New copper(II) complexes as efficient catalysts for olefin aziridination: The effect of ligand steric hindrance on reactivity

Article abstract of DOI:10.1002/ejic.200600490

Three new copper(II) complexes derived from the tridentate N₃ ligands 4-methyl-1-(pyrid-2-ylmethyl)-1,4-diazacycloheptane (L1), 1-(quinol-2-ylmethyl)-1,4-diazacycloheptane (L2), and 4-methyl-1-(quinol-2- ylmethyl)-1,4-diazacycloheptane (L3) have been prepared and examined as copper catalysts for olefin aziridination. In the X-ray crystal structures of the complexes [Cu(L2)Cl₂] (2) and [Cu(L3)Cl₂] (3) copper(II) adopts a trigonal-bipyramidal distorted square-based pyramidal geometry (TBDSBP) as seen from the values of the trigonality index τ (0.08 for 2 and 0.48 for 3). The enhanced trigonal distortion in 3 is due to the presence of an N-Me group, the lone pair orbital of which is not oriented exactly along the d _{x2_y2} orbital of copper(II). While [Cu(L1)(H₂O)](ClO ₄)₂ (1) assumes a tetragonal geometry in solution, complexes 2 and 3 adopt a distorted tetragonal geometry, as revealed by UV/Vis and EPR spectral studies. The complexes undergo quasi-reversible Cu ^II/Cu^I redox behavior in methanol solution. The ability of the complexes to mediate nitrene transfer from PhINTs to olefins to form N-tosylaziridines has been studied. The complexes are found to be efficient catalysts (in 5 mol-% amounts) for the aziridination of the reactive olefin styrene, with yields varying from 80 to 90% (with respect to PhINTs). They exhibit significant catalytic nitrene transfer reactivity (yields of 30 to 60%) also towards the less reactive olefins cyclohexene and cyclooctene. A remarkable observation is the significantly accelerated rate of aziridination by 3, which is ascribed to the steric crowding around copper(II) imposed by the bulky quinolyl and N-Me groups of the tridentate ligand L3. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejic.200600490

FULL PAPER
DOI: 10.1002/ejic.200600490
New Copper(II) Complexes as Efficient Catalysts for Olefin Aziridination: The
Effect of Ligand Steric Hindrance on Reactivity
Thirumanasekaran Dhanalakshmi,[a] Eringathodi Suresh,[b] Helen Stoeckli-Evans,[c] and
Mallayan Palaniandavar*[a]
Keywords: Aziridination / Copper / N ligands / Tridentate ligands / Steric hindrance
Three new copper(II) complexes derived from the tridentate
N3 ligands 4-methyl-1-(pyrid-2-ylmethyl)-1,4-diazacyclohep-
tane (L1), 1-(quinol-2-ylmethyl)-1,4-diazacycloheptane (L2),
and 4-methyl-1-(quinol-2-ylmethyl)-1,4-diazacycloheptane
(L3) have been prepared and examined as copper catalysts
for olefin aziridination. In the X-ray crystal structures of the
complexes [Cu(L2)Cl2] (2) and [Cu(L3)Cl2] (3) copper(II)
adopts a trigonal-bipyramidal distorted square-based pyram-
idal geometry (TBDSBP) as seen from the values of the trigo-
nality index τ (0.08 for 2 and 0.48 for 3). The enhanced trigo-
nal distortion in 3 is due to the presence of an N-Me group,
the lone pair orbital of which is not oriented exactly along
the dx2–y2 orbital of copper(II). While [Cu(L1)(H2O)](ClO4)2 (1)
assumes a tetragonal geometry in solution, complexes 2 and
3 adopt a distorted tetragonal geometry, as revealed by UV/
Vis and EPR spectral studies. The complexes undergo quasi-
reversible CuII/CuI redox behavior in methanol solution. The
ability of the complexes to mediate nitrene transfer from
PhINTs to olefins to form N-tosylaziridines has been studied.
The complexes are found to be efficient catalysts (in 5 mol-%
amounts) for the aziridination of the reactive olefin sty-
rene, with yields varying from 80 to 90% (with respect to
PhINTs). They exhibit significant catalytic nitrene transfer re-
activity (yields of 30 to 60%) also towards the less reactive
olefins cyclohexene and cyclooctene. A remarkable observa-
tion is the significantly accelerated rate of aziridination by 3,
which is ascribed to the steric crowding around copper(II)
imposed by the bulky quinolyl and N-Me groups of the tri-
dentate ligand L3.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
catalyzed olefin aziridination reactions, the most widely em-
ployed reaction (Scheme 1) has been nitrene transfer from
(tosylimino)phenyliodinane (PhINTs).[7–9]
Introduction
Similar to epoxides, aziridine rings enjoy special atten-
tion in organic synthesis, particularly as intermediates[1,2] in
the synthesis of amines and amino alcohols. Furthermore,
naturally occurring azinomycin and mitomycin, which con-
tain aziridines in their core, exhibit cytotoxic properties.[3]
The development of new catalysts for the preparation of
aziridines has therefore been an active area of investigation.
Of the two different catalytic processes that lead to the for-
mation of aziridines,[4–6] that involving addition of an “NR”
nitrene unit to a C=C double bond (alkene; Scheme 1) has
been more extensively studied than the other, which in-
volves the addition of a “CR” carbene unit to a C=N (im-
ine) bond. This reaction is often catalyzed by transition
metals through coordination of the nitrene[4] or by Lewis
acids through substrate activation.[5] In transition-metal-
Scheme 1.
