Color difference caused by chirality

Article abstract of DOI:10.1016/j.inoche.2010.09.016

Synthesis, crystal structure and spectra properties of enantiopure and corresponding racemic Cu(II)-dipyridine amido complexes are reported. The different coordination styles result in the unexpected chiral-unrelated color changes between chiral and racem

Full text of DOI:10.1016/j.inoche.2010.09.016

Inorganic Chemistry Communications 14 (2011) 13–16
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Color difference caused by chirality
a,b,
b
c
Zhaoqiong Jiang a, Hua Lu b, Di Wu a,b, , Xiangge Zhou
⁎
⁎
, Zhen Shen , Nagao Kobayashi
a
Institute of Homogeneous Catalysis, College of Chemistry, Sichuan University, Chengdu 610064, China
State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210093, China
Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Synthesis, crystal structure and spectra properties of enantiopure and corresponding racemic Cu(II)-
dipyridine amido complexes are reported. The different coordination styles result in the unexpected chiral-
unrelated color changes between chiral and racemic complexes.
Received 4 August 2010
Accepted 14 September 2010
Available online 22 September 2010
© 2010 Elsevier B.V. All rights reserved.
Keywords:
Chiral coordination complex
Color
Crystal structure
Copper
Enantiopure compounds are of central importance in many
domains of chemistry. Compared with organic compounds, chirality
in coordination compounds has received relatively fewer attention.
[1] In recent times, interest in inorganic stereochemistry has rised
rapidly partially due to the stereochemically well-defined building
blocks for the assembly of supramolecular structures, a field that has
found great interest in coordination chemistry. Many examples exist
where molecular chirality is used to influence the supramolecular
stereochemistry of containers, helical fibers, micelles, bilayers, and
crystals. [2] For example, molecular chirality on the packing style of
supramolecular systems affects the thermodynamic properties, and
some chiral-related optical properties such as circular dichroism (CD),
circularly polarised luminescence (CPL), optical rotation, chiroptical
switching processes, and magneto-chiral effects. [3] However, the
effects of molecular chirality on the chiral-unrelated properties of
supramolecular systems, such as electronic absorption spectra, are
rarely observed. [4] We report herein the synthesis, crystal structure
and spectra properties of enantiopure and corresponding racemic Cu
complexes, which shows quite different colors caused by different
coordination styles.
plexes 1 were obtained as red, red and purple crystals as shown in
Fig. 1. Mixing the solution of (R,R) and (S,S)-1 in the molar ratio of 1:1,
and then crystallizing or simply removing the solvent, the purple solid
appeared, showing the formation of the racemic-1.
The new complexes 1 are generally air- and moisture-stable. They
are characterized by MS, IR, elemental analysis, UV–vis absorption
spectra in solution and UV–vis solid reflection spectra. The single
crystal structures of complexes 1 have been determined. The infrared
spectra of the complexes display, in general, a large number of intense
peaks in the range of 1600–600 cm−1. Disappearance of N–H
stretching bands (3318 cm−1) in IR spectra supports the formation
of Cu(II) complexes.
Enantiopure solid differs from racemic solid in the solid UV–vis
reflection spectra mainly in the long-wavelength region (see
supporting information, Fig. S3). (R,R)-1 and (S,S)-1 reflect much
more ratio of visible light above 550 nm than racemic-1.
Spectroscopic and computational studies at the molecular level
have been carried out to figure out the reason of the color difference.
As expected, UV–vis absorption profiles of (R,R)-1, (S,S)-1 and
racemic-1 solutions are not distinguishable from each other. Consid-
erable broadening absorption bands at 400–600 nm are due to the
excitation of metal and O=C–N to bipyridine group charge transfer
(see supporting information, Fig. S5). In this region, the calculated
spectrum of (R,R)-1 shows two peaks at 532.8 and 530.8 nm with the
oscillator strengths of 0.0002 and 0.0027, respectively (see supporting
information, Fig. S4 and Table S1), which indicates strong *→LUMO-1
character. An analysis of the relative orbitals (Fig. S5) clearly shows that
those excitations correspond to an electron transfer from the metal and
O=C–N to the bipyridine ligands.
Started from 2-methyl-pyridine and 1,2-diaminocyclohexane, the
ligand 1,2-[bis(6′-pyridine-2′-carboxamido)-ethane]cyclohexane 2
was synthesized according to our previous studies as shown in
Scheme 1. [5,6] The corresponding copper(II) complexes 1 were then
obtained by the reaction of the ligand 2 and copper acetate in ethanol.
[7] Dependent on different chirality of the starting materials, i.e. (R,R),
(S,S), and racemic-1,2-diaminocyclohexane, the corresponding com-
The (R,R)-1 and (S,S)-1 exhibit converse CD signals in the UV/vis
region as shown in Fig. 2. To investigate insightfully the relationship
⁎
Corresponding authors. Fax: +86 28 8541 2290.
E-mail address: wood@scu.edu.cn (Di Wu).
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.09.016

