Preferential formation of homochiral helical sandwich-shaped architectures through the metal-mediated assembly of tris(imidazoline) ligands with a set of d3-d10 transition-metal ions

Article abstract of DOI:10.1002/chem.200801154

A novel type of chiral trismonodentate imidazolinyl ligands ((S,S,S)-4 and (R,R,R)-4) has been achieved in good yields. The ligands show a strong tendency to induce the generation of the discrete sandwich-shaped M₃L ₂ architectures with programmed helicity through the edge-directed complexation with a series of d³-d¹⁰ transition-metal ions, while taking advantage of the steric hindrance of the bulky substituents of the imidazoline rings to avoid the formation of extended metal-organic frameworks (MOFs). In spite of different coordination geometries, monovalent metal ions (e.g. Ag⁺), divalent metal ions (e.g. Pd²⁺, Cu²⁺, Cd²⁺, Zn²⁺, Co²⁺, Mn ²⁺, and Ni²⁺), and even trivalent metal ions (e.g. Fe ³⁺ and Cr³⁺) exhibit isostructural coordination. Installation of stereo-centers fused onto the imidazoline rings results in favored handedness of the self-assemblies through the expression of molecular chirality into supramolecular helicity. In the crystal structures of [M ₃{(S,S,S)-4}₂], the self-assembly has to adopt the M form to relax the van der Waals repulsions of the phenyl and isopropyl groups. The replacement of (S,S,S)-A with (R,R,R)-4 exclusively affords the opposite helicity (P). These results should provide important insights for the design of chiral helical capsule-like assemblies.

Full text of DOI:10.1002/chem.200801154

FULL PAPER
DOI: 10.1002/chem.200801154
Preferential Formation of Homochiral Helical Sandwich-Shaped
Architectures through the Metal-Mediated Assembly of Tris(imidazoline)
Ligands with a Set of d3–d10 Transition-Metal Ions
Liwei Yan,[a] Zhen Wang,[a] Ming-Tsz Chen,[b] Ningjie Wu,[a] Jingbo Lan,[a] Xin Gao,[a]
Jingsong You,*[a] Han-Mou Gau,[b] and Chi-Tien Chen*[b]
Abstract: A novel type of chiral tris-
monodentate imidazolinyl ligands
spite of different coordination geome-
tries, monovalent metal ions (e.g.
the self-assemblies through the expres-
sion of molecular chirality into supra-
molecular helicity. In the crystal struc-
((S,S,S)-4 and (R,R,R)-4) has been ach-
ieved in good yields. The ligands show
a strong tendency to induce the genera-
tion of the discrete sandwich-shaped
M3L2 architectures with programmed
helicity through the edge-directed com-
plexation with a series of d3–d10 transi-
tion-metal ions, while taking advantage
of the steric hindrance of the bulky
substituents of the imidazoline rings to
avoid the formation of extended
metal–organic frameworks (MOFs). In
Ag+), divalent metal ions (e.g. Pd2+
,
Cu2+, Cd2+, Zn2+, Co2+, Mn2+, and
Ni2+), and even trivalent metal ions
(e.g. Fe3+ and Cr3+) exhibit isostructur-
al coordination. Installation of stereo-
centers fused onto the imidazoline
rings results in favored handedness of
tures of [M3ACTHNGUTERN{NUG (S,S,S)-4}2], the self-assem-
bly has to adopt the M form to relax
the van der Waals repulsions of the
phenyl and isopropyl groups. The re-
placement of (S,S,S)-4 with (R,R,R)-4
exclusively affords the opposite helicity
(P). These results should provide im-
portant insights for the design of chiral
helical capsule-like assemblies.
Keywords: helical structures · imi-
dazoline · sandwich complexes ·
self-assembly · transition metals
Introduction
chemistry to direct the assembly of small-component mole-
cules into aesthetically appealing helical architectures.[2,3]
Despite significant progress, the creation of nonracemic hel-
icity of assemblies continues to be a challenging and fasci-
nating field of research.[4] One of the key issues in this area
is how to control the expression of helicity from predesigned
organic and inorganic building blocks and to prevent helix
racemization.