Many copper-based catalysts have been reported for ni-
trene-transfer reactions from PhINTs to alkenes, although
they have rarely been structurally characterized.[4,10–14]
Halfen et al. have systematically investigated[10] the struc-
tural and electronic factors that regulate the catalytic ac-
tivity of copper complexes in such nitrene-transfer reac-
tions. They used the complex [Cu(iPr3TACN)(O2CCF3)2] to
effect the near-quantitative aziridination of styrene deriva-
tives. They also reported aziridination of olefins catalyzed
by a series of square-based copper(II) complexes in which
tetradentate pyridyl-appended diazacycloalkane ligands are
meridionally coordinated.[13] In these complexes the axial
coordination site(s) are either vacant or occupied by a read-
ily displaced solvent or counterion, which facilitates the azi-
ridination with the nitrene source PhINTs. The reactivity
of the copper(II) complexes is significantly enhanced for
aziridination of styrene when the ligand denticity is lowered
from tetradentate to tridentate.[15] Very recently Vadernikov
[a] School of Chemistry, Bharathidasan University,
Tiruchirappallli-620024, India
E-mail: palani51@sify.com; palanim51@yahoo.com
[b] Analytical Science Discipline, Central Salt and Marine Chemi-
cals Research Institute,
Bhavnagar 364 002, India
[c] Department of Chemistry, University of Neuchatel,
Neuchatel, Switzerland
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T. Dhanalakshmi, E. Suresh, H. Stoeckli-Evans, M. Palaniandavar
FULL PAPER
et al. have reported that coordinative unsaturation at cop- 2:1 electrolyte (190 Ω–1 cm2 –1) while 2 and 3 behave as 1:1
per is responsible for the very high catalytic activity of cer- electrolytes (2, 98 and 3, 86 Ω–1 cm2 –1) in methanol solu-
tain four-coordinate copper(II) chloride complexes of pyri- tion, corresponding to dissociation of the axial chloride to
dine-based ligands.[16]
give [Cu(L)Cl]+ ions. The longer equatorial chloride can
We decided to prepare copper(II) complexes of tridentate dissociate in solution in the presence of the substrates and
N3 ligands and study them systematically to identify the facilitate aziridination reaction.
key ligand structural and electronic properties for optimum
copper catalyst performance. As the highly oxidized copper-
nitrene species are suggested[10] to be stabilized by electron-
rich and sterically hindered ligands, sterically hindering N–
Me and quinolyl moieties were incorporated into the triden-
tate N3 ligands (Scheme 2) chosen for the present study. The
ring system of the ligand would provide resistance[13]
towards oxidative degradation of the catalyst by the
strongly oxidizing nitrene transfer agent PhINTs. The coor-
dinative unsaturation as well as steric crowding around cop-
per(II) in the present complexes is expected to lead to im-
proved selectivity, reaction rates, and broad substrate toler-
ance. The X-ray crystal structures of two of the complexes
were determined to establish the degree of coordinative un-
saturation and distortion in the complexes. The solution
structures of the complexes have been probed by employing
spectroscopic and electrochemical methods to understand
the effect of the ligand architecture on the copper(II) coor-
dination geometry. All the complexes examined were found
to be fast and efficient catalysts for the aziridination of sty-
rene, cyclohexene, and cyclooctene when a high amount (5–
10 equiv.) of olefin was treated with PhINTs (1 equiv.).
Description of the Crystal Structures
An ORTEP view of the structures of complexes
2·CH3CN and 3, along with the atom numbering schemes,
are shown in Figures 1 and 2 respectively. Selected bond
lengths and bond angles relevant to the copper coordina-
tion spheres are given in Table 1. In the complex 2·CH3CN
copper(II) is bound to two chloride ions and two nitrogen
atoms (N2 and N3) of the homopiperazine moiety and the
nitrogen atom (N1) of the quinoline moiety of the triden-
tate ligand L2. The value of the structural index[17] τ (0.08)
suggests that the pentacoordinate complex is very slightly
distorted square pyramidal and the coordination geometry
around copper(II) may be best described as trigonal-bipy-
ramidal distorted square-based pyramidal (TBDSBP)[18] in
which the square plane is constituted by the three nitrogen
atoms (N1, N2, N3) of L2 and one chloride ion (Cl1) and
the apical position occupied by the other chloride ion (Cl2).
Two crystallographically independent complex molecules
with the same chemical formula are present in the asymmet-
ric unit cell of complex 3. In both these molecules cop-
per(II) is coordinated by three nitrogen atoms, two (N2 and
N3) from the homopiperazine moiety and one (N1) from
the quinoline moiety of ligand L3, and two chloride ions
(Cl1 and Cl2), as in 2·CH3CN. The value of the structural
index[17] τ (0.48) reveals that the coordination geometry
around copper(II) in 3 is best described as TBDSBP in
which the corners of the square plane are occupied by the
three nitrogen atoms of L3 and one chloride ion (Cl1), and
the apical position by the other chloride ion (Cl2). In fact,
the coordination geometry lies exactly midway between tri-
gonal-bipyramidal and square-pyramidal geometries. The
value of the trigonality index τ for 3 is much higher than
Scheme 2. Ligands employed for the present study.
Results and Discussion
Synthesis of Ligands and Complexes
The tridentate ligands L1–L3 were prepared as oils in
good yields (50–70%) by stirring the appropriate diazacy-
cloheptane (homopiperazine or N-methylhomopiperazine)
with one equivalent of 2-picolyl chloride or 2-quinolyl chlo-
ride and two equivalents of triethylamine in ethanol at
room temperature for three days. The copper(II) complexes
of the ligands were prepared by the addition of either
CuCl2·H2O or Cu(ClO4)2·6H2O in methanol to a meth-
anolic solution of the ligands; they were characterized by
analytical and spectroscopic methods and studied as cata-
lysts for aziridination reactions. The ligands occupy three
coordination positions in the X-ray crystal structures of
both 2 and 3, with a chloride ion in the equatorial position.