14
Z. Jiang et al. / Inorganic Chemistry Communications 14 (2011) 13–16
Scheme 1. Synthetic routine of ligand 2 and complexes 1.
between observed CD spectrum and molecular geometry, rotatory
strengths for the optimized structure of (R,R)-1 are calculated at the
B3LYP/6-31G(d) level. The results suggest the main features in the CD
spectra observed are well matched by the computations for the (R,R)-
1. For examples, positive and negative CD signals are predicted to be
531 and 352 nm respectively, which are in accordance with the
observed positive and negative CD at 534 and 350 nm.
copper ion adopts a square pyramidal 5-coordinated style in racemic-
1 while a square 4-coordinated style in the homochiral (R,R)-1.
Different coordination environment in crystal results in a considerable
difference in the energy levels of the electronic structure, and
consequently the disparity in the energy absorption to lift the electrons
through d–d transition.
On the crystallographic aspect, it is the steric hindrance of
molecular structure that induces the different patterns of molecular
packing between racemic and homochiral 1. The molecules exist in a
distorted plane, sharing nearly the same dihedral angles C(12)–C(13)–
C(14)–C(15) around 110° both in the racemic and homochiral
assemblies. The homochiral molecules distort unidirectionally
and the repulsion in between each other set the Cu and O atoms
apart from each other for at least 3.19(5)Å, much longer than the sum
of their ionic radii. While in the case of racemic molecular packing, the
(R,R) and (S,S)-molecules distort in the opposite direction, (dihedral
angles C(12)–C(13)–C(14)–C(15) are 108.1(3)° for (R,R)-molecules
and −108.1(3)° for (S,S)-molecules, respectively), which allows a
couple of molecules with the mirror handness packing much closer
than they could with the same chirality, thus make it possible for the
Cu and O atoms to be close enough to form bonds.
The reason of different color between homochiral and racemic
solid complexes is assumed from the different crystal structures
(Fig. 3). As shown in Fig. 4 and Table 1, apical Cu–O bonds are found
only in the racemic-1. The Cu–N coordination bond lengths in the
equatorial square plane are 1.917–2.033 Å for racemic-1, similar to
the literature data, [8] but a little longer than that for optically active
(R,R)-1(1.901–2.011 Å), manifesting a little looser of the coordination
between Cu and the ligand in racemic-1. The difference also exists in
the distortion angles of the equatorial square plane. N3 deviates off
O
the N(1)–N(4)–Cu(1) plane about 164.2(5) for racemic-1, signifi-
cantly more than that for (R,R)-1 (179.3(1)O). Looser coordination of
Cu-ligand and more distorted Cu centered plane are adopted for the
racemic-1 to make it possible to form an intermolecular apical
coordination bond. The distance of Cu(1)–O(1) in racemic-1 was
elongated to 2.45(1)Å, as shown in Fig. 4, longer than the typical
intramolecular Cu–O planar bond length around 1.95 Å, [8,9] but
similar to Cu–O apical bond distances around 2.3 Å and some
intermolecular Cu–O bond lengths around 2.373 and 2.858 Å. [10]
Meanwhile, in the case of (R,R)-1, the distance of Cu–O atoms (3.19
(5)Å) seems to be too long to form effective bond. As a result, the
Fig. 2. CD spectra (up) and UV–vis absorption spectrum (bottom) of the (R,R)-1 in
Fig. 1. The different crystal colors of (R,R)-1, (S,S)-1(red) and racemic-1 (purple).
CH2Cl2.

