Steric, geometrical and solvent effects on redox potentials in salen-type copper(ii) complexes

Article abstract of DOI:10.1039/b906013h

Two diastereomers of the Schiff base ligand [N,N′-bis(2′- hydroxyphenyl)phenylmethylidene]-1,2-diamino-1,2-diphenylethane (H₂L) were used for the analyses of the redox behaviour of the copper(ii) complexes [Cu(L_R,R/S,S)] (rac-1) and

Full text of DOI:10.1039/b906013h

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www.rsc.org/dalton | Dalton Transactions
Steric, geometrical and solvent effects on redox potentials in salen-type
copper(II) complexes†
Masakazu Hirotsu,*a Naoto Kuwamura,a Isamu Kinoshita,a Masaaki Kojima,b Yuzo Yoshikawab and
Keiji Uenoc
Received 25th March 2009, Accepted 17th June 2009
First published as an Advance Article on the web 27th July 2009
DOI: 10.1039/b906013h
Two diastereomers of the Schiff base ligand [N,N¢-bis(2¢-hydroxyphenyl)phenylmethylidene]-1,2-
diamino-1,2-diphenylethane (H2L) were used for the analyses of the redox behaviour of the copper(II)
complexes [Cu(LR,R/S,S)] (rac-1) and [Cu(LR,S)] (meso-1). Both complexes were structurally characterised
by X-ray crystallographic studies and showed square planar geometries. The reduction potential of CuII
to CuI for meso-1 was higher than that for rac-1. This is due to the steric effect of the phenyl
substituents and the geometrical change in the copper(I) state, which is supported by DFT calculations.
A red shift of the absorption spectrum was observed for meso-1 in the visible region by the change of
solvent from dichloromethane to pyridine, while rac-1 did not show a significant change. The effect of
solvent on the reduction potential was found to be relatively small. The geometrical effect is more
important for understanding the electrochemical behaviour in this system.
change around the metal center and the coordination of solvent
molecules. These factors are closely related to each other and make
the catalyst design difficult.5
Introduction
The application of transition metal catalysts in organic syn-
thesis is currently one of the most important areas in chem-
istry. Salen-type Schiff base ligands, where salen denotes N,N¢-
bis(salicylidene)ethylenediamine, have been used as a versatile
ligand system for complex catalysts.1 The synthetic advantages
are that the salen skeleton is readily prepared by the 2 : 1
condensation of salicylaldehyde and ethylenediamine derivatives,
and the complexation is well-established for all first-row transition
metals.2 The salen-type ligands are able to incorporate a metal ion
into the tetradentate chelate with a N2O2 donor set, and stabilises
various oxidation states. In salen-type metal complexes, the steric
environment around the metal center is easily designed, and
stereogenic carbon atoms can be introduced at the N–N chelate
moiety or as chiral substituents. Chiral salen-type complexes have
been utilised as catalysts for asymmetric reactions in the last two
decades and have provided high enantioselectivity.1,3
Recently, it has been reported that chiral salen-type cop-
per(II) complexes are effective phase transfer catalysts for
asymmetric alkylation reactions.6 Furthermore, various func-
tional groups have been introduced in the salen copper(II)
complex to investigate the new functionality in addition to
the structural, spectroscopic and electrochemical properties.7–12
In this paper, we focus on the electrochemical behaviour of
salen-type copper(II) complexes. Diastereomers of the Schiff
base ligand [N,N¢-bis(2¢-hydroxyphenyl)phenylmethylidene]-1,2-
diamino-1,2-diphenylethane (H2L, Chart 1) were very useful for
analysing the factors that affect the reduction potential of the
copper(II) complexes. The effects of solvent, intramolecular steric
interactions and the coordination geometry were evaluated.