The self-assembly through metal–ligand-directed interac-
tion to provide discrete monodispersed and specific struc-
tures has received a great deal of attention due to potential
applications in selective ionophores,[5] chiral memory[6] and
amplification,[7] and molecular capsulation.[8] In particular,
multi-monodentate nitrogen-containing heterocyclic rings
with arene cores have recently been of increasing interest in
the design and construction of a variety of sandwich-like
topologies.[9] Among these discrete architectures, however,
there are only a few examples based on helical structures,
and in most cases the helical assemblies feature the racemic
mixture of right- and left-handed forms.
Self-organization of molecules into helical, nonracemic ar-
chitectures provides one of the most significant structural
features relative to those represented by natural biomole-
cules such as double-stranded helices of nucleic acids and a-
helical polypeptides.[1] In recent years, considerable effort
has therefore been made to introduce helicity into artificial
systems. One promising approach is to utilize coordination
[a] L. Yan, Z. Wang, N. Wu, Dr. J. Lan, X. Gao, Prof. Dr. J. You
Key Laboratory of Green Chemistry and Technology of
Ministry of Education, College of Chemistry,
and State Key Laboratory of Biotherapy, West China Medical School
Sichuan University, 29 Wangjiang Road, Chengdu 610064 (China)
Fax : (+86)28-8541-2203
E-mail: jsyou@scu.edu.cn
[b] M.-T. Chen, Prof. Dr. H.-M. Gau, Prof. Dr. C.-T. Chen
Department of Chemistry, National Chung Hsing University
Taichung 402 (Taiwan)
E-mail: ctchen@mail.nchu.edu.tw
Recent studies of the coordination interaction of 1,3,5-
tris(1-imidazolyl)benzene (1) with transition metals have re-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801154.
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11601

vealed the preferential forma-
tion of a range of extended
metal–organic
frameworks
(MOFs) rather than discrete ar-
chitectures (Scheme 1).[10] In
sharp contrast, the self-assem-
bly of disk-shaped tris-mono-
dentate ligands 2[9a] or 3[9b,f,h,i]
with appropriate metal ions has
resulted in the generation of
helical sandwich-shaped com-
plexes. Three methyl or p-tolyl
groups were speculated to force
Figure 1. Schematic representation of the formation of the sandwich-shaped (M)-[M3(LS)2] complexes from
(S,S,S)-4 and a series of different d3–d10 transition-metal ions.
Results and Discussion
Synthesis of ligands: The tris(imidazoline)benzene (S,S,S)-4
was prepared according to our previously described route in
excellent yield (89%).[13] The synthesis is a straightforward
process, starting from the tris-amido alcohol (S,S,S)-6, which
was easily prepared from readily available (S)-valinol and
benzene-1,3,5-tricarbonyl trichloride 5. Treatment of (S,S,S)-
6 with SOCl2 afforded chloroethyl imidoyl chloride (S,S,S)-
Scheme 1. Disk-shaped C3-symmetric tris-monodentate ligands.
7, followed by chloride displacement with aniline
(Scheme 2). Starting from (R)-valinol, the ligand (R,R,R)-4
was prepared by following the same procedure as described
above for (S,S,S)-4. In addition, there were no atropisomers
of the ligand observed. We rationalized that it might be at-
the neighboring N-containing heterocyclic rings away from
the plane of the central aromatic ring so that metal ions
could be arrayed on the disk plane to form the M3L2 sand-
wich. However, these crystals were obtained as a racemate
of the M and P isomers.[9b,f]
ꢀ
tributed to the rather high rotation barrier along the Caryl
Cimidazoline axis due to the steric buttressing effects by the
phenyl and isopropyl groups of the imidazoline rings.
It is reasonable to assume that installation of stereocen-
ters fused onto the N-containing heterocyclic rings of the
disk-shaped tris-monodentate ligands should not only force
the generation of sandwich-shaped M3L2 architectures
through the edge-directed complexation of the metal ions
with three ditopic tectons[11] while taking advantage of the
steric hindrance of the substituents to avoid the formation
of extended three-dimensional networks, but also lead to fa-
vored handedness of the self-assemblies through the expres-
sion of molecular chirality into supramolecular helicity. Imi-
dazolines are ubiquitously biochemical, biological, and me-
dicinal structures and function as important synthetic inter-
mediates and auxiliaries.[12] However, to date, the use of
metal-mediated assembly of imidazolines to construct supra-
molecular architectures is scarce. Quite recently, we de-
scribed a new addition to the rational design of sterically
and electronically easily tunable chiral bis(imidazoline) li-
gands and further demonstrated their application for highly
enantioselective parent Henry reactions.[13] Herein we pres-
ent a novel type of chiral tris-monodentate imidazolinyl li-
gands ((S,S,S)-4 and (R,R,R)-4) that favor the formation of
sandwich-shaped M3L2 architectures with predetermined
chiral helicity (Figure 1). More interestingly, the self-assem-
bly protocols exhibit isostructural coordination with a set of
d3–d10 transition-metal ions in spite of different coordination
geometries.[8g]
Single crystals of (S,S,S)-4 suitable for X-ray analysis were
obtained by recrystallization from ethyl acetate (EtOAc).