Z. Jiang et al. / Inorganic Chemistry Communications 14 (2011) 13–16
15
Fig. 3. Crystal structures of (R,R)-1 (left) and the (R,R) part of racemic-1(right).
Another evidence is the different color of chiral and racemic
complexes in solvent. For example, both of racemic-1 and homochiral
1 are red in CH2Cl2 and purple in methanol, which indicated the
possibility of coordination of methanol with central metal. [11]
In conclusion, molecular chirality could not only influence the
thermodynamic and chiral-related optical properties, but also
influence chiral-unrelated optical properties. Circularly unpolarized
visible light could interact in different way with chiral supramolecular
system and corresponding racemic one, which caused the color
changes between them.
Acknowledgements
We appreciate the Natural Science Foundation of China (Nos.
20672075, 20771076, 20901052), Sichuan Provincial Foundation
(08ZQ026-041) and Ministry of Education (NCET-10-0581) for the
financial support.
Appendix A. Supplementary material
CCDC 779242–779244 contains the supplementary crystallographic
data for (R,R)-1, racemic-1 and (S,S)-1 respectively. The data can be
obtained free of charge via bhttp://www.ccdc.cam.ac.uk/conts/retrieving.
htmlN, or from the Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB2 1EZ, UK; fax:(+44)1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk. Additional structural figure, spectrum and
computational detail are available as electronic supplementary informa-
tion in the online version, at doi:10.1016/j.inoche.2010.09.016.
References
Fig. 4. Crystal packing drawing of (R,R)-1 (up) and racemic-1 (bottom).
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Table 1
Selected bond lengths [Å] and torsion angles [°].
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R,R-1
Racemic-1
Cu(1)–N(1)
Cu(1)–N(4)
Cu(1)–N(3)
Cu(1)–N(2)
1.909(3)
1.919(3)
1.978(3)
2.011(3)
1.236(5)
1.241(5)
3.19(5)
179.3(1)
31.8(6)
−109.4(5)
1.917(2)
1.941(2)
1.991(2)
2.033(2)
1.235(4)
1.261(3)
2.45(1)
164.2(5)
30.7(0)
108.1(3)
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O(1)–C(20)
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[6] Synthesis of 1,2-[bis(6′-pyridine-2′-carboxamido)-ethane]cyclohexane (2) A
solution of 0.114 g (1 mmol) 1,2-diaminocyclohexane in 60 mL dry pyridine was
slowly added to a solution of the acid 3 0.303 g (1 mmol) in 60 mL pyridine. After
heating the solution to 50 °C, 0.62 g (2 mmol) triphenylphosphite was added
O(2)–C(7)
O(1)–Cu(1)a
N(1)–N(4)–Cu(1)–N(3)
N(1)–N(4)–Cu(1)–N(2)
C(12)–C(13)–C(14)–C(15)
a
Intermolecular O(1)–Cu(1).