Electronic effects of the ligands are also important for con-
trolling the reactivity and selectivity of the complex catalyst.4
The electronic effect on the metal center in complexes is usually
evaluated by the redox potentials, which is tunable by the para-
substituent of the phenol ring in the salen framework. The redox
potential, however, depends on factors such as the geometrical
aDepartment of Material Science, Graduate School of Science, Osaka
City University, Sumiyoshi-ku, Osaka 558-8585, Japan. E-mail: mhiro@
sci.osaka-cu.ac.jp; Fax: 816 6690 2753; Tel: 816 6605 2546
bDepartment of Chemistry, Faculty of Science, Okayama University, 3-1-1
Tsushima-naka, Okayama 700-8530, Japan
cDepartment of Chemistry and Biochemistry, Graduate School of Engineer-
ing, Gunma University, Kiryu 376-8515, Japan
† Electronic supplementary information (ESI) available: Cyclic voltam-
mograms and the results of DFT calculations. CCDC reference numbers
725415 and 725416. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/b906013h
Chart 1 Schiff base ligands.
7678 | Dalton Trans., 2009, 7678–7683
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◦
˚
Table 2 Absorption spectral data for rac-1, meso-1 and [Cu(salen)] (2),
and CD spectral data for S,S-1 and R,R-3, respectively
Table 1 Selected bond distances (A) and angles ( ) of rac-1 and meso-1
rac-1 meso-1
1.8748(16) 1.8752(15)
Complex Solvent
Absorption: lmax/nm (e/dm3 mol-1 cm-1)
Cu–O(1)
rac-1
meso-1
CH2Cl2
CH2Cl2
556 (436), 374 (13 900), 277 (28 700), 247 (40 900)
574 (282), 495 (435),a 378 (12 600), 290 (23 000),a
251 (38 500)
Cu–O(2)
Cu–N(1)
Cu–N(2)
O(1)–Cu–O(2)
O(1)–Cu–N(1)
O(1)–Cu–N(2)
O(2)–Cu–N(1)
O(2)–Cu–N(2)
N(1)–Cu–N(2)
N(1)–C(14)–C(34)–N(2)
1.8719(16)
1.9419(18)
1.9489(17)
90.14(7)
1.8946(17)
1.9677(18)
1.9346(15)
88.61(7)
2
CH2Cl2
567 (405), 365 (11 200), 275 (24 400), 237 (40 600)
rac-1
meso-1
2
Pyridine 563 (425), 379 (14 600)
93.00(7)
92.97(7)
Pyridine 608 (281), 500 (435),a 384 (12 300)
Pyridine 600 (306), 371 (11 900)
168.05(7)
168.06(7)
93.43(7)
172.89(9)
178.39(6)
92.15(7)
85.83(7)
86.33(7)
-40.0(2)
Complex Solvent
CD: l/nm (De/dm3 mol-1 cm-1)
-51.70(19)
SS-1
CH2Cl2
605 (-1.79), 478 (1.79), 404 (-30.8), 365 (12.6),
282 (-22.4)
Results and discussion
SS-1
RR-3
Pyridine 615 (-1.47), 473 (1.54), 407 (-30.6), 368 (12.4)
CH2Cl2
580 (-1.62), 481 (1.30), 393 (-27.9), 357 (14.8),
275 (-31.6), 250 (-21.6)
Synthesis and characterisation of complexes
a Shoulder.
Complexes with the chiral Schiff base ligand LR,R or LS,S have been
reported for manganese(III), cobalt(II) and copper(II).5,8 We pre-
pared the copper(II) complex of LR,R/S,S by a slight modification of
the method of Belokon, North, et al.8 The racemic ligand H2LR,R/S,S
readily reacted with Cu(CH3COO)2·H2O to afford purple crystals
of [Cu(LR,R/S,S)] (rac-1). The diastereomer H2LR,S was newly
prepared by the condensation reaction of 2-hydroxybenzophenone
and (R,S)-1,2-diamino-1,2-diphenylethane in 70% yield. The
reaction of H2LR,S with Cu(CH3COO)2·H2O afforded the meso
complex [Cu(LR,S)] (meso-1) as green crystals.
X-Ray diffraction studies of rac-1 and meso-1 were carried out.
ORTEP drawings are shown in Fig. 1 and selected bond distances
and angles are given in Table 1. Both complexes have four-
coordinate square planar geometries, and no other interaction
with the copper center was observed. Complex rac-1 has the
expected C2 symmetrical structure. The Cu–O and Cu–N distances
in rac-1 are similar to those found in related salen-type copper(II)
complexes.9,10 Two phenyl groups on the N–N chelate moiety are
axially oriented to minimise steric repulsion. The N(1)–C(14)–
C(34)–N(2) torsion angle of -51.70(19)◦ in rac-1 is larger than
that in [Cu(salen)] (41◦).9 The N2O2 donor atoms around Cu
are tetrahedrally distorted from the square planar geometry: the
angle between CuO(1)N(1) and CuO(2)N(2) planes is 16.19◦. The
structure of meso-1 is deformed from the Cs symmetry by the
different orientation of the two phenyl rings on the N–N chelate.
One is in an equatorial orientation and the other is in an axial
orientation as shown in Fig. 1(c). The equatorial phenyl group
gives rise to a severe steric interaction with a phenyl substituent
=
on the C N group, and the N(1)–C(14)–C(34)–N(2) torsion angle
(-40.0(2)◦) is smaller than that in rac-1. The Cu–N(1) distance is
somewhat larger than Cu–N(2), and the O(2)–Cu–N(1) angle is
almost linear. As a result, the coordination geometry of meso-1 is
close to planar: the angle between CuO(1)N(1) and CuO(2)N(2)
planes in meso-1 is 7.07◦. These structural features suggest that
the unsymmetrical distortion around Cu is caused by the steric
repulsion of the phenyl substituents.
The colour of a dichloromethane solution of rac-1 is purple,
while that of meso-1 is green. Each colour is similar to that in
the solid state. It has been reported that salen-type copper(II)
complexes exhibit solvatochromism.9 For example, [Cu(salen)] (2)
shows a purple colour in dichloromethane and a green colour in
pyridine. The solvatochromism is related to the fifth (and sixth)
coordination of solvent molecules to the Cu center. The absorption
spectra of rac-1, meso-1 and 2 were measured in dichloromethane
and pyridine, and the data are summarised in Table 2. As shown in
Fig. 2, a dichloromethane solution of rac-1 shows an absorption
band at 556 nm, which is similar to that of 2. On the other
hand, meso-1 shows a broad band at 574 nm and a shoulder at
495 nm. The different spectral feature of meso-1 is attributable to
the unsymmetrical distortion around Cu found in the solid state
structure. In pyridine, a significant red shift was observed in the
visible region for meso-1 (34 nm) and 2 (33 nm), while the shift for
rac-1 was small (7 nm). This indicates that in rac-1 the coordination
Fig. 1 ORTEP drawings of (a) rac-1 and (b) meso-1 with thermal ellipsoids at the 30% probability level. Hydrogen atoms are omitted for clarity. (c) A
side view of meso-1. Two phenyl rings are omitted for clarity.
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Table 3 Electrochemical data of rac-1, meso-1 and 2a
E1/2 vs. E◦¢(Fc+/0)/V, (DEp/mV)c
Solvent
DNb
rac-1
meso-1
2
CH2Cl2
CH3CN
DMSO
Pyridine
-1.78 (130)
-1.67 (85)
-1.64 (90)
-1.73 (126)
-1.62 (148)
-1.51 (86)
-1.49 (106)
-1.58 (184)
-1.71 (172)
-1.66 (80)
-1.64 (107)
-1.71 (159)
14.1
29.8
33.1
E1/2 - E1/2(rac-1)/V
Solvent
DNb
rac-1
meso-1
2
CH2Cl2
CH3CN
DMSO
Pyridine
0
0
0
0
0.16
0.16
0.15
0.15
0.07
0.01
0.00
0.02
14.1
29.8
33.1
Fig. 2 Absorption spectra of rac-1 ( ), meso-1 (---) and 2 (-·-) in
a Data from cyclic voltammetric measurements with 0.1 M Bu4NBF4 as a
supporting electrolyte at room temperature; scan rate, 100 mV s-1; working
electrode, glassy carbon; auxiliary electrode, platinum wire; reference
dichloromethane.
electrode, Ag/Ag+. b Donor number.13 E1/2 values are given vs. Fc+/Fc;
c
of pyridine molecules is prevented by the axially oriented phenyl
groups on the N–N chelate.
DEp, peak to peak separation.
The circular dichroism (CD) spectrum of [Cu(LS,S)] (S,S-1)
in dichloromethane showed
a similar spectral pattern to
that of [Cu{sal-(R,R)-chxn}] (R,R-3), where sal-(R,R)-chxn
is the dianion of (R,R)-[N,N¢-bis-(2¢-hydroxybenzylidene)]-1,2-
diaminocyclohexane (Table 2). Because the conformation of the
N–N chelate in R,R-3 is fixed to l in both the solid and solution
states, S,S-1 is also expected to have the l conformation, where the
phenyl substituents on the N–N chelate are in the axial direction
as found in the crystal structure of rac-1.11 A pyridine solution
of S,S-1 exhibits the CD spectral feature similar to that for the
dichloromethane solution, not only in sign but also in intensity.
The N–N chelate conformation of S,S-1 is locked into l and is
not disturbed by the pyridine donor.
Electrochemistry
The electrochemical properties of rac-1, meso-1 and 2 were studied
by cyclic voltammetry in CH2Cl2, CH3CN, DMSO and pyridine.
The cyclic voltammograms of rac-1, meso-1 and 2 in CH2Cl2 are
shown in Fig. 3. Half-wave potentials and peak to peak separations
are summarised in Table 3. Each copper(II) complex showed a
one-electron redox couple in the range of -1.78 to -1.49 V vs.
E◦¢(Fc+/0), which is attributed to the CuII/I process.9,12 The peak to
peak separations were larger than 59 mV expected for a reversible
one-electron process and dependent on the scan rates (see ESI†),
which suggests that the reduction process of the complexes is quasi-
reversible.
The redox potential of meso-1 was higher than those of rac-1 and
2, which shows that meso-1 is more easily reduced to the copper(I)
complex [meso-1]-. This trend did not change in all solvents studied
here. On the other hand, the absorption spectra of meso-1 and
2 are affected by the solvent. In rac-1, the coordination of the
solvent is highly prevented by the axially oriented phenyl groups.
Therefore the solvent effect was analysed by using the CuII/I couple
of rac-1 as a standard potential. The calculated data for each
solvent are included in Table 3. The positive shift of meso-1 is
insensitive to solvents, while the shift values for 2 in coordinating
solvents are smaller than that in CH2Cl2. Probably the CuII state
of the less hindered complex 2 is stabilised by the coordination of
Fig. 3 Cyclic voltammograms of rac-1 ( ), meso-1 (---) and 2 (-·-)
in dichloromethane containing 0.1 M Bu4NBF4; scan rate, 100 mV s-1;
working electrode, glassy carbon; auxiliary electrode, platinum wire;
reference electrode, Ag/Ag+. The observed potentials were corrected using
the redox potential of ferrocenium/ferrocene (Fc+/Fc) obtained under the
same conditions.
solvent molecules compared with the CuI state. Although an axial
coordination site is available in meso-1, the data show that the
influence of the solvent on the redox potential of meso-1 is small.
This implies that the difference of the redox potentials between
rac-1 and meso-1 is mainly caused by geometrical factors.
DFT calculations
Density functional theory (DFT) calculations at the B3LYP/
LANL2DZ level were performed for rac-1, meso-1 and 2. The
complexes were optimised for both the copper(II) and copper(I)
oxidation states, and the structural data are summarised in Table 4.
The crystal structures were reproduced well in the optimised
structures of the CuII state: rac-1 and 2 have C2 symmetrical
7680 | Dalton Trans., 2009, 7678–7683
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Table 4 Data of the DFT-optimised structures (B3LYP/LANL2DZ) for
rac-1, meso-1, 2, [rac-1]-, [meso-1]- and [2]-
Copper(II) complexes
rac-1
meso-1
2
˚
Bond lengths/A
Cu–O1
Cu–O2
Cu–N1
1.917
1.917
1.985
1.985
1.917
1.917
1.992
1.964
1.930
1.930
1.982
1.982
Cu–N2
Bond angles/◦
O1–Cu–N2
O2–Cu–N1
Torsion angle/◦
N–C–C–N
173.38
173.38
166.67
165.11
171.17
171.16
-48.95
-39.46
43.03
Copper(I) complexes
Fig. 4 A DFT-calculated structure of [meso-1]-.
[rac-1]-
[meso-1]-
[2]-
˚
Bond lengths/A
Cu–O1
Cu–O2
Cu–N1
Conclusions
2.030
2.031
2.135
2.134
1.978
2.034
2.370
2.014
2.074
2.074
2.124
2.124
Comparison of the diastereomeric salen-type copper(II) complexes
rac-1 and meso-1 provided useful information about the electro-
chemical behaviour. The higher potential value of the CuII/I redox
couple for meso-1 is mainly attributable to the geometrical change
to the trigonal structure in the copper(I) state, which is caused
by the steric interactions of the phenyl substituents. The present
study also showed that solvent effect on redox potentials is much
smaller than the geometrical effect in this CuII/I system.
Cu–N2
Bond angles/◦
O1–Cu–N2
O2–Cu–N1
Torsion angle/◦
N–C–C–N
DE/eVa
137.49
137.37
150.92
144.58
157.65
157.64
-52.17
1.335
-57.63
1.566
47.84
1.178
a Energy difference between copper(II) and copper(I) complexes: DE(M) =
E([M]) - E([M]-).
Experimental
structures, while meso-1 has an unsymmetrical structure. The
elongation of one of two Cu–N bonds in the calculated structure of
meso-1 is consistent with the crystal structure. The DFT-calculated
structure for the copper(I) complexes [rac-1]- and [2]- showed
the enhanced tetrahedral distortion around Cu (see ESI†). The
torsion of N–C–C–N in [rac-1]- is slightly larger than that in
[2]- to avoid the steric repulsion between the phenyl groups. The
copper(I) state calculated for [meso-1]- was quite different from
[rac-1]- and [2]-. One of two Cu–N bonds is largely elongated to
form a distorted trigonal planar geometry as shown in Fig. 4: O1–
Cu–O2, 116.19◦; O1–Cu–N2, 150.92◦; O2–Cu–N2, 91.80◦. The
calculated structure of [meso-1]- is indicative of the reason why the
CuII/I redox potential of meso-1 is higher than that of rac-1. The
geometrical change to the trigonal structure reduces the repulsive
interactions of the phenyl groups to stabilise the CuI state. Such
stabilisation could not be achieved in the cases of rac-1 and 2.
The energy difference between the copper(II) and copper(I)
complexes (DE) can be used for estimating the redox potential
of the CuII/I couple. The DE value for meso-1 (1.566 eV) is
larger than that for rac-1 (1.335 eV), which suggests that meso-1
is easily reduced to [meso-1]- compared with rac-1. The value
0.23 V obtained from the energy differences, DE(meso) -
DE(rac-1), is relatively larger than the difference of the redox
potentials in dichloromethane (0.16 V). The redox potential
estimated for 2 is lower than that for rac-1 by 0.16 V, which does not
match with the experimental data. Probably, the simplified evalua-
tion of the redox potential by DFT calculations is applicable only
for complexes with analogous structures, such as diastereomers.
Materials and physical measurements
rac-1,2-Diamino-1,2-diphenylethane,14
(S,S)-1,2-diamino-1,2-
diphenylethane15 and (R,S)-1,2-diamino-1,2-diphenylethane16
were prepared according to the literature procedures. All other
reagents and solvents were purchased from commercial sources
and used as received. NMR spectra were recorded on a JEOL
JNM-AL300, a JEOL Lambda 300, or a Bruker AVANCE 300
FT-NMR spectrometer at room temperature. UV-vis spectra were
measured on a JASCO V-570 spectrometer. Circular dichroism
spectra were measured on a JASCO J-720W spectropolarimeter.
Elemental analyses were performed by the Analytical Research
Service Center at Osaka City University on Perkin-Elmer
240C or FISONS Instrument EA108 elemental analysers.
Cyclic voltammetric measurements were performed using an
ALS/CHI600A volammetric analyser (Bioanalytical Systems
Inc.). Cyclic voltammograms were recorded at room temperature.
A glassy carbon disk electrode with a diameter of 3 mm was used
as a working electrode. A platinum wire auxiliary electrode and
a Ag/Ag+ (Ag/0.01 M AgNO3, 1 M = 1 mol dm-3) reference
electrode were used in a three electrode system. Sample solutions
containing 0.1 M Bu4NBF4 as a supporting electrolyte were
prepared in the concentration of 0.5–2.4 mM and deoxygenated
with pure argon before measurements. The electrode potentials
reported here were corrected using the redox potential of
ferrocenium/ferrocene E◦¢(Fc+/0) as an external standard redox
couple.
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Preparation of ligands
diffractometer using the w-2q scan technique with graphite-
monochromated Mo Ka radiation. The data were corrected
for Lorentz and polarisation effects. The analyses were carried
out using the CrystalStructure 3.8 crystallographic software
package.17 An empirical absorption correction based on y scans
of three reflections was applied. The structures were solved
using direct methods (SIR9718) and refined by full-matrix least-
squares on F2 using CRYSTALS.19 All non-hydrogen atoms were
refined anisotropically. A hydrogen atom on O of an ethanol
molecule in rac-1 was found in difference Fourier maps and refined
isotropically. H atoms of a disordered dichloromethane molecule
in meso-1 were not included in the model. Other H atoms were
located on calculated positions and refined as the riding model.
[N,N¢-Bis-(2¢-hydroxyphenyl)phenylmethylidene]-(S,S)-1,2-dia-
mino-1,2-diphenylethane (H2LS,S
(H2LR,R/S,S) were prepared according to literature procedures.5
) and its racemic mixture
[N,N¢-Bis-(2¢-hydroxyphenyl)phenylmethylidene]-(R,S)-1,2-dia-
mino-1,2-diphenylethane (H2LR,S). A yellow solution of 2-hydro-
xybenzophenone (833 mg, 4.2 mmol) and (R,S)-1,2-diamino-1,2-
diphenylethane (425 mg, 2.0 mmol) in ethanol (5 mL) was refluxed
for 24 h, and then cooled to room temperature. The resulting yel-
low precipitate was collected by filtration and washed with ethanol.
The product was recrystallised from dichloromethane/ethanol
to give the desired product as yellow crystals (814 mg, 70%).
Anal. Calc. for C40H32N2O2·0.1CH2Cl2: C, 82.87; H, 5.58; N, 4.82.
Found: C, 82.91; H, 5.60; N, 4.90%. 1H NMR (300 MHz, CDCl3):
d 15.28 (s, 2H, OH), 7.45 (m, 2H), 7.34 (m, 4H), 7.29–7.17 (m,
8H), 7.13–7.09 (m, 4H), 6.90 (m, 2H), 6.73 (d, 2H, J = 7.1 Hz),
6.57–6.50 (m, 4H), 6.25 (d, 2H, J = 7.5 Hz), 4.79 (s, 2H, NCH).
Crystal data for rac-1. C42H36CuN2O3, M = 680.29, mono-
˚
clinic, a = 12.7373(13), b = 19.8056(13), c = 13.754(2) A, b =
◦
3
˚
92.992(11) , V = 3465.0(7) A , T = 295 K, space group P21/n
(no. 14), Z = 4, Dcalcd = 1.304 Mg m-3, m = 0.672 mm-1, F(000) =
1420.00, 8291 reflections measured, 7945 unique (Rint = 0.014)
which were used in all calculations. The final R indices were R1
[I > 2s(I)] = 0.0398 and wR2 (all data) = 0.1381. GOF = 1.007.
1
13C{ H} NMR (75.5 MHz, CDCl3): d 174.2, 162.6, 140.1, 133.5,
132.4, 131.8, 129.1, 128.5, 128.3, 128.2, 128.1, 127.6, 127.5, 127.0,
119.8, 117.53, 117.48, 72.0 (NCH).
Crystal data for meso-1. C41H32Cl2CuN2O2, M = 719.16,
˚
Preparation of complexes
triclinic, a = 12.5803(13), b = 12.8121(13), c = 11.3593(11) A,
◦
3
˚
a = 103.360(7), b = 103.987(8), g = 81.908(9) , V = 1721.5(3) A ,
The procedure reported for [Cu(LR,R)] (R,R-1) was modified and
used for [Cu(LR,R/S,S)] (rac-1) and [Cu(LS,S)] (S,S-1).8 [Cu(salen)]
(2) and [Cu{sal-(R,R)-chxn}] (R,R-3) were prepared by a similar
procedure using H2salen and H2sal-(R,R)-chxn, respectively.
T = 295 K, space group P1 (no. 2), Z = 2, Dcalcd = 1.387 Mg m-3,
¯
m = 0.828 mm-1, F(000) = 742.00, 8278 reflections measured, 7917
unique (Rint = 0.009) which were used in all calculations. The final
R indices were R1 [I > 2s(I)] = 0.0442 and wR2 (all data) = 0.1535.
GOF = 1.004.
[Cu(LR,R/S,S)] (rac-1). To a blue suspension of Cu(CH3COO)2·
H2O (60 mg, 0.30 mmol) in ethanol (15 mL) was added a solution
of H2LR,R/S,S (172 mg, 0.30 mmol) in dichloromethane (3 mL). The
solution was stirred at room temperature for 1 h, during which
time the colour changed to purple. The resulting purple precipitate
was collected by filtration, washed with ethanol, and recrystallised
from dichloromethane/ethanol (5 mL : 10 mL) to afford rac-2 as
dark purple plate crystals. Yield: 172 mg (84%). Anal. Calc. for
C40H30CuN2O2·C2H5OH: C, 74.15; H, 5.33; N, 4.12. Found: C,
74.28; H, 5.33; N, 4.17%. A piece of crystal was used for single
crystal X-ray structure analysis.
Computational details
Structure optimisation of the complexes was performed using the
Gaussian 03 program package.20 The B3LYP density functional
method and the LANL2DZ basis set were used for complexes
rac-1, meso-1 and 2. The initial models for rac-1 and meso-1 were
obtained from the crystal structures analysed in this work. The
initial model for 2 was obtained from the crystal structure in ref. 9.
The calculations of copper(I) complexes [rac-1]-, [meso-1]- and
[2]- were performed using the optimised structures of the corre-
sponding copper(II) complexes as the initial model. The optimised
structures and their molecular coordinates are given in ESI.†
[Cu(LS,S)] (S,S-1). This complex was prepared by the same
procedure for rac-1 except that H2LS,S was used in place of
H2LR,R/S,S. Yield: 130 mg (68%). Anal. Calc. for C40H30CuN2O2: C,
75.75; H, 4.77; N, 4.42. Found: C, 75.41; H, 4.70; N, 4.40%.
References
[Cu(LR,S)] (meso-1). To a blue suspension of Cu(CH3COO)2·
H2O (60 mg, 0.30 mmol) in ethanol (15 mL) was added a solution
of H2LR,S (172 mg, 0.30 mmol) in dichloromethane (3 mL).
The resulting green suspension was refluxed for 1 h. The green
precipitate was collected by filtration, washed with ethanol, and
recrystallised from dichloromethane/ethanol (5 mL : 10 mL) to
afford meso-1 as dark green rod crystals. Yield: 165 mg (79%).
Anal. Calc. for C40H30CuN2O2·0.5CH2Cl2·H2O: C, 70.02; H, 4.79;
N, 4.03. Found: C, 70.09; H, 4.78; N, 3.96%. A piece of crystal was
used for single crystal X-ray structure analysis.
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