condi-
tions. It should be noted that a Cu(I)–alkene complex has been iso-
lated and structurally characterized [49]. The Cu(I) species thus
generated in situ would then react with PhINTs to form the reac-
tive Cu(III)–nitrene intermediate [9–13] constituting a Cu(I)/Cu(III)
reaction cycle and the reaction proceeds with the consecutive nit-
rene transfer from the radical-like intermediate to olefins. The ste-
rically hindered ligands in 3 and 4 with higher Cu(II)/Cu(I) redox
potentials would be expected to facilitate Cu(I)-alkene and hence
the metallo-nitrene intermediate formation more than 1 and 2
do leading to their higher catalytic ability. Similarly, the diaza-
pane-based complex 7 with a Cu(II)/Cu(I) redox potential higher
than the present complexes functions as a more efficient catalyst.
Also, the present complexes of sterically hindering ligands would
form less stable adducts with the less reactive alkenes leading to
lower reactivity.
The copper(II) complexes 1–4 were examined for their ability to
catalyze the aziridination of olefins, especially styrene, cis-cyclooc-
tene and n-hexene as model substrates using PhINTs and chlora-
mine-T trihydrate as the nitrene sources and the results are
collected in Table 5. When a 5:1 olefin:PhINTs ratio is employed
with a catalyst loading of 5 mol%, 1 and 2 give only 24–32% yield
while 3 and 4 give higher yields (85–90%) for the aziridination of
styrene the most reactive olefin. However, the reaction is slow
and is completed only after 12 h. When a 5:1 olefin:PhINTs ratio
was employed with a catalyst loading of 5 mol% for the less reac-
tive olefins like cis-cyclooctene and n-hexene, 1 and 2 fail to give
the desired aziridine but yield p-toluenesulfonamide as the major
product. On the other hand, 3 and 4 gave yield of about 30–56%
of the desired aziridine. However, the reaction is slow and is com-
pleted after 24 h only. When the aziridination of olefins by the
complexes is performed after the removal of both the coordinated
chloride ions by using AgNO3, the yield decreases further (results
not reported here).
When a 5:1 olefin:chloramine-T trihydrate ratio is employed
with a catalyst loading of 5 mol% for the reactive olefin styrene,
the desired aziridine is not formed and mainly p-toluenesulfon-
amide is obtained for 1 and 2. In fact, it has been reported already
that some side products like styrene epoxides may be formed if
chloramine-T trihydrate is used as the nitrene source. On the other
hand, 3 and 4 gave about 63–75% yield of the desired aziridine.
Also, the reaction required three days for completion. When a
5:1 olefin:chloramine-T trihydrate ratio is employed with a cata-
lyst loading of 5 mol% with the less reactive olefins like cis-cyclooc-
tene and n-hexene, 1 and 2 fail to give the desired aziridine while
low yields of aziridine (30–50%) are observed for 3 and 4. Thus,
PhINTs is a nitrene transfer reagent much better than chlora-
mine-T trihydrate for the present complexes to obtain the aziri-
dines in good yield. Though chloramine-T trihydrate is harmless
and easily available, the yields of the aziridines are less than the
side products. Also, though PhINTs is a better source for the nitrene
transfer catalysis by the present complexes, good yields are ob-
tained only for styrene.
According to the recently proposed mechanism for copper-cat-
alyzed aziridination [30], the coordination of PhINTs to copper(II)
is required for intramolecular electron transfer leading to the for-
mation of PhI and a highly reactive copper-nitrene radical-like
intermediate [(L)CuII@NTs], which is then involved in consecutive
nitrene transfer to olefin (Scheme 3). We have shown [37] that
the steric crowding around copper(II) imposed by the bulky quino-
lyl moiety and the N-Me group of certain diazapane-derived tri-
dentate 3N ligands would confer steric constraints on Cu(II)
coordination geometry and hence facilitate the quick transfer of
the @NTs from the metallo-nitrene intermediate to the olefins
leading to the observed decrease in the reaction time with an in-
Ts
N
4. Conclusions
[LCuII]
PhINTs
step1
Copper(II) complexes of four new sterically hindering linear tri-
dentate 3N ligands have been isolated. All of them have been struc-
turally characterized to possess square pyramidal coordination
geometry involving meridional coordination of the ligands. The
incorporation of sterically hindering quinolyl or 6-methylpyridyl
moieties or ꢁNMe2 donor group in the ligand fails to confer any
significant distortion on the copper(II) coordination geometries
but imposes varying tendencies on them to destabilize Cu(II) spe-
cies. Also, the complexes exhibit varying catalytic nitrene transfer
reactivity towards three olefins when PhINTs and chloramine-T
[LCuII(PhINTs)]
[LCuII(NTs)]
[LCuIII(NTs)]
step2
PhI
Scheme 3. Proposed mechanism [30] of aziridination by Cu(II) complexes.

T. Dhanalakshmi et al. / Inorganica Chimica Acta 365 (2011) 143–151
151
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We sincerely thank the Council of Scientific and Industrial Re-
search, New Delhi for a Senior Research Fellowship to T.D. We
thank Department of Science and Technology, New Delhi for the
Award of Ramanna Fellowship to M.P. and also for supporting this
research [Scheme No. SR/S1/IC-45/2003 and SR/S5/BC-05/2006].
We thank Prof. K. Natarajan, Bharathiar University, Coimbatore
for CHN analysis.
Appendix A. Supplementary material
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CCDC 680240, 680241, 680242 and 680243 contain the supple-
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