16
Z. Jiang et al. / Inorganic Chemistry Communications 14 (2011) 13–16
dropwise with continuous stirring. The temperature was then raised and kept at
α = 90°, β = 90°, γ = 90°, V = 3520.7(5)Å3
,
Z = 8, μ = 1.265 mm− 1
,
100 °C for 9 h, the mixture was cooled down. After removal of pyridine under
reduced pressure, the resulting brown oil was dissolved in dichloromethane,
washed with water and sodium bicarbonate solution, and then dried over
magnesium sulphate. After filtration and removal of solvent, The residue was
finally purified by column chromatography on silica gel using hexane-ethyl
acetate mixture as eluent(yield 12%). Selected IR data (KBr, cm-1):v=3318 (N-H),
1674 (C=O), 1656 (C–N), 1590, 1547, 1452, 1385. Elemental Anal. Calc. for
Dc =1.554 Mg m−3, F(00 0)=1704, 25327 reflections collected, 4020 unique
with Rint=0.1691; final R1= 0.0576, wR2= 0.0993, GOF= 1.000 for all data. (S,S)-1,
Color: red; Selected IR data (KBr, cm-1):v=1623.82 ((R,R)-1),C=O); Elemental
Anal. Calc. for C20H20N4O2Cu: C, 58.27; H, 4.86; N, 13.60. Found: C, 58.25 ; H, 4.76 ; N,
13.62 %. MS (ESI): 411.9 (M+). Crystal data for (S,S)-1, CCDC 779244, are almost the
same with (R,R)-1, CCDC 779242. Data collection was performed on a Rigaku
Mercury CCD X-ray diffractometer (Mo Ka, λ=0.71073 Å). The structure has been
solved using direct methods using the SHELX97 program package. [12] All non-
hydrogen atoms were refined anisotropically. The positions of metal atoms and their
first coordination spheres were located from direct-methods E-maps; other non-
hydrogen atoms were found in alternating difference Fourier syntheses and least-
squares refinement cycles and, during the final cycles, refined anisotropically.
Hydrogen atoms were placed in calculated position and refined as riding atoms with
C
20H22N4O2: C, 68.49; H, 6.28; N, 15.98. Found: C, 68.17; H, 6.37; N, 15.79%.
MS (EI): 350.4 (M+). 1H NMR (400 MHz, CDCl3): δ=1.52(m, 2H, Hcylcohexane),
1.86(m, 4H, Hcylcohexane), 2.63(m, 2H, Hcylcohexane), 3.37(m, 4H, -PyCH2CH2Py-),
3.67(s, 2H, Hcylcohexane), 7.23(d, J1 =7.6 Hz, 2H, HPy), 7.67(m, 2H, HPy), 7.81
(d, J2 =7.2 Hz, 2H, HPy), 8.45(s, 2H, Hamide)ppm. 13C NMR(CDCl3, 100 MHz)
δ 166.9, 159.6, 150.1, 137.6, 125.3, 119.3, 56.5, 35.6, 32.3, 25.1 ppm.
[7] General Synthesis of 1: A batch of 0.105 g (0.30 mmol) ligand 2 was dissolved in
3 mL hot ethanol and a solution of 0.060 g (0.30 mmol) of Cu(OAc)2•H2O in 5 mL
of ethanol was then added. The color of the mixture turned into purple
immediately. After the mixture was refluxed for 3 h, the corresponding copper
(II) complex 1 were collected by filtration, washed with ethanol, and dried under
vacuum (99 mg, yield 80%). Crystals suitable for X-ray diffraction were grown
via diffusion of Et2O into a CH3OH/CH2Cl2 solution of the complex. (R,R)-1, Color:
red; Selected IR data (KBr, cm-1):v=1623.82 ((R,R)-1),C=O); Elemental Anal.
Calc. for C20H20N4O2Cu: C, 58.27; H, 4.86; N, 13.60. Found: C, 58.25 ; H, 4.76 ; N,
13.62%. MS (ESI): 411.9 (M+). Crystal data for (R,R)-1, CCDC 779242,
a uniform value of Uiso.
[8] L. Rigamonti, F. Demartin, A. Forni, Copper(II) complexes of salen analogues with
two differently substituted (push-pull) salicylaldehyde moieties. A study on the
modulation of electronic asymmetry and nonlinear optical properties, Inorg.
Chem. 45 (2006) 10976–10989.
[9] M.J. Katz, C.J. Shorrock, R.J. Batchelor, [Au(CN)(4)]− as both an intramolecular and
intermolecular bidentate ligand with [(tmeda)Cu(mu-OH)] dimers: from anti-
ferro- to ferromagnetic coupling in polymorphs, Inorg. Chem. 45 (2006)
1757–1765.
[10] P.A. Papanikolaou, P.C. Christidis, A.T. Chaviara, An experimental and density
functional study of the interaction of CuII complexes of diethylenetriamine (Dien)
with pyridine, nicotinic acid, and nicotinamide: the crystal structure of [Cu(dien)
(nicotinamide)(NO3)2], Eur. J. Inorg. Chem. 10 (2006) 2083–2095.
[11] V. Chandrasekhar, T. Senapati, E.C. Sañudo, R. Clérac, Tri-, tetra-, and hexanuclear
copper(II) phosphonates containing N-donor chelating ligands: synthesis,
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6192–6204.
C
20H20N4O2Cu: Orthorhombic, space group P212121, Mr =411.94, a=7.9450(3)
Å, b=19.4403(6)Å, c=22.5762(7)Å, α=90°, β=90°, γ=90°, V=3487.0(2)Å3,
Z=8, μ=1.277 mm−1, Dc =1.569 Mg m−3, F(00 0)=1704, 24700 reflections
collected 7910 unique with Rint =0.1320; final R1 =0.0642, wR2 = 0.1006,
GOF=0.945 for all data. Racemic-1, Color: red; Selected IR data (KBr, cm-1):
v=1636.92 (racemic-1, C=O); Elemental Anal. Calc. for C20H20N4O2Cu: C, 58.27;
H, 4.86; N, 13.60. Found: C, 58.25 ; H, 4.76 ; N, 13.62 %. MS (ESI): 411.9 (M+).
Crystal data for racemic-1, CCDC 779243, C20H20N4O2Cu: Orthorhombic, space
group Pbca, Mr =411.94, a=19.3385(14)Å, b= 8.2962(6)Å, c=21.9447(17)Å,
[12] G.M. Sheldrick, SHELX97, program for crystal structure refinement, University of
Göttigen, Germany, 1997.