Figure 1. ORTEP drawing of [Cu(L2)Cl2] (2) showing the atom
numbering scheme and the thermal motion ellipsoids (50% prob-
Conductivity studies revealed that complex 1 behaves as a ability level).
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New Copper(II) Complexes as Efficient Catalysts for Olefin Aziridination
FULL PAPER
that for 2·CH3CN, thus revealing that incorporation of the than the equatorial chloride ions (2.272 Å in 2 and 2.304 Å
sterically hindering N–Me group in 3 leads to a greater geo- in 3). This is obviously because of the presence of two elec-
metrical constraint at copper(II). The Cu–Namine distances trons in the dz2 orbital of copper(II) in the square-based
in both 2·CH3CN and 3 are in the range expected[15] and, environments of both complexes. It is interesting to note
interestingly, they are longer in 3 (2.052, 2.068 Å) than that both 2·CH3CN and 3 are monomeric, which is in con-
those (2.036, 2.0392 Å) in 2·CH3CN, thus revealing that the trast to the dimeric structure of the complex [(L4)Cu(µ-Cl)2-
lone pair orbitals of the N-Me group in 3 are not oriented Cu(L4)], where L4 is 5-methyl-1-(2-pyridylmethyl)-1,5-di-
exactly along the dx2–y2 orbital of copper(II).[19] Further, the azacyclooctane, the pyridine analogue of ligands L2 and
axial chloride ions in both 2·CH3CN and 3 are located fur- L3. The incorporation of the sterically hindering quinolyl
ther away from the metal (2.456 Å in 2 and 2.462 Å in 3) moiety in place of the pyridyl moiety in L4 and that of the
ethylene linker instead of propylene one in the homopipera-
zine backbone of L4 prevent the dimerization of both 2
and 3. We have previously shown[20] that incorporation of
a sterically hindering NMe2 group in the place of a pyridine
donor enhances the Fe–O–C bond angle in iron(III) com-
plexes of certain tetradentate monophenolate ligands from
128.5° to 136.1°. The equatorial chloride ion in 2·CH3CN
and 3 can be replaced by PhINTs, which leads to catalysis
of aziridination reactions, as observed for copper(II) com-
plexes of didentate ligands.[16]
Spectral Properties
The solid-state reflectance spectra of all the complexes
show a broad ligand-field feature (540–850 nm) in the vis-
ible region, which appears to contain more than one band;
this is typical of CuII located in a square-based environ-
ment, as evidenced from the X-ray crystal structures of
complexes 2 and 3 (Table 2). On dissolution in methanol,
only one ligand-field feature is observed (640–760 nm, Fig-
ure 3) for all the complexes, which suggests changes in their
coordination geometries such as dissociation of the axial
chloride ion in solution. The ligand-field energies of com-
plexes 2 and 3 are much lower, with higher molar absorp-
tivities, than those of 1, thereby indicating that the replace-
ment of the pyridyl moiety in 1 by the bulky quinolyl moi-
ety in 2 and 3 leads to an enhancement of the distortion in
the square-pyramidal geometry towards trigonal bipyrami-
dal. Further, the ligand-field energy of 3 is lower, and the
molar absorptivity higher, than those of 2, as expected from
the weaker σ-coordination of N(3), as explained above, and
due to the steric hindrance imposed by the N-Me group
leading to a more distorted coordination geometry, as evi-
dent from the X-ray structures of the complexes. A similar
decrease in ligand-field energy has been observed upon re-
placing an NHMe group by an NMe2 group in the coordi-
nation sphere of copper(II) complexes of tridentate pyri-
dine-based ligands.[19]
Figure 2. ORTEP drawing of [Cu(L3)Cl2] (3) showing the atom
numbering scheme and the thermal motion ellipsoids (50% prob-
ability level).
Table 1. Selected bond lengths [Å] and angles [°] for 2 and 3.