The structure revealed that the three N1 atoms of each imi-
dazoline ring in (S,S,S)-4 are the coordinate sites and are on
the same side of the central benzene plane (Figure 2). As
expected, each imidazoline ring is out of the plane of the
central aromatic ring as a result of the steric interaction that
stems from the phenyl and isopropyl groups of the imidazo-
line rings. The dihedral angles between the three imidazo-
line rings and the central benzene ring are about 388. As a
consequence, such ligands should show a strong tendency to
form dimeric sandwich-shaped architectures with program-
med M or P helicity through the coordination of a metal ion
with two N1 atoms from the different ligands. The metal
ions would then be arranged on the disk plane with metal–
metal distances of several angstroms.
Formation of trinuclear [M3(LS)2] complexes: In this study,
the solution behavior of (S,S,S)-4 with AgNO3 was initially
1
investigated. The H NMR spectrum of a mixture of (S,S,S)-
4 and AgNO3 in a 2:3 ratio shows one set of new signals in
a simple, highly symmetrical pattern, and the signals for the
metal-free ligand disappear completely (Figure 3c). These
results indicate the quantitative formation of a single self-as-
sembled architecture. Surprisingly, we found that these new
signals are independent of the (S,S,S)-4/AgNO3 ratio. When
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Homochiral Helical Sandwich Complexes
FULL PAPER
suggest that a discrete, highly
symmetrical
trinuclear
[Ag3(Ls)2A(NO3)3] complex can
CTHUNGTRENNUNG
be exclusively formed by
mixing (S,S,S)-4 and AgNO3 in
solution.
Although the metal-mediated
assembly of organic ligands to
form discrete architectures has
been investigated extensively,
the number of metals used to
maintain the shapes of the
metal-assembled architectures
is still limited owing to different
coordination geometries. Re-
cently, a series of 10 isostructur-
al octahedron-shaped coordina-
tion nanocapsules were con-
structed from disk-shaped tris-
monodentate ligands and 10 di-
Scheme 2. Synthesis of chiral tris(imidazoline)benzene (S,S,S)-4.
Figure 2. X-ray crystal structure of (S,S,S)-4. Color code: C (light gray),
N (black), and H (omitted).
1
the ratio of (S,S,S)-4/AgNO3 is less than 2:3, the H NMR
spectrum observed is similar to that for a mixture of (S,S,S)-
4 and AgNO3 in a 2:3 ratio. As shown in Figure 3d, further
addition of AgNO3, up to three equivalents, did not change
1
the H NMR spectrum, although the amount of metal ion is
enough for the formation of an extended metal complex.
The sterically hindered interactions arising from the phenyl
and isopropyl groups of the imidazoline rings appear to pre-
vent the generation of polymeric structures. On the other
hand, when the ratio of (S,S,S)-4/AgNO3 is more than 2:3,
the signals for the metal-free ligand clearly remains. For in-
stance, in the presence of an equimolar amount of Ag+ ion,
two sets of signals are assigned to the self-assembled com-
plex and the metal-free ligand (Figure 3b). A further in-
crease in (S,S,S)-4 was not capable of varying the intrinsic
characteristic of the spectrum. The electrospray ionization
time-of-flight (ESI-TOF) mass spectrum of a mixture of
(S,S,S)-4 and AgNO3 in a 2:3 ratio shows a signal at m/z
Figure 3. 1H NMR spectra for (S,S,S)-4 with AgNO3 in CDCl3 at 293 K
(a: the metal-free ligand; a’: complex): a) the metal-free ligand (S,S,S)-4,
b) (S,S,S)-4/AgNO3 =3:3, c) (S,S,S)-4/AgNO3 =2:3, and d) (S,S,S)-4/
AgNO3 =1:3.
valent d5–d10 transition-metal ions by Shionoya et al.[8g] In
addition to monovalent Ag+, we further aimed to use other
d3–d10 transition-metal ions involving divalent and trivalent
metal ions. Remarkably, monovalent metal ions (e.g. Ag+),
divalent metal ions (e.g., Pd2+, Cu2+, Cd2+, Zn2+, Co2+
,
Mn2+, and Ni2+), and even trivalent metal ions (e.g., Fe3+
and Cr3+) exhibit strong isostructural coordination with the
ligand (S,S,S)-4. These self-assembled M3(Ls)2 complexes
could be confirmed by NMR spectroscopy in combination
with ESI-TOF mass measurements, elemental analysis, and/
1720.0, which is assigned to the species [Ag3(Ls)
N
ꢀ
with the loss of one NO3 ion. This is in good agreement
with the convergence ratio of 2:3 for the (S,S,S)-4/Ag+ ob-
served in the 1H NMR study. Thus, these results strongly
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11603

J. You, C.-T. Chen et al.
or X-ray analysis. To the best of our knowledge, examples of
self-assemblies with isostructural coordination for a broad
range of metals are, hitherto, relatively rare. The 1H NMR
studies indicated that the signals for the metal-free ligand
(S,S,S)-4 disappeared completely when 1.5 equivalents of a
metal ion were added (see the Supporting Information). The
1H NMR spectrum of a mixture of (S,S,S)-4 and a diamag-
netic divalent transition-metal ion such as Pd2+, Zn2+, or
Cd2+ at a 2:3 ratio shows a similar pattern to that for the
monovalent Ag+ ion. Although paramagnetic metal com-
plexes are known to give electronic relaxation rates large
enough to prevent NMR observation,[14] in this study the
paramagnetic divalent Ni2+ and Cu2+ as well as the trivalent
Cr3+ afforded one set of relatively sharp signals, which are
similar to those for diamagnetic transition-metal ions. This is
probably the signature of tetrahedral coordination of the
metals, which gives rise to pseudo-triply degenerated (T)
electronic ground states in Td symmetry compatible with
fast electronic relaxation, thus making NMR detection pos-
Figure 4. 1H NMR spectrum for (S,S,S)-4 with CrCl3 in CD3OD at 293 K:
a) the metal-free ligand (S,S,S)-4, and b) (S,S,S)-4/CrCl3 =2:3.
and [Cr3(Ls)2Cl4]5+, respectively (see the Supporting Infor-
mation).
1
sible. As shown in Figure 4b, the H NMR spectrum for a
mixture of (S,S,S)-4 and CrCl3 in a 2:3 ratio suggests the
formation of a trinuclear [Cr3(Ls)2Cl9] complex. Notably,
even the broadened proton signals for the paramagnetic
Mn2+, Co2+, and Fe3+ ions could approximately demon-
strate the quantitative formation of trinuclear [M3(Ls)2]
complexes.[8g]
X-ray analysis: The structures of the self-assembled com-
plexes of (S,S,S)-4 with AgNO3, PdCl2, and CuCl2·2H2O (8–
10, respectively) were determined by X-ray crystallographic
analyses, which provide unambiguous evidence for the for-
mation of the sandwich-shaped trinuclear [Ag3(Ls)2ACHTUNGTRENNUNG(NO3)3],
The ESI-TOF spectra of the complexes of (S,S,S)-4 and
PdCl2, ZnCl2·2H2O, CdCl2·5H2O, CuCl2·2H2O, CoCl2·6H2O,
MnCl2·4H2O, NiCl2·6H2O, FeCl3·6H2O, and CrCl3·6H2O in
a 2:3 ratio show the peaks at m/z 1769.3, 1771.2, 1821.4,
1787.7, 1755.3, 1740.2, 1751.2, 1816.1, and 1569.4, which cor-
respond to the species [Pd3(Ls)2Cl5]+, [Zn3(Ls)2Cl6·5H2O],
[Pd3(Ls)2Cl6], and [Cu3(Ls)2Cl6] architectures with M helicity
(see Table 1 for X-ray data collection parameters). In these
self-assemblies, the two chiral ligands adopt a face-to-face
orientation and are joined together by three metal ions to
generate the individual sandwich-shaped configuration.
For [Ag3(Ls)2ACTHNUTRGNEUNG(NO3)3] ((M)-8), the two central phenyl rings
[Cd3(Ls)2Cl6],
[Cu3(Ls)2Cl6·6H2O],
[Co3(Ls)2Cl6·5H2O],
are nearly parallel to each other with an angle of 2.548
[Mn3(Ls)2Cl6.5H2O], [Ni3(Ls)2Cl6·5H2O], [Fe3(Ls)2Cl9·3H2O],
(Figure 5), which is different from those for the [Pd3(Ls)2Cl6]
Table 1. X-ray data collection parameters for complexes (M)-8, (M)-9, (M)-10, and (P)-10.[16]
(M)-8
(M)-9
(M)-10
(P)-10
formula
Fw
color
crystal system
space group
a [ꢁ]
b [ꢁ]
c [ꢁ]
a [8]
C84H96Ag3N15O9
1799.37
colorless
monoclinic
P2(1)
18.622(16)
18.604(18)
30.459(3)
90
C84H96Cl6Pd3N12
1805.64
orange
trigonal
R3(2)
18.416(11)
18.416(11)
33.875(2)
90
C84H96Cl6Cu3N12
1677.06
olive-drab
cubic
P2(1)3
20.974(8)
20.974(8)
20.974(8)
90
C84H96Cl6Cu3N12
1677.05
olive-drab
cubic
P2(1)3
20.944(6)
20.944(6)
20.944(6)
90
b [8]
99
90
90
90
g [8]
90
120
9950.3(10)
18
90
90
9187(3)
4
V [ꢁ3]
Z
10414.1(17)
4
1.148
9227.2(6)
12
1.207
1calcd [gcmꢀ3
]
0.904
1.204
F
(000)
3704
2772
3492
3444
T [K]
293(2)
0.0541
0.1189
0.869
0.612
0.01(2)
293(2)
0.0421
0.1016
0.995
0.555
0.01(5)
293(2)
0.0530
0.1212
1.024
0.905
0.05(2)
296(2)
0.0808
0.1906
1.003
0.908
ꢀ0.06(5)
R1[a] (I>2d)
[b]
wR2
GOF
m [mmꢀ1
]
Flack parameter[17]
2
2
[a] R1=ꢀ(jjFojꢀjFc jj)/ꢀjFo j. [b] wR2=[ꢀ(w
G
(Fo )2]1/2
ACHTUNGTERNNUNG .
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Homochiral Helical Sandwich Complexes
FULL PAPER
Figure 5. Crystal structure of (M)-[Ag3(Ls)2ACTHNUTGRNEG(UN NO3)3] ((M)-8). a) side view,
and b) top view. Color code: C (light gray), N (black), Ag (dark gray). H
and NO3ꢀ were omitted for clarity.
and [Cu3(Ls)2Cl6] complexes in which the two central aro-
matic rings are parallel. The face-to-face distance between
the two central aromatic rings is about 4.85 ꢁ, which is
longer than for aromatic p–p stacking,[9b] suggesting that the
formation of the dimeric sandwich-shaped structure can ap-
parently be attributed to metal–ligand-directed coordina-
tion. The Ag+ ion is almost linearly coordinated with the
two nitrogen atoms of the two different tripodal ligands
with N-Ag-N angles of 172.318, 174.348, 179.138, and the
Figure 6. Variable-temperature 1H NMR spectra of the [Ag3(LS)
2ACHTUNGTRENNUNG(NO3)3]
complex ((M)-8) (600 MHz, CD2Cl2): a) 293 K, b) 273 K, c) 253 K, d)
233 K, e) 213 K, and f) 193 K.
ꢀ
Ag N bond lengths are between 2.09(1) and 2.12(6) ꢁ. The
ꢀ
intermetallic Ag Ag distances range from 6.42 to 6.55 ꢁ,
and all nitrate ions are positioned outside the cage near the
three Ag centers. The imidazoline donors are directed
toward the out-of-plane of the C3-symmetric facial ligand,
and the tilting angle of about 48.308 between the central
benzene and the imidazoline ring gives rise to the curvature
needed for the formation of a helical structure. This capsule
has to be “chirally” twisted into a left-handed form accord-
ingly, in which the three N-Ag-N linkages are distorted from
the helix axis by approximately 348. