Chiral copper-bipyridine complexes: Synthesis, characterization and mechanistic studies on asymmetric cyclopropanation

Article abstract of DOI:10.1016/j.poly.2010.10.021

Chiral bipyridine ligands of different steric properties when reacted with CuCl₂ formed orange, yellow or green solids of new copper(II) complexes, [Cu(L)Cl₂] (L = L2-6), in good yield. Together with [Cu(L1)Cl₂], these complexes were characterized in solution by UV-Vis spectroscopy and cyclic voltammetry. The complexes give d-d transitions between 860 and 970 nm, and exhibit one quasi-reversible Cu(II)/Cu(I) couple between +0.405 V and +0.516 V versus NHE. Two of the copper(II) complexes, [Cu(L5)Cl₂] and [Cu(L6)Cl₂], and a copper(I) complex of L1, [Cu(L1)Cl], were characterized by X-ray crystallography. The triflate derivatives of both the Cu(I) and Cu(II) complexes are active catalysts towards the cyclopropanation of ethyl diazoacetate with styrene. The asymmetric induction suffers when the size difference between the alkyl and alkoxyl groups was minimized. The mechanism of the cyclopropanation was studied with kinetic and competition experiments. The rate is first order in catalyst and ethyl diazoacetate, inverse order with styrene and is strongly affected by the counterion.

Full text of DOI:10.1016/j.poly.2010.10.021

Polyhedron 30 (2011) 178–186
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Chiral copper–bipyridine complexes: Synthesis, characterization
and mechanistic studies on asymmetric cyclopropanation
Wing-Sze Lee a, Chi-Tung Yeung a, Kiu-Chor Sham a, Wing-Tak Wong b, Hoi-Lun Kwong a,
⇑
a Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong SAR, China
b Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Chiral bipyridine ligands of different steric properties when reacted with CuCl2 formed orange, yellow or
green solids of new copper(II) complexes, [Cu(L)Cl2] (L = L2–6), in good yield. Together with [Cu(L1)Cl2],
these complexes were characterized in solution by UV–Vis spectroscopy and cyclic voltammetry. The
complexes give d–d transitions between 860 and 970 nm, and exhibit one quasi-reversible Cu(II)/Cu(I)
couple between +0.405 V and +0.516 V versus NHE. Two of the copper(II) complexes, [Cu(L5)Cl2] and
[Cu(L6)Cl2], and a copper(I) complex of L1, [Cu(L1)Cl], were characterized by X-ray crystallography.
The triflate derivatives of both the Cu(I) and Cu(II) complexes are active catalysts towards the cycloprop-
anation of ethyl diazoacetate with styrene. The asymmetric induction suffers when the size difference
between the alkyl and alkoxyl groups was minimized. The mechanism of the cyclopropanation was stud-
ied with kinetic and competition experiments. The rate is first order in catalyst and ethyl diazoacetate,
inverse order with styrene and is strongly affected by the counterion.
Received 21 July 2010
Accepted 12 October 2010
Available online 5 November 2010
Keywords:
Chiral bipyridines
Copper–bipyridine complexes
Asymmetric catalysis
Cyclopropanation
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
plexes with ligands having different steric properties. Herein, we
report the synthesis, spectroscopic characterisation and redox
Copper–diimine complexes have been of great interest in a
number of research areas, such as photochemistry [1,2], supramo-
lecular chemistry [3,4] electrochemistry [5,6] and catalysis [7–9].
In enantioselective reactions, chiral versions of the complexes have
also been used successfully in catalysis that involved cyclopropa-
nation [10–13] and allylic oxidation [14–16]. The results led us
to systematically investigate a family of chiral copper(II) com-
behaviours of a series of copper(II)–bipyridine complexes, as well
as their use in the asymmetric cyclopropanation of styrene with
ethyl diazoacetate (EDA). X-ray crystal structures for [Cu(L5)Cl2]
and [Cu(L6)Cl2] and a copper(I) complex, [Cu(L1)Cl], are described.
The results of mechanistic studies and kinetic studies under
pseudo-first order conditions on the asymmetric cyclopropanation
catalysts are also presented.
N
N
N
N
N
N
N
N
OMe
MeO
O
O
OH
OH
HO
PhCH2
CH2Ph
CH2Ph
L1
L4
L2
L5
L3
L6
N
N
N
N
HO
O
PhCH2
O
OMe
MeO
⇑
Corresponding author. Tel.: +852 27887304; fax: +852 27887406.
E-mail address: bhhoik@cityu.edu.hk (H.-L. Kwong).
0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.10.021

W.-S. Lee et al. / Polyhedron 30 (2011) 178–186
179
13C NMR (CDCl3): d 22.7, 70.9, 78.8, 78.9, 119.7, 127.5, 127.6,
128.3, 137.5, 138.3, 155.3, 162.6. Anal. Calc. for C28H28O2N2: C,
79.22; H, 6.65; N, 6.60. Found: C, 79.29; H, 6.54; N, 6.34%.
2. Experimental
2.1. General methods
2.3. Procedure for the preparation of [Cu(L)Cl2]
Toluene was distilled under N2 over sodium. Dichloromethane
and acetonitrile were distilled over calcium hydride. Diethyl ether
and THF were distilled under N2 over sodium/benzophenone.
Chemicals were of reagent-grade quality and were obtained com-
mercially. Infrared spectra in the range 500–4000 cmꢀ1 using a
Nujol matrix or KBr plates were recorded on a Perkin–Elmer Model
FTIR-1600 spectrometer. The electronic absorption spectra were
measured on a Perkin–Elmer Lambda 19 double-beam UV–Vis-
NIR spectrophotometer. 1H and 13C NMR spectra were recorded
on a Varian 300 MHz Mercury instrument. Positive ion FAB mass
spectra as a 3-nitrobenzylalcohol matrix were recorded on a Finna-
gin MAT 95 spectrometer. ESI-MS were taken by a PE SCIEX API
365 mass spectrometer. Electron ionization mass spectra were re-
corded on a Hewlett–Packard 5890II GC instrument coupled with a
5970 mass selective detector. Elemental analyses were performed
on a Vario EL elemental analyzer. Optical rotations were measured
by a JASCO DIP-370 digital polarimeter. Melting points were mea-
sured by an electrothermal digital apparatus. The chiral bromopyr-
idine intermediates and bipyridines L1, L3 and L6 were prepared
according to the literature procedures [17]. [Cu(L1)Cl2] and
[Cu(L1)Cl] were synthesized according to our previously reported
procedure [11].
Chiral bipyridine L (0.4 mmol) in CH2Cl2 (5 ml) was added drop-
by-drop to a solution of CuCl2ꢁ2H2O (0.4 mmol, 0.068 g) in absolute
ethanol (5 ml). The mixture was refluxed for a few hours. Addition
of ether to the cooled reaction mixture led to the formation of a
microcrystalline solid. The mixture was placed in the refrigerator
overnight and the microcrystalline solid was filtered and washed
with ether.
2.3.1. [Cu(L2)Cl2]
Recrystallization from CH2Cl2/EtOH/Et2O gave 0.136 g (53%) of
an orange solid: Anal. Calc. for CuN2Cl2C34H40O2: C, 63.49; H,
6.22; N, 4.36. Found: C, 64.10; H, 6.12; N, 4.48%. UV–Vis-NIR spec-
trum (CH2Cl2), kmax (nm) (e
/Mꢀ1 cmꢀ1): 245 (16 000), 311 (15 000),
390 sh (852), 887 (150); MS (+FAB): 607(M+ꢀCl) and 572
(M+ꢀ2Cl).
2.3.2. [Cu(L3)Cl2]
Recrystallization from CH2Cl2/EtOH/Et2O gave 0.152 g (83%) of a
green solid: IR (KBr, cmꢀ1): 3268 s; Anal. Calc. for CuN2Cl2-
C20H28O2ꢁ(H2O): C, 49.94; H, 6.24; N, 5.83. Found: C, 49.98; H,
6.27; N, 5.79%. UV–Vis-NIR spectrum (MeOH), kmax (nm) (e/
Mꢀ1 cmꢀ1): 304 (11 400), 246 (10 700), 257 (9300), 871 (70); MS
2.2. Synthesis of bipyridines L2, L4 and L5
(API): 390 (M+ꢀHClꢀCl).
At 70 °C and under N2, to NiCl2ꢁ6H2O (6 mmol, 1.43 g) in de-
gassed DMF (30 ml), triphenylphosphine (24 mmol, 6.30 g) was
added to give a blue solution. Zinc powder (13 mmol, 0.87 g) was
then added and the resulting mixture was stirred for 1 h, resulting
in the formation of a dark-brown mixture. The suitable bromopyr-
idine (5 mmol) in degassed DMF (5 ml) was added slowly and the
mixture was stirred for another 3 h. The mixture was then allowed
to cool to room temperature and 5% aqueous NH3 (50 ml) was
added. The layers were separated, and the aqueous layer was ex-
tracted three times with CH2Cl2 (70 ml ꢂ 3). The combined organic
layers were washed three times with water (50 ml ꢂ 3) and once
with brine (50 ml). Drying with Na2SO4 and removal of the solvent
under reduced pressure yielded a pale yellow solid. This was puri-
fied by column chromatography (petroleum ether–ethyl acetate)
to give a white solid.
2.3.3. [Cu(L4)Cl2]
Recrystallization from CH2Cl2/EtOH/Et2O gave 0.109 g (67%) of a
yellow solid: Anal. Calc. for CuN2Cl2C16H20O2: C, 47.23; H, 4.92; N,
6.89. Found: 47.57; H, 4.83; N, 7.01%. UV–Vis-NIR spectrum
(CH2Cl2), kmax (nm) (e
/Mꢀ1 cmꢀ1): 315 (15 400), 301 (14 500),
246 (12 600), 366 sh (770), 996 (110); MS (+FAB): 370 (M+ꢀCl)
and 335 (M+ꢀ2Cl).
2.3.4. [Cu(L5)Cl2]
Recrystallization from CH2Cl2/EtOH/Et2O gave 0.132 g (59%) of a
yellow solid: Anal. Calc. for CuN2Cl2C28H28O2: C, 60.16; H, 5.01; N,
5.01. Found: 61.13; H, 4.91; N, 5.11%. UV–Vis-NIR spectrum
(CH2Cl2), kmax (nm) (e
/Mꢀ1 cmꢀ1): 307 (16 000), 317 (14 900),
246 sh (11 000), 377 sh (738), 980 (130); MS (+FAB): 522
(M+ꢀCl) and 587 (M+ꢀ2Cl).
2.2.1. Bipyridine L2
2.3.5. [Cu(L6)Cl2]
Yield: 0.66 g (52%); 1H NMR (CDCl3): d 1.02 (s, 18H), 4.33 (s, 2H),
4.32–4.50 (m, 4H), 7.27–7.35 (m, 10H), 7.47 (d, 2H, J = 7.5 Hz), 7.81
(t, 2H, J = 7.5 Hz), 8.29 (d, 2H, J = 7.5 Hz); 13C NMR (CDCl3): d 26.3,
26.3, 26.4, 35.7, 71.3, 90.4, 119.4, 121.6, 121.6, 127.1, 128.0, 136.5,
138.7, 154.8, 159.6. Anal. Calc. for C34H40O2N2: C, 80.28; H, 7.93; N,
5.51. Found: C, 80.48; H, 7.70; N, 5.24%.
Recrystallization from MeOH/Et2O gave 0.064 g (42%) of a green
solid: IR (KBr, cmꢀ1): 3534 versus; Anal. Calc. for CuN2Cl2C14H16O2:
C, 44.39; H, 4.23; N, 7.40. Found: 44.44; H, 4.15; N, 7.49%. UV–Vis-
NIR spectrum (MeOH), kmax (nm) (e
/Mꢀ1 cmꢀ1): 308 (10 300), 258
(12 000), 318 (7900), 438 (100), 860 (70); MS (+FAB): 342 (M+ꢀCl).
2.4. X-ray crystallography analysis for [Cu(L5)Cl2], [Cu(L6)Cl2] and
[Cu(L1)Cl]
2.2.2. Bipyridine L4
Yield: 0.38 g (56%); 1H NMR (CDCl3): d 1.53 (d, 6H, J = 6.6 Hz),
3.36 (s, 6H), 4.50–4.56 (m, 2H), 7.43 (d, 2H, J = 7.8 Hz), 7.83 (t,
2H, J = 7.8 Hz), 8.33 (d, 2H, J = 7.5 Hz); 13C NMR (CDCl3): d 22.3,
56.8, 80.8, 119.5, 119.6, 137.3, 155.3, 162.2. Anal. Calc. for
For [Cu(L5)Cl2] and [Cu(L1)Cl], diffraction data were obtained
on a Rigaku AFC7R diffractometer at 296 and 301 K, respectively.
Absorption corrections based on the PSI scans technique were ap-
plied on both of these complexes. The structures were solved by
using direct methods (SHELXS97) and refined on F2 against all reflec-
tions. The absolute configurations of [Cu(L1)Cl] at C11 and C17
were found to be R, as confirmed by the Flack parameter of
0.000(14). For [Cu(L6)Cl2], diffraction data were obtained on a
Burker SMART 1000 CCD diffractometer at 298 K. The multi-scan
method was applied for absorption correction. The structure was
C16H20O2N2: C, 70.56; H, 7.40; N, 10.29. Found: C, 70.70; H, 7.38;
N, 9.99%.
2.2.3. Bipyridine L5
Yield: 0.52 g (49%); 1H NMR (CDCl3): d 1.59 (d, 6H, J = 6.6 Hz),
4.47–4.58 (m, 4H), 4.72–4.79 (m, 2H), 7.30–7.37 (m, 10H), 7.54
(d, 2H, J = 7.5 Hz), 7.84 (t, 2H, J = 7.8 Hz), 8.35 (d, 2H, J = 7.5 Hz);

180
W.-S. Lee et al. / Polyhedron 30 (2011) 178–186
Table 1
Crystallographic data for [Cu(L5)Cl2], [Cu(L6)Cl2] and [Cu(L1)Cl].
Complex
[Cu(L5)Cl2]
[Cu(L6)Cl2]
[Cu(L1)Cl]
Formula
C
28H28Cl2CuN2O2
C14H16Cl2CuN2O2
378.73
green, block
0.22 ꢂ 0.22 ꢂ 0.10
trigonal
C22H32ClCuN2O2
455.49
brown, rod
Molecular weight
Crystal color, habit
Crystal dimensions (mm)
Crystal system
Lattice
558.96
yellow, rod
0.12 ꢂ 0.07 ꢂ 0.07
0.07 ꢂ 0.21 ꢂ 0.23
orthorhombic
monoclinic
a (Å)
b (Å)
c (Å)
12.947(3)
25.011(3)
8.044(4)
90
10.965(1)
10.965(1)
22.247(2)
90.00
9.7263(13)
11.3273(14)
11.5636(15)
90
a
(°)
b (°)
90
90
90.00
120.00
2316.4(4)
P3121 (no. 144)
6
112.837(9)
90
1174.1(3)
P21
c
(°)
V (Å3)
2604.7(15)
P212121 (no. 19)
4
1.425
301
Space group
Z value
2
Dcalc (g/cm3)
T (K)
1.629
298
1.288
296(2)
Radiation, k (Å)
l
2hmax (°)
F(0 0 0)
Mo K
1.07
50.0
1156
a
, 0.71073
Mo K
1.76
55.0
1158
a
, 0.71073
Mo K
1.06
45.0
480
a, 0.71073
(Mo Ka )
) (mmꢀ1
Reflections: measured/independent/with I > 2
Rint
r(I)
2224/2224/689
0.000
14385/3490/2682
0.040
3482/3078/2718
0.022
R-factor: R1; wR
Flack parameter
0.046; 0.166
–
0.032; 0.070
0.000(14)
0.033; 0.090
0.000(14)
solved by using the heavy atoms Patterson method (PATTY) and re-
fined on F2 against all reflections. The absolute configurations of
the molecule were found to be S for C2 and C13, as confirmed by
the Flack parameter of 0.000(14). Crystal data and details of the
measurements for these complexes are summarized in Table 1.
column. For competition experiments, an equal amount of styrene
(2 mmol) and substituted styrene (2 mmol) was used.
2.7. Kinetic experiments (EDA decomposition)
All the kinetics experiments reported here were carried out by
the following procedure: to a two-neck round bottom flask were
added [Cu(L1)Cl] (0.0046 g, 0.01 mmol), degassed chloroform
(5.0 ml) and silver(I) salt (0.01 mmol) under nitrogen. The solution
was stirred at room temperature for 10 min and filtered through a
packed filter paper to another flask containing a solution of chloro-
benzene, an internal standard (0.01 M) in degassed chloroform
(95 ml). EDA (0.01 mmol) was then added and the reaction was
monitored by GC until no EDA was present. For cyclopropanation
with styrene, styrene was added together with the internal stan-
dard. For the investigation of the counterion, suitable silver salts
were used.
2.5. Cyclic voltammetry analysis
Cyclic voltammetry was performed using a CH Instruments
Electrochemical Workstation CHI750A. Experiments were carried
out under nitrogen in a three-electrode cell with glassy-carbon as
the working electrode, a Pt-wire as the counter electrode and sat-
urated Ag/AgNO3 as the reference electrode. Ferrocene was used as
the internal standard and 0.1 M tetrabutylammonium hexafluoro-
phosphate as a supporting electrolyte. Cyclic voltammograms were
scanned at room temperature in the potential range between
+0.6 V and ꢀ0.2 V at the sweep rate of 0.1 V/s.
2.6. Procedure for copper-catalyzed cyclopropanation
3. Results and discussion
Under nitrogen, [Cu(L)Cl2] (0.02 mmol), AgOTf (0.04 mmol) and
CH2Cl2 (2 ml) were added to a two-neck round bottom flask and
stirred at room temperature for 2 h. The mixture was filtered
through a packed filter-paper and styrene (4 mmol) was added.
The catalyst was activated by the addition of 0.2 equiv of diazoace-
tate. Some of the complexes required heating during the activation
process. Using a syringe pump, a solution of diazoacetate (1 mmol)
in CH2Cl2 (0.5 ml) was added to the reaction mixture over a period
of 4 h. After the addition of diazoacetate, the mixture was allowed
to stir for 16 h at room temperature. The solvent was then removed
and the crude product obtained was purified by column chroma-
tography (hexane/EtOAc). All the cyclopropanes obtained are
known compounds and were characterized by 1H NMR, 13C NMR,
IR and GC–MS. The enantiomeric excesses of the cyclopropanes
were determined by HPLC with a Daicel Chiralcel OJ column. Abso-
lute configurations were determined by comparing the order of
elution with samples of a known configuration [18]. Diastereose-
lectivities (cis/trans ratio) were measured by GC with an Ultra
3.1. Ligands and complexes syntheses
The chiral bipyridines L1–6, having different substituent
groups, were prepared either as reported previously (L1, L3 and
L6) or following a similar synthetic route (L2, L4 and L5) [17].
The copper(II) complexes [Cu(L)Cl2] (L = L1–6) were obtained in
moderate to good yields by the reaction of an equal molar ratio
mixture of L and CuCl2ꢁ2H2O in dichloromethane–ethanol solution
at reflux temperature. All the complexes were air-stable in the so-
lid state and in solution. Elemental analyses indicate that these
compounds are 1:1 ligand-to-copper complexes with two chloride
ions. The positive ion FAB mass spectra for these complexes show
parent ion peaks with masses that matched with the formulas cal-
culated by CHN analysis. The colors of the complexes are different:
[Cu(L1)Cl2] and [Cu(L2)Cl2] are orange; [Cu(L4)Cl2] and [Cu(L5)Cl2]
are yellow; and [Cu(L3)Cl2] and [Cu(L6)Cl2] are green. Both the
orange and yellow colors are unusual for Cu(II) complexes [19].
Copper(I) complexes of these ligands have also been prepared
2-crosslinked
5%
PhMesilcone
(25 m ꢂ 0.2 mm ꢂ 0.33
lm)

W.-S. Lee et al. / Polyhedron 30 (2011) 178–186
181
R1
R2
R2
R1
N
N
L1-6 + CuCl2
Cu
CH Cl /ethanol
2
2
OR
RO
reflux
Cl
Cl
[Cu(L1)Cl2]: R1 = t-Bu, R2 = H, R = CH3
[Cu(L2)Cl2]: R1 = t-Bu, R2 = H, R = CH2Ph
[Cu(L3)Cl2]: R1 = t-Bu, R2 = H, R = H
[Cu(L4)Cl2]: R1 = H, R2 = CH3, R = CH3
[Cu(L5)Cl2]: R1 = H, R2 = CH3, R = CH2Ph
[Cu(L6)Cl2]: R1 = H, R2 = CH3, R = H
Scheme 1. Synthesis of [Cu(L)Cl2].
L1
L2
L3
L4
L5
L6
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.16
0.12
0.08
0.04
0.00
600
800
1000
1200
nm
400
600
800
1000
Wavelength (nm)
Fig. 1. Electronic absorption spectra of (a) [Cu(L)Cl2] (L = L1–6) at a concentration
of 3.8 ꢂ 10ꢀ5 M at room temperature (CH2Cl2 was used for L1, L2, L4 and L5, MeOH
was used for L3 and L6). The inset shows spectra in the region 600–1200 nm at a
concentration of 1.0 ꢂ 10ꢀ3 M.
but these are mostly unstable, except for [Cu(L1)Cl] which is a red-
dish brown solid (Scheme 1).
Fig. 2. Cyclic voltammograms of [Cu(L)Cl2] (L = L1–6 from top to bottom) in
acetonitrile with a scan rate of 0.1 V/s. All the potentials were measured against the
Ag/AgNO3 reference electrode.
3.2. Spectroscopic properties
As the color of the complexes [Cu(L)Cl2] vary from orange
(L = L1 and L2) to yellow (L = L4 and L5) to green (L = L3 and L6),
their spectroscopic behaviors were explored. UV–Vis-NIR elec-
tronic absorption spectra of [Cu(L)Cl2] (L = L1–6) are shown in
Fig. 1. Intense absorption bands at about 252 and 310 nm in the
UV-region may be attributed to p–p* transitions of the bpy ligands
[20]. These absorption bands extend to the visible region with dif-
ferent molar absorptivities. For the complexes with L1 and L5, rel-
Table 2
Cyclic voltammetric parameters for [Cu(L)Cl2].
Ep (mV)a
ip,c/ip,a
b
Complex
E1/2 (V)
D
[Cu(L1)Cl2]
[Cu(L2)Cl2]
[Cu(L3)Cl2]
[Cu(L4)Cl2]
[Cu(L5)Cl2]
[Cu(L6)Cl2]
+0.482
+0.516
+0.462
+0.405
+0.424
+0.436
155
134
107
166
103
121
1.03
0.49
1.04
0.83
1.05
1.08
atively strong absorptions at 406 (
e
ꢃ 800 Mꢀ1 cmꢀ1) and 377 nm
ꢃ 700 Mꢀ1 cmꢀ1) were observed respectively. These absorptions
a
(e
Cathodic peak to anodic peak separation.
Ratio of cathodic peak current to anodic peak current.
b
are generally believed to originate from LMCT (Clꢀ ? CuII) [20]. For
[Cu(L6)Cl2], the absorption for this transition is weak (ꢄ438 nm,
e
ꢃ 100 Mꢀ1 cmꢀ1). In addition to the above, all the complexes ex-
hibit a very broad band in the lower energy region (>600 nm), ten-
tatively assigned to a Cu(II) d–d transition [20]. Many studies have
shown that the energies of these d–d transitions correlate with the
copper coordination geometries [21] and that the intensity maxi-
mum is shifted to lower energy when the geometry is changed
from a regular square pyramid to a regular trigonal bipyramid
[21–23]. This was also observed in our study, as [Cu(L6)Cl2], which
(vide infra). In addition, the absorption at 980 nm for [Cu(L5)Cl2]
may be assigned to the transition of dxz or dyz ? d2 and the absorp-
z
tion at 860 nm for [Cu(L6)Cl2] may be assigned to the transition of
d2z ? d2x ꢀ y2 [21,24]. For [Cu(L1)Cl2], as reported previously [11],
the absorption at 919 nm indicates a pseudo-tetrahedral geometry
[25]. Complex [Cu(L2)Cl2], which has a color similar to [Cu(L1)Cl2],
has an absorption at 887 nm and might also have a tetrahedral
geometry. When compared with [Cu(L6)Cl2] and [Cu(L5)Cl2],
respectively, the similar absorptions of [Cu(L3)Cl2] and [Cu(L4)Cl2]
possibly indicate similarities in geometries.
is a distorted square pyramid with
a higher energy than [Cu(L5)Cl2] (860 versus 980 nm), which has
comparatively more of a trigonal bipyramidal form with = 0.48
s = 0.28 (vide infra), absorbed at
s

182
W.-S. Lee et al. / Polyhedron 30 (2011) 178–186
Fig. 3. Crystal structures for (a) [Cu(L5)Cl2], (b) [Cu(L6)Cl2] and (c) [Cu(L1)Cl] including the atom numbering schemes. All hydrogen atoms have been omitted for clarity.
3.3. Redox behaviour
and as the cathodic to anodic peak current ratios, ip,c/ip,a, deviate
from unity, all complexes exhibit quasi-reversible electron transfer
behavior.
The E1/2 values for the complexes with ligands that contain the
bulkier t-butyl substituents (L1–3) are higher than the copper
complexes with the less bulky corresponding methyl substituents
(L4–6). This observation provides evidence that the steric
The redox behaviour of the Cu(II/I) couple in acetonitrile was
assessed by cyclic voltammetry. For comparison, the cyclic voltam-
mograms are shown in Fig. 2 and data summarized in Table 2. As
the cathodic and anodic peak separations (
are larger than those of a fully reversible process (
D
Ep = 103 ꢀ 166 mV)
D
Ep = 59 mV),

W.-S. Lee et al. / Polyhedron 30 (2011) 178–186
183
Table 3
the
complex.
[Cu(L6)Cl2] also has a distorted trigonal bipyramidal geometry
s index of 0.48 for [Cu(L5)Cl2] reflects the distorted nature of its
Selected bond distances and angles for [Cu(L5)Cl2], [Cu(L6)Cl2] and [Cu(L1)Cl].
[Cu(L5)Cl2]
[Cu(L6)Cl2]
[Cu(L1)Cl]
similar to [Cu(L5)Cl2]: one of the nitrogen atoms N(1) of the bipyr-
idine ligand and two chloride ions occupy the equatorial plane
while the oxygen atom O(1) and the remaining nitrogen atom
N(2) are axial. The copper–nitrogen bond distances (Cu(1)–
N(1) = 1.959(3) Å; Cu(1)–N(2) = 2.064(3) Å) are slightly shorter
than those of [Cu(L5)Cl2]. The two copper–chloride bonds are dif-
ferent (Cu(1)–Cl(1) = 2.274(1) Å and Cu(1)–Cl(2) = 2.424(1) Å).
The copper–oxygen bond is 2.095(2) Å. The bite angle of 80.0°
made by the bipyridine ligand and copper is comparable to that
of [Cu(L5)Cl2]. The O(1)–Cu(1)–N(2) angle of 157.4° (Table 2)
shows the O–Cu–N linkage is not perfectly linear. The bond angles
for Cl(1)–Cu(1)–Cl(2), Cl(1)–Cu(1)–N(1) and Cl(2)–Cu(1)–N(1) are
108.0°, 141.3° and 110.0°, respectively. As the angles around the
copper sum up to 359.3°, the equatorial plane of the complex is
Atoms
Bond lengths (Å)
Cu(1)–Cl(1)
Cu(1)–Cl(2)
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–O(1)
2.311(5)
2.302(5)
2.009(13)
2.065(14)
2.135(13)
2.274(1)
2.424(1)
1.959(3)
2.064(3)
2.095(2)
2.129(3)
–
2.067(6)
2.054(10)
–
Bond angles (°)
Cl(1)–Cu(1)–Cl(2)
Cl(1)–Cu(1)–O(1)
Cl(1)–Cu(1)–N(1)
Cl(1)–Cu(1)–N(2)
Cl(2)–Cu(1)–O(1)
Cl(2)–Cu(1)–N(1)
Cl(2)–Cu(1)–N(2)
O(1)–Cu(1)–N(1)
O(1)–Cu(1)–N(2)
N(1)–Cu(1)–N(2)
118.86(19)
95.97(37)
114.18(144)
95.52(40)
89.75(40)
126.23(39)
103.37(40)
76.44(54)
155.58(52)
79.19(57)
107.99(3)
93.27(7)
141.28(9)
100.37(8)
92.13(6)
109.98(9)
100.55(8)
78.11(12)
157.41(10)
80.00(11)
–
–
138.7(1)
140.69(9)
–
–
–
–
–
almost planar. Given its
s index of 0.28, the structure of [Cu(L6)Cl2]
80.61(12)
is more distorted and closer to a square pyramid.
Dihedral angles (°)
The copper(I) centre of [Cu(L1)Cl] is three-coordinated and has
a Y-shaped planar geometry, with the metal ion surrounded by two
pyridine nitrogen atoms and one chloride anion. The copper–
nitrogen bond distances are 2.054(10) and 2.067(6) Å. These bond
distances are slightly longer than the copper–nitrogen bond
distances for other achiral three-coordinate copper(I) phenanthro-
line complexes. The bite angle of 80.6° made by the bipyridine li-
gand and copper is small when compared with the value of 82.9°
cited in the literature [31]. The bond angles for Cl(1)–Cu(1)–N(1)
and Cl(1)–Cu(1)–N(2) are 138.7° and 140.7°, respectively. The an-
gles around copper sum up to be 360° which suggests that the
complex is planar in geometry. The copper–chloride bond distance
is 2.129 Å. There are a few examples of three-coordinate Y-shaped
copper(I) complexes reported in the literature [32–34]. The large
difference in geometry between [Cu(I)(L1)Cl] and [Cu(I)(L1)Cl2]
probably explains the quasi-reversible nature of the Cu(II)/Cu(I) re-
dox couple.
N(1)–C(x)–C(y)–N(2)
ꢀ9.75(232)
x = 7, y = 8
ꢀ6.14(42)
x = 7, y = 8
ꢀ6.04(48)
x = 5, y = 6
bulkiness of L1–3 distorts the copper(II) state and hence makes the
complexes easier to reduce than those with ligands that are less
bulky. Among the three structurally characterized complexes (vide
infra), the E1/2 for the tetrahedral complex, [Cu(L1)Cl2], is higher
than that for the square pyramid complex, [Cu(L6)Cl2]. The E1/2 va-
lue for the trigonal bipyramidal complex, [Cu(L5)Cl2], is the lowest.
The high E1/2 value for [Cu(L1)Cl2] is most probably due to the min-
imal alteration to the preferred tetrahedral geometry when the
complex is reduced to Cu(I) [23,26,27]. Along the same line, in
the case of the two five-coordinated complexes, the higher E1/2 va-
lue of [Cu(L6)Cl2] can be attributed to be the smaller change in its
square pyramidal geometry during reduction from Cu(II) to Cu(I),
whereas the lower E1/2 value of [Cu(L5)Cl2] likely reflects a signif-
icant change in geometry upon reduction.
3.5. Catalytic asymmetric cyclopropanation
3.4. Structural characterization
The use of [Cu(L)Cl2] (L = L2–6) in the cyclopropanation of
styrene with EDA was studied and the results are summarized in
Table 4. The active form of the catalyst was generated in situ by
reacting 2 mol% of [Cu(L)Cl2] with AgOTf, which was then filtered
and reacted with a few equivalents of EDA to reduce the copper(II)
species to copper(I). Copper catalysts generated from L1–5 re-
quired no heating and were much more easily activated than the
one from L6, which required heating at 40 °C for a few minutes.
As L6 has the lowest E1/2 value, the result is consistent with our
observation on the correlation between the Cu(II)/Cu(I) redox po-
tential of the copper(II) chloride complex and the E1/2 value.
All these copper–bipyridine complexes were active catalysts,
producing cyclopropyl esters in yields between 85% and 99% (en-
tries 1–6). The best results in both enantioselectivity and diastere-
oselectivity (trans:cis = 80:20) were achieved with L1, which gave
90% ee and 82% ee for trans and cis-isomers, respectively (entry
1). Trans/cis ratios ranged from 66:34 to 80:20 (entries 1–6). When
L2 and L3 were employed, the absolute configurations of the cyclo-
propane esters were found to be (1R, 2R) and (1R, 2S) for the trans
and cis isomers, respectively, similar to L1. Employment of L4–6 re-
sulted in the opposite absolute configurations (trans = 1S, 2S;
cis = 1S, 2R). This can be attributed to the use of ligands with oppo-
site configuration, thus the sense of asymmetric induction ob-
served is also consistent with L1 [11]. In general, better
enantioselectivity and trans/cis ratio are observed with the t-butyl
series of ligands. Decreasing the size difference between the alkyl
group and the alkoxy/hydroxy group has a negative effect. The
The crystal structures of [Cu(L5)Cl2], [Cu(L6)Cl2] and [Cu(L1)Cl]
are shown in Fig. 3a–c. Selected bond lengths and angles are
listed in Table 3. The copper(II) centres of both [Cu(L5)Cl2] and
[Cu(L6)Cl2] are five-coordinated and have a distorted trigonal bipy-
ramidal geometry: one of the nitrogen atoms N(1) of the bipyridine
ligand and two chloride ions occupy the equatorial plane while the
oxygen O(1) atom and the remaining nitrogen atom N(2) are axial.
The marked difference between these two structures and the pre-
viously reported [Cu(L1)Cl2] [11] demonstrate that the coordina-
tion environment is affected by the steric property of the ligand.
In the structure of [Cu(L5)Cl2], the copper–nitrogen bond dis-
tances for the two nitrogen atoms are different (Cu(1)–
N(1) = 2.009(13) Å, Cu(1)–N(2) = 2.065(14) Å) but they are within
the range generally found for other copper(II) bipyridine com-
plexes [28,29]. The copper–chloride bonds are 2.311(5) and
2.302(5) Å and the copper–oxygen bond is 2.135(13) Å. The bite
angle of 79.2° made by the bipyridine ligand and copper is small
when compared with that of 83.0° for [Cu(L1)Cl2] [11]. As the
O(1)–Cu(1)–N(2) angle is 155.6°, the O–Cu–N linkage for the com-
plex is not perfectly linear. The bond angles around the Cu(1) cen-
tre Cl(1)–Cu(1)–Cl(2), Cl(1)–Cu(1)–N(1) and Cl(2)–Cu(1)–N(1) are
118.9°, 114.2° and 126.2°, respectively. As the sum of these angles
is 359.3°, the equatorial plane of the complex is almost planar. As
the degree of distortion in the geometry of a five-coordinated com-
plex can be represented by the value of its trigonality
s index [30],

184
W.-S. Lee et al. / Polyhedron 30 (2011) 178–186
Table 4
Asymmetric cyclopropanation of styrene with ethyl diazoacetate, catalyzed by [Cu(L)(OTf)2].
2 mol% Cu(OTf)2
O
H
H
Ph
H
2.2 mol% L
H
+
+
OEt
Ph
COOEt
H
COOEt
CH2Cl2 r.t., N2
N2
Entrya
Ligand
Yield (%)b
trans/cis
% ee. (config.)c
trans
cis
1
2
3
4
5
6
L1
L2
L3
L4
L5
L6
85
86
92
88
92
99
80:20
76:24
74:26
66:34
66:34
70:30
90 (1R, 2R)
83 (1R, 2R)
73 (1R, 2R)
30 (1S, 2S)
43 (1S, 2S)
44 (1S, 2S)
82 (1R, 2S)
57 (1R, 2S)
38 (1R, 2S)
25 (1S, 2R)
23 (1S, 2R)
26 (1S, 2R)
a
b
c
Diazoacetate (0.2 equivalent) was used for the reduction of the Cu(II) catalysts before the start of the cyclopropanation reaction.
Isolated yield after column chromatography.
Enantiomeric excesses were determined by HPLC with a Daicel Chiralcel OJ column, and the absolute configurations were determined by comparing the order of elution of
samples with a known configuration [18].
Table 5
Catalytic asymmetric cyclopropanation with [Cu(L1)X] as the catalyst.
O
H
1mol% [Cu(L)Cl]
1 mol% AgX
Ph
H
H
H
+
+
OEt
CHCl3, r.t., N2
H
COOEt
Ph
COOEt
N2
Entry
AgX
Solvent
trans:cis
% ee (trans)a
% ee (cis)a
Yield %b
1
2
3
4
5
6
7
8
CHCl3
CH2Cl2
Toluene
THF
CH3CN
CHCl3
THF
CHCl3
THF
CHCl3
THF
80:20
79:21
83:17
82:18
–
75:25
75:25
71:29
75:25
74:26
76:24
–
90 (1R, 2R)
91 (1R, 2R)
92 (1R, 2R)
93 (1R, 2R)
–
86 (1R, 2R)
79 (1R, 2R)
87 (1R, 2R)
91 (1R, 2R)
85 (1R, 2R)
90 (1R, 2R)
–
82 (1R, 2S)
85 (1R, 2S)
78 (1R, 2S)
85 (1R, 2S)
–
85 (1R, 2S)
70 (1R, 2S)
83 (1R, 2S)
84 (1R, 2S)
83 (1R, 2S)
86 (1R, 2S)
–
67
68
70
76
–
88
54
65
74
56
43
–
AgOTf
AgPF6
AgBF4
AgSbF6
AgClO4
9
10
11
12
CHCl3
a
Enantiomeric excesses were determined by HPLC with a Daicel Chiralcel OJ column, and the absolute configurations were determined by comparing the order of elution of
samples with a known configuration [18].
b
Isolated yield after chromatography.
group on the oxygen has a relatively small effect on the enantiose-
lectivity, probably because of the flexibility of the group and its
distance away from the metal centre.
Clꢀ. This is probably due to the strong coordination of the perchlo-
rate ion to the metal center.
The role of the counterion was further investigated by a compe-
tition experiment ꢀwith substituted styrenes. With [Cu(L1)X]
(X = ꢀOTf, PF6ꢀ, BF4 and SbF6 as the catalyst, competitions were
ꢀ
3.6. Effect of anion and competition experiments
carried out in either chloroform or THF. In all cases the reaction rates
increased with electron-donating substituted styrenes, i.e. the 4-
methoxy- and 4-methylstyrenes, while the electron-withdrawing
substituted styrenes, such as 4-chloro- and 3-nitrostyrenes, de-
The use of a silver salt to generate the active catalyst provides
an easy way to study the effect of the counter ion. Since L1 gives
the best results and the copper(I) complex is the active form of
the catalyst and requires no activation, [Cu(L1)Cl] was used as
the precursor for our further study. The results with different coun-
terions and different solvents are listed in Table 5. With triflate as
the anion, trans/cis ratios, enantioselectivies and yields in different
solvents of different polarity were similar (entries 1–4). No signif-
icant solvent effect was observed. However, when the coordinating
solvent acetonitrile was used,ꢀno reaction occurred (entry 5).
creased the reaction rate. In chloroform, good
r
correlationsꢀwere
obtained with
q
= ꢀ1.35 for ꢀOTf (Fig. 4), ꢀ1.36 for both BF4 and
SbF6ꢀ and ꢀ1.65 for PF6ꢀ (Figs. S1–S3). The negative value of
q indi-
cates the formation of an electrophilic metal–carbene complex
intermediate. The similarity in
qvalues obtained with catalysts hav-
ing different counterions, together with the similarity in enantiose-
lectivities obtained with these catalysts, provide evidence that the
counterion is not involved in the active intermediate.
Catalysts with ꢀOTf, BF4ꢀ, PF6 and SbF6 as counterions gave
ꢀ
similar trans/cis ratios, enantioselectivities and isolated yields, no
matter whether the reaction took place in chloroform or THF (en-
tries 1, 4, 6–11). These results suggest the existence of a common
intermediate for the catalysts with different counterions. For the
catalyst with ClO4ꢀ, no reaction took place (entry 12), just like
3.7. Kinetic rate law
Kinetic studies for the copper-catalyzed cyclopropanation of
olefins have been reported previously [35–38]. These studies were

W.-S. Lee et al. / Polyhedron 30 (2011) 178–186
185
-4
-5
-6
-7
-8
4-MeO
4-Me
ρ = -1.35
0.4
0.0
H
4-Cl
-0.4
-0.8
3-NO2
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8
0
5
10
15
20
Hammett constant σ
Time (min)
Fig. 7. EDA decomposition in tꢀhe absence of olefin, catalyzed by [Cu(L)X] (X = ꢀOTf
Fig. 4. Hammett plot for the cyclopropanation of styrene with EDA in CHCl3 using
[Cu(L1)OTf] as the catalyst.
ꢀ
ꢀ
(d), BF4
(N
), PF6
(j
) and SbF6 (ꢀ)).
-4
26
24
22
20
18
16
1
kobsd = 0.126 min
-5
-6
-7
-8
-9
k
0.04 0.08 0.12 0.16 0.20
[styrene] (M)
0
5
10 15 20 25 30
Time (min)
Fig. 8. Variation of kobsd versus different concentrations of styrene for EDA
decomposition using 1 mol% of [Cu(L1)OTf].
Fig. 5. EDA decomposition catalyzed by [Cu(L1)OTf].
[Cu(L)X]
H
H
0.3
0.2
+
X
-
X
EtO2C
EtO2C
CO2Et
H
[Cu(L)]
+
EDA
N2
H
CO2Et
k2
0.1
k1
k
EDA
H
0.0
0.5
1.0
1.5
2.0
2.5
(L)Cu
CO2Et
mol% of Cu catalyst
Fig. 6. Variation of kobsd versus [Cu]total for EDA decomposition using [Cu(L1)OTf].
Scheme 2.
k1½Cuꢅtotal½EDAꢅ
1 þ K½styreneꢅ
Rate ¼
based on achiral catalysts while we employed chiral catalysts [39].
In the absence of styrene, the rates of EDA decomposition were
found to be first order in both [EDA] (Fig. 5) and [Cu]total (Fig. 6),
similar to the previous study by Pérez et al. [37,38]. A comparison
of the k1 values indicated our catalyst (k1 = 658 Mꢀ1 minꢀ1) is more
active than the reported Cu(I)Bp catalyst (where Bp = dihydrid-
ibis(1-pyrazoyl)borate) (k1 = 52 Mꢀ1 minꢀ1) [37,38]. With catalysts
having different counterions, the rates of EDA decomposition were
also found to be affected strongly by the counterions (Fig. 7). EDA
The equilibrium constant K is calculated to be 2.4. The relatively
small value obtained here when compared with reported value
with Cu(I)Bp (K = 77 29) [38] indicates that [Cu(L)OTf] is mostly
in the catalytically active form during cyclopropanation. This
means more concentrated styrene solutions can be used with our
catalyst. Previously Kochi et al. [35] had also observed similar
binding and showed catalytically effective copper(I) species were
deactivated by the multiple coordination of olefin, which in turn
inhibited the EDA decomposition.
decompositionꢀ rates were the fastest for SbF6 (1.91 minꢀ1),
ꢀ
slower for BF4 (1.01 minꢀ1) and PF6 (1.18 minꢀ1), and slowest
ꢀ
for ꢀOTf (0.126 minꢀ1). These results seem to indicate the presence
of an equilibrium between the catalyst and the counterion. With
these results, a scheme similar to the one proposed by Pérez
et al. is proposed for EDA decomposition (Scheme 2).
3.8. Mechanism of copper-catalyzed asymmetric cyclopropanation
A general mechanistic scheme with copper-catalyzed cyclo-
propanation, similar to that proposed by Pérez and his co-workers
[38], is shown in Scheme 3. The active catalyst here, a 14-electron
fragment of [Cu(L1)]+, can undergo two possible reactions: (i) with
styrene to form a copper–olefin complex, which has a retardation
In the presence of excess styrene, the reaction rates were found
to decrease with [styrene]. A plot of 1/kobsd versus [styrene] is
linear (Fig. 8). The rate law for cyclopropanation under pseudo-first
order conditions is therefore

186
W.-S. Lee et al. / Polyhedron 30 (2011) 178–186
+
These data can be obtained free of charge via http://www.ccdc.ca-
X-
(L1)Cu
(L1)Cu
X
m.ac.uk/conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. Sup-
plementary data associated with this article can be found, in the
online version, at doi:10.1016/j.poly.2010.10.021.
Ph
-X-
- styrene
K
+X-
+ styrene
[Cu(L1)]+
Fumarate
References
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Maleate
Ph
COOEt
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the ligands not only affect the colour and coordination geometries
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Financial support from the Hong Kong Research Grants Council
GRF (CityU 101108) and the Area of Excellence Scheme established
under the University Grants Committee of the Hong Kong SAR,
China (Project No. AoE/P-10/01) and the City University of Hong
Kong are gratefully acknowledged.
Appendix A. Supplementary data
CCDC 158090, 732089 and 732090 contains the supplementary
crystallographic data for [Cu(L1)Cl], [Cu(L5)Cl2] and [Cu(L6)Cl2].
[39] For the derivation of the rate law, see supporting information.