2
3
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–N(3)
Cu(1)–Cl(1)
Cu(1)–Cl(2)
2.135(2)
2.0392(17) Cu(1)–N(3)
2.036(2) Cu(1)–N(2)
2.2722(9) Cu(1)–Cl(1)
2.4556(9) Cu(1)–Cl(2)
Cu(2)–N(21)
Cu(1)–N(1)
2.039(2)
2.052(2)
2.068(2)
2.3043(8)
2.4620(8)
2.045(2)
2.056(2)
2.068(2)
2.3183(8)
2.4299(8)
161.28(10)
83.05(9)
78.23(10)
100.72(7)
92.69(7)
133.78(8)
91.92(7)
94.24(7)
111.03(8)
114.82(3)
83.36(9)
Cu(2)–N(22)
Cu(2)–N(23)
Cu(2)–Cl(21)
Cu(2)–Cl(22)
N(1)–Cu(1)–N(2)
N(1)–Cu(1)–N(3) 154.90(7)
N(2)–Cu(1)–N(3) 76.59(7)
Cl(1)–Cu(1)–N(1) 103.67(5)
Cl(1)–Cu(1)–N(2) 149.96(5)
Cl(1)–Cu(1)–N(3)
Cl(2)–Cu(1)–N(1)
Cl(2)–Cu(1)–N(2) 101.96(5)
Cl(2)–Cu(1)–N(3) 102.04(5)
Cl(1)–Cu(1)–Cl(2) 107.19(2)
81.19(6)
N(1)–Cu(1)–N(3)
N(1)–Cu(1)–N(2)
N(3)–Cu(1)–N(2)
N(1)–Cu(1)–Cl(1)
N(3)–Cu(1)–Cl(1)
N(2)–Cu(1)–Cl(1)
N(1)–Cu(1)–Cl(2)
N(3)–Cu(1)–Cl(2)
N(2)–Cu(1)–Cl(2)
Cl(1)–Cu(1)–Cl(2)
N(21)–Cu(2)–N(22)
90.23(5)
93.78(4)
The polycrystalline EPR spectra of complexes 1 and 2
are axial while that of 3 is rhombic, which is consistent with
the distorted square-based geometry found in the X-ray
crystal structure of 3. The frozen solution spectra (Figure 4)
of all the complexes are axial [gʈ Ͼ gЌ Ͼ 2.0, G = (gʈ –
2)/(gЌ – 2) = 3.0–4.3],[21] which is usual for mononuclear
tetragonal copper(II) complexes with a dx2–y2 ground
state.[22] Copper(II) complexes with a square-based CuN4
coordination sphere exhibit gʈ and Aʈ values of around
N(21)–Cu(2)–N(23) 161.08(10)
N(22)–Cu(2)–N(23) 78.23(10)
N(21)–Cu(2)–Cl(21) 102.24(7)
N(22)–Cu(2)–Cl(21) 131.95(8)
N(23)–Cu(2)–Cl(21)
N(21)–Cu(2)–Cl(22)
93.37(7)
92.38(7)
N(22)–Cu(2)–Cl(22) 117.75(7)
N(23)–Cu(2)–Cl(22) 92.33(8)
Cl(21)–Cu(2)–Cl(22) 109.72(3)
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T. Dhanalakshmi, E. Suresh, H. Stoeckli-Evans, M. Palaniandavar
Table 2. Electronic and EPR spectroscopic data for the copper(II) complexes.
FULL PAPER
Complexes
Electronic spectra[a]
EPR spectra[b]
λmax [nm] (εmax [–1 cm–1])
Solid
Solution (in methanol)
Solid
Frozen[c] solution
[Cu(L1)(H2O)](ClO4)2 (1)
545–645
642 (150)
gʈ 2.223
gЌ 2.109
gʈ 2.227
Aʈ 186
268 (34310)[d]
gЌ 2.066
gʈ/Aʈ 119
G 3.43
[Cu(L2)Cl2] (2)
[Cu(L3)Cl2] (3)
650–750
700–850
730 (195)
gʈ 2.198
gЌ 2.147
gʈ 2.225
Aʈ 146
gЌ 2.052
gʈ/Aʈ 152
G 4.32
270 (33280)[d]
235 (35845)[d]
203 (34710)[d]
758 (270)
g3 2.162
g2 2.136
g1 2.078
gʈ 2.231
Aʈ 130
gЌ 2.077
gʈ/Aʈ 171
G 3.00
275 (27430)[d]
238 (36005)[d]
[a] Concentration: 2×10–3  for ligand-field and 2×10–5  for ligand-based transitions. [b] Aʈ in 10–4 cm–1. [c] Methanol/acetone (4:1,
v/v) glass at 77 K. [d] π–π* transitions within the ligand.
Figure 3. Electronic absorption spectra of complexes 1–3 in meth-
anol solution. Concentration of the complexes: 5×10–3 .
2.200 and 200×10–4 cm–1, respectively. Replacement of a
coordinated nitrogen in this chromophore by oxygen (sol-
vent methanol) is expected to enhance the gʈ value and de-
crease the Aʈ value. Thus, the gʈ (2.225–2.231) and Aʈ (180–
130×10–4 cm–1) values of 1–3 are suggestive of the presence
of a square-based [Cu(N3)Cl]+ chromophore.[19] Interest-
ingly, the Aʈ values of 2 and 3 are much lower than that of
1, which suggests that the coordination geometries in them
are strongly distorted from planarity. In fact, the values of
their gʈ/Aʈ quotient (152 cm for 2 and 171 cm for 3) is
higher than that for 1 (117 cm). This suggests a large devia-
tion from a perfectly square-planar geometry (gʈ/Aʈ = 105–
135 cm)[23] because of the incorporation of sterically de-
Figure 4. X-band EPR spectra of complexes 1–3 at 77 K in meth-
anol/acetone (4:1 v/v) glass.
Electrochemical Properties
The electrochemical data obtained for the present com-
manding quinolyl moiety in 2 and 3 in place of the pyridyl plexes in methanol solution using TBAP as supporting elec-
moiety in 1. Further, the gʈ and Aʈ values of 2 are lower and trolyte are collected in Table 3. The cyclic (CV) and
higher, respectively, than those for 3, and the gʈ/Aʈ value for differential pulse voltammograms (DPV) were recorded
2 is lower than that for 3. This reveals that the distortion using Pt as working electrode and Ag/AgCl as reference
from a square-based geometry in 3 is higher than that in 2 electrode. For all the complexes the CuII to CuI reduction
because of the sterically hindering N-Me substitution, (Epc, –0.325 to –0.475 V) is associated with a reoxidation
which is consistent with the above X-ray structures and peak (Epa, –0.244 to –0.318 V) in the reverse scan. The
electronic spectral results.
value of the limiting peak-to-peak separation (∆Ep = 158–
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New Copper(II) Complexes as Efficient Catalysts for Olefin Aziridination
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Table 3. Electrochemical data[a] for copper(II) complexes at 25 0.2 °C in methanol solution.
Complexes
Epc [V]
Epa [V]
E1/2 [V]
CV
∆Ep [mV]
ipa/ipc
D [10–6 cm2 s–1]
DPV[b]
[Cu(L1)(H2O)](ClO4)2 (1)
[Cu(L2)Cl2] (2)
[Cu(L3)Cl2] (3)
–0.476
–0.454
–0.326
–0.318
–0.244
–
–0.397
–0.349
–0.164[c]
–0.378
–0.336
–0.252
158
210
–
0.6
0.9
–
3.4
4.2
5.8
[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), ∆Ep = 88 mV; scan rate: 50 mVs–1; supporting electrolyte: tetrabutylammonium perchlorate (0.1 );
complex concentration: 1×10–3 . [b] Differential pulse voltammetry (DPV); scan rate: 1 mVs–1; pulse height: 50 mV. [c] Potential at
half-height Ep1/2
.
210 mV) is higher than that for the Fc/Fc+ couple (∆Ep = rene (within an hour) for complex 3. The presence of labile
88 mV) under identical conditions. This suggests that the ligands and the coordinative unsaturation around cop-
heterogeneous electron-transfer process in the present com- per(II) promotes faster coordination of PhINTs to cop-
plexes is far from reversible and that considerable stereo- per(II) (otherwise slow) and is responsible for the high yield
chemical reorganization of the coordination sphere occurs and reduced reaction times. In fact, on treatment of 3 with
on electron transfer. The CuII/CuI redox potentials follow excess PhINTs in acetonitrile solution, the ligand-field band
the trend 3 Ͼ 2 Ͼ 1 (Figure 5). This reveals the importance is shifted from 799 to 794 nm with enhanced absorptivity,
of the sterically hindering quinolyl moiety in destabilizing thus suggesting the coordination of PhINTs to copper(II).
the CuII form of 2, thereby facilitating the electron transfer. Further, the persistence of the blue or green color of the
Similarly, the incorporation of the sterically hindering and reaction mixtures after dissolution of PhINTs reveals that
weakly σ-bonding N-Me group in 2 to obtain 3 further de- the resting state of the catalyst is always CuII. According to
stabilizes the CuII oxidation state, which is consistent with the very recently proposed mechanism for copper-catalyzed
the X-ray structures of the complexes. Similar results have aziridination,[16] the coordination of =NTs to copper(II) is
been observed by incorporating an NMe2 group in the cop- required for intramolecular electron transfer. This leads to
per(II) complexes of certain pyridine-based linear N3 li- the formation of PhI and a highly reactive copper-nitrene
gands.[19]
radical-like intermediate [(L3)CuII=NTs], which is then in-
volved in consecutive nitrene transfer to olefin. Obviously,
the steric crowding around copper(II) imposed by the bulky
quinolyl moiety and the N-Me group (see above) of the tri-
dentate ligand would stabilize this intermediate and facili-
tate the aziridination. Also, it would prevent any side reac-
tions like recoordination of chloride. In fact, when aziridin-
ation was performed for complex 3 after the removal of two
chloride ions by treatment with AgNO3, the reaction time
decreased further, as expected, but with no change in yield.
It is interesting to note that the incorporation of a sterically
hindering NMe2 group in the place of a pyridine donor
enhances the dioxygenase activity of iron(III) complexes of
certain tetradentate monophenolate ligands.[20] No change
in yield was observed with a decrease in the olefin:PhINTs
ratio; however, the reaction becomes faster (completed
within 30 min).
Figure 5. Differential pulse voltammograms of complexes 1–3 in
methanol solution at 25 °C at a scan rate of 0.05 Vs–1. Complex
concentration: 0.001 .
Aziridination of the less reactive cyclohexene was per-
formed with 5 mol-% catalyst loading. When the ole-
Application to Catalytic Olefin Aziridination Reactions
The newly synthesized copper(II) complexes 1–3 were ex- fin:PhINTs ratio was 5:1, all the complexes showed a good
amined for their ability to catalyze the aziridination of ole- yield of more than 60%. It is remarkable that the yield of
fins, especially styrene, cyclohexene, and cyclooctene, using aziridine observed for cyclohexene is higher than those for
PhINTs as the nitrene source; the results are collected in other complexes available in the literature (except [Cu-
Table 4. When a 10:1 olefin:PhINTs ratio was employed (MeCN)4]ClO4, 77%).[4] Also, and interestingly, no allylic
with a catalyst loading of 5 mol-%, all the complexes gave sulfonamide insertion product was obtained. The com-
high yields (85–90%) for the aziridination of the most reac- plexes also catalyze the aziridination of the other less reac-
tive olefin styrene. However, when the olefin:PhINTs ratio tive olefin cis-cyclooctene, although the yield is very low
was decreased to 5:1, slightly lower yields (75–80%) were (30–45%). When the olefin:PhINTs ratio was decreased
obtained. When this ratio was decreased from 5:1 to 1:1 as from 10:1 to 5:1, the yield was also reduced. Again, com-
for 3, the yield decreased further. A remarkable observation plex 3 catalyzes the aziridination (within an hour) faster
is the significantly accelerated rate of aziridination of sty- than the other two complexes.
Eur. J. Inorg. Chem. 2006, 4687–4695
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T. Dhanalakshmi, E. Suresh, H. Stoeckli-Evans, M. Palaniandavar
FULL PAPER
Table 4. Results of catalytic aziridination studies using copper(II)
complexes 1–3.
Experimental Section
Materials and Methods: CuCl2·2H2O, cyclohexene, triethylamine,
(Merck, India), p-toluenesulfonamide, iodosobenzene diacetate
(Merck, Germany), tetra-butylammonium bromide (G. F. Smith),
N-methylhomopiperazine, homopiperazine, 2-picolylchloride hy-
drochloride, 2-quinolylchloride hydrochloride, Cu(ClO4)2·6H2O,
and cis-cyclooctene (Aldrich) were used as received. Styrene was
distilled 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 prepared by a modified literature procedure and was
stored in the dark.[7] tetra-Butylammonium perchlorate (TBAP)
was prepared by the addition of sodium perchlorate to a hot etha-
nol solution of tetra-butylammonium bromide. The product was
recrystallized from aqueous ethanol and was tested for the absence
of bromide. 1H NMR spectra were obtained using a Bruker
200 MHz spectrometer at room temperature. 1H NMR chemical
shifts are reported relative to TMS and are referenced to residual
solvent peaks. Electronic absorption spectra were acquired using a
Varian 300 spectrophotometer (200–900 nm). FTIR spectra (4000–
400 cm–1 range, KBr pellets) were recorded with a Jasco FT-IR
spectrometer. EPR spectra were recorded with a JEOL JES-TE 100
X-band spectrometer, the field being calibrated with diphenylpic-
rylhydrazyl (dpph). The g0 and A0 values were estimated at ambient
temperature and gʈ and Aʈ at 77 K. The values of gЌ and AЌ were
computed as 1/2(3g0 – gʈ) and 1/2(3A0 – Aʈ) respectively. Electro-
chemical 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 methanol using 0.1  TBAP as supporting
electrolyte. Elemental analyses were performed at Bharathiar Uni-
versity, Coimbatore. Conductivity measurements on methanolic
solution of the complexes were made using an Elico conductivity
bridge.
[a] 5 mol-% vs. PhINTs. [b] Isolated yields of N-tosylaziridine after
purification with respect to 0.3 m of PhI=NTs used.
Caution! Perchlorate salts of transition metal complexes containing
organic ligands are potentially explosive and should be prepared in
small quantities and handled with appropriate precautions. While
no difficulties were encountered with the complexes reported
herein, due caution should be exercised.
Conclusions
Copper(II) complexes of three new tridentate N3 ligands
have been isolated. Two of them have been structurally
characterized to contain the meridionally coordinated li-
gand, and the incorporation of a sterically hindering quino-
lyl moiety and N-Me donor group in the ligand leads to
a strong trigonal distortion of the copper(II) coordination
geometries. All the complexes exhibit significant catalytic
nitrene transfer reactivity with PhINTs towards three ole-
fins. While good yields (above 80%) were obtained for the
reactive olefin styrene, yields exceeding 60% were obtained
for less reactive cyclohexene; however, lower yields (30–
45%) were obtained for cyclooctene. The present findings
confirm that both the degree of coordinative unsaturation
and the steric congestion in copper-based systems are im-
portant factors that determine their catalytic activity. It is
fascinating to note that the use of low-coordinate copper(II)
species with distorted geometries as catalysts dramatically
accelerates the aziridination (reaction times 0.5–1 h) for re-
active as well as less reactive olefinic substrates. This finding
could be expected to pave the way for the rational design of
new and more active catalysts for aziridination and related
group-transfer reactions.
4-Methyl-1-(pyrid-2-ylmethyl)-1,4-diazacycloheptane
(L1):
An
aqueous solution (5 mL) of NaOH (0.40 g, 10 mmol) was added
dropwise to an aqueous solution (10 mL) of 2-picolyl chloride hy-
drochloride (0.82 g, 5 mmol) at 0 °C. An aqueous solution (10 mL)
of N-methylhomopiperazine (0.57 g, 5 mmol) was then added to
this mixture over 15 min. The mixture was stirred in a loosely se-
aled flask at room temperature for three days. The reaction mixture
was then extracted with CHCl3 (3×50 mL). The organic layer was
washed with saturated sodium hydrogen carbonate solution, evapo-
rated, and then dried (Na2SO4). The organic solvent was removed
on a rotary evaporator to yield the crude product as a pale-yellow
oil. The pure product was obtained by extracting the oil once again
with ethyl acetate. The product obtained was used as such for the
complex preparation. Yield: 0.58 g (56%). C12H19N3 (205.16):
calcd. C 70.20, H 9.33, N 20.47; found C 70.15, H 9.48, N 20.37.
1H NMR (200 MHz, CDCl3): δ = 8.64–7.29 (m, 4 H), 3.95 (s, 2
H), 2.48 (m, 4 H), 2.36 (m, 4 H), 1.49 (quint, 2 H), 2.27 (br. s, 3
H) ppm. IR: ν = 3023 (m), 2983 (s), 2888 (w), 1467 (m), 1407 (s),
˜
1240 (m), 941 (s), 779 (s) cm–1.
1-(Quinol-2-ylmethyl)-1,4-diazacycloheptane (L2): The ligand L2
was synthesized by a slightly modified procedure that was reported
for the preparation of the monosubstituted diazocyclooctane li-
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New Copper(II) Complexes as Efficient Catalysts for Olefin Aziridination
FULL PAPER
gands.[24] Triethylamine (1.01 g, 10 mmol) was added to 2-quinolyl
chloride hydrochloride (1.07 g, 5 mmol) in ethanol (25 mL) at 0 °C,
and a solution of homopiperazine (2.00 g, 20 mmol) in ethanol
(10 mL) was added slowly to this mixture with stirring. The whole
mixture was refluxed for 2 h. The solution was stirred for 2 d at
room temperature and the ethanolic solution was removed on a
rotary evaporator to dryness. Water was added to the residue and
the pH of the solution was adjusted to 10 with sodium carbonate.
The solution was extracted with chloroform and the extracts were
dried with sodium sulfate. The ligand was isolated as a yellow oil
after rotary evaporation of the chloroform extracts. Yield: 0.75 g
Catalytic Aziridinations. Styrene: Aziridinations were performed by
stirring mixtures of PhINTs (0.3–0.4 mmol), styrene (10 to 1 equiv.
vs. PhINTs), and the copper catalyst (5 mol% vs. PhINTs) in 2 mL
of anhydrous CH3CN under a dry N2 atmosphere as reported pre-
viously.[10] At the completion of the reactions, the clear, pale-green
solutions were passed through a short column of neutral alumina
to remove copper species, eluted with ethyl acetate, and the eluates
were evaporated to yield oily residues. These crude product mix-
tures (of iodobenzene, 2-phenyl-1-tosylaziridine, and p-toluenesul-
fonamide) were treated with hexane to produce the crude aziridine
products, which were recrystallized from hexane/diethyl ether mix-
(61%). C15H19N3 (241.16): calcd. C 74.65, H 7.94, N 17.41; found tures at –20 °C. The 2-phenyl-1-tosylaziridine obtained was charac-
1
1
C 74.55, H 8.13, N, 17.32. H NMR (200 MHz, CDCl3): δ = 8.02–
terized by H NMR spectroscopy and the data were identical with
7.36 (br. m, 6 H), 3.93 (s, 2 H), 2.65 (m, 2 H), 2.55 (m, 2 H), 2.45 those reported in the literature.[13]
(m, 2 H), 2.33 (m, 2 H), 1.51 (quint, 2 H), 2.91 (br. s, NH) ppm.
IR: ν = 3432 (br), 3010 (s), 2884 (s), 1432 (s), 1230 (w), 760 (s),
Cyclooctene: Aziridinations were performed as described above by
˜
stirring mixtures of PhINTs (0.3–0.4 mmol), the olefin (10 to
5 equiv. vs. PhINTs), and the copper catalyst (5 mol-% vs. PhINTs)
in 2 mL of anhydrous CH3CN. At the completion of the reactions,
the clear, pale-green solutions were passed through a short column
of neutral alumina to remove copper species, eluted with ethyl ace-
tate, and the eluates were evaporated to yield semi-solid residues.
These crude product mixtures were treated with hexane, and the
resultant mixtures were filtered to remove p-toluenesulfonamide.
The filtrates were evaporated and dried overnight under vacuum
to yield the pure aziridine products. 1H NMR spectroscopic data
confirmed the formation of the aziridine.
629 (s) cm–1.
4-Methyl-1-(quinol-2-ylmethyl)-1,4-diazacycloheptane
(L3):
A
slightly different procedure was followed for the synthesis of L3.
Triethylamine (1.01 g, 10 mmol) was added to 2-quinolyl chloride
hydrochloride (1.07 g, 5 mmol) in ethanol (25 mL) at 0 °C. A solu-
tion of N-methylhomopiperazine (0.57 g, 5 mmol) in ethanol
(10 mL) was added to this mixture slowly with stirring. The whole
mixture was refluxed for 2 h and then stirred for 2 d at room tem-
perature. The ethanolic solution was removed on a rotary evapora-
tor to dryness, water was added to the residue, and the pH of the
solution was adjusted to 10 with sodium carbonate. The solution
was extracted with chloroform and the extracts were dried with
sodium sulfate. The ligand was isolated as dark yellow oil after
rotary evaporation of the chloroform extracts. Yield: 0.98 g (77%).
C16H21N3 (255.17): calcd. C 75.26, H 8.29, N 16.46; found C 75.33,
Cyclohexene: Aziridinations were performed as described above by
stirring mixtures of PhINTs (0.3–0.4 mmol), the olefin (5 equiv. vs.
PhINTs), and the copper catalyst (5 mol-% vs. PhINTs) in 2 mL
of anhydrous CH3CN. At the end of the reaction, flash column
chromatography (3×18 cm silica, 4:1 hexane/ethyl acetate) yielded
the aziridine as a white crystalline solid. The N-(p-tolylsulfonyl)-7-
1
H 8.15, N 16. 52. H NMR (200 MHz, CDCl3): δ = 8.27–7.32 (br.
m, 6 H), 3.98 (s, 2 H), 2.46 (m, 4 H), 2.36 (m, 4 H), 1.49 (quint, 2
1
azabicyclo-[4.1.0]heptane obtained was characterized by H NMR
H), 2.27 (br. s, 3 H) ppm. IR: ν = 3054 (s), 2981 (s), 2865 (s), 1459
˜
(s), 1215 (w), 766 (s), 628 (s) cm–1.
spectroscopy and the data were identical with those reported in the
literature.[4e]
[Cu(L1)(H2O)](ClO4)2 (1): A solution of L1 (0.21 g, 1 mmol) in
methanol (15 mL) was treated with Cu(ClO4)2·6H2O (0.37 g,
1 mmol) to obtain a deep-blue color. After 15 min of stirring, di-
ethyl ether was allowed to diffuse into the solution. After a period
of several days blue crystals of the product were deposited. Yield:
0.32 g (56%). C12H21Cl2CuN3O9 (484.01): calcd. C 28.07, H 3.85,
X-ray Crystallography: The single-crystal diffraction experiments
for 2·CH3CN were carried out on a Bruker SMART APEX CCD
diffractometer at room temperature. The SMART[25] program was
used for collecting frames of data, indexing reflections, and de-
termining the lattice parameters; SAINT[25] was used for integra-
tion of the intensity of reflections and scaling; SADABS[25] was
used for absorption correction, and the SHELXTL[25] program for
space group and structure determination and least-squares refine-
ments on F2. The structure was solved by the heavy-atom method.
Other non-hydrogen atoms were located in successive difference
Fourier syntheses. The final refinement was performed by full-ma-
trix least-squares. Most of the hydrogen atoms were located from
the difference Fourier map and refined isotropically, except for the
hydrogen atoms of the lattice acetonitrile molecule, which was
placed at a geometrical position. For 3, the intensity data were
collected at 173 K on a Stoe Image Plate Diffraction System.[26]
Image plate distance: 70 mm; φ oscillation scans: 0–200°; step: ∆φ
N 8.93; found C 28.11, H 3.78, N 8.98. IR: ν = 3451 (br), 2923 (w),
˜
–
–
2869 (w), 1610 (s), 1089 (br, ClO4 ), 890 (s), 626 (s, ClO4 ) cm–1.
[Cu(L2)Cl2] (2): A procedure analogous to that used to prepare 1
was followed, using CuCl2·2H2O (0.17 g, 1 mmol) and L2 (0.24 g,
1 mmol) instead of L1. The pure product was isolated as blue crys-
tals. Yield: 0.19 g (47%). C15H19Cl2CuN3 (374.04): calcd. C 47.94,
H 5.10, N 11.18; found C 47.78, H 5.23, N 11.21. IR: ν = 3442
˜
(br), 3235 (s), 2927 (s), 2884 (s), 1481 (w), 786 (s), 634 (s) cm–1. X-
ray diffraction quality crystals of 2·CH3CN were obtained by the
vapor diffusion of diethyl ether into a solution of the complex in
acetonitrile.
[Cu(L3)Cl2] (3): This complex was prepared by the addition of L3
(0.26 g, 1 mmol) in methanol (15 mL) to a methanolic solution of
CuCl2·2H2O (0.17 g, 1 mmol) with stirring. After 15 min of stir-
ring, the solution was layered with diethyl ether. Green crystals
were deposited after two days. Yield: 0.26 g (65%). C16H21Cl2CuN3
(388.05): calcd. C 49.30, H 5.43, N 10.78; found C 49.42, H 5.38,
= 1.0°; exposure time: 3 min; 2θ range: 3.27–52.1°; dmin.–dmax. =
12.45–0.81 Å. The structure was solved by direct methods using the
programme SHELXS-97.[27] The refinement and all further calcula-
tions were carried out using SHELXL-97.[28] The H atoms were
included in calculated positions and treated as riding atoms using
the SHELXL default parameters. The non-H atoms were refined
N 10.71. IR: ν = 3442 (br), 3079 (w), 2981 (m), 2869 (m), 1610 (s), anisotropically, using weighted full-matrix least-squares on F2.
˜
782 (s), 634 (s) cm–1. Suitable single crystals of 3 for X-ray diffrac-
tion were obtained by slow evaporation of the methanolic solution
of the complex.
There are two independent molecules per asymmetric unit (Z = 8,
ZЈ = 2). A multi-scan absorption correction was applied using the
MULscanABS routine in PLATON;[29] transmission factors:
Eur. J. Inorg. Chem. 2006, 4687–4695
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4693

T. Dhanalakshmi, E. Suresh, H. Stoeckli-Evans, M. Palaniandavar
FULL PAPER
Table 5. Crystal data and structure refinement details for [Cu(L2)Cl2] (2) and [Cu(L3)Cl2] (3).
2·CH3CN
3
Empirical formula
Formula weight
C15H19Cl2CuN3·C2H3N
416.84
C16H21Cl2CuN3
389.80
Crystal system
Crystal size [mm]
Space group
a [Å]
b [Å]
c [Å]
α [°]
monoclinic
0.10×0.16×0.24
P21/C (no. 14)
13.641(4)
11.138(3)
12.613(3)
90.000
monoclinic
0.27×0.23×0.23
P21/n
16.6495(11)
10.7723(6)
18.9392(14)
90.000
β [°]
107.966(4)
90
1822.9(8)
4
105.767(8)
90
3269.0(4)
8
γ [°]
V [Å]3
Z
λ [Å] (Mo-Kα)
0.71073
1.519
1.032
10552
4231
0
294
0.71073
1.584
1.040
25289
6382
0
399
Dcalc [gcm–3]
Goodness-of-fit on F2
Number of reflections measured
Number of reflections used
Number of L.S. restraints
Number of refined parameters
Final R indices [IϾ2σ(I)]
[a]
R1
0.0334
0.0931
0.0332
0.0837
[b]
wR2
2
2 2
[a] R1 = [Σ(||Fo| – |Fc||)/Σ|Fo|]. [b] wR2 = {[Σ[w(Fo – Fc ) ]/Σ(wFo4)]1/2}.
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[6]
Acknowledgments
We sincerely thank the Council of Scientific and Industrial Re-
search, New Delhi, for a Junior Research Fellowship to T. D. We
also thank the Department of Science and Technology, New Delhi,
for supporting this research (scheme no. SR/S1/IC-45/2003). We
thank Dr. P. Sambasiva Rao, Pondicherry University, Pondicherry,
for providing the EPR facilities and Prof. K. Natarajan, Bharathiar
University, Coimbatore, for CHN analysis.
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New Copper(II) Complexes as Efficient Catalysts for Olefin Aziridination
FULL PAPER
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Received: May 26, 2006
Published Online: September 22, 2006
Eur. J. Inorg. Chem. 2006, 4687–4695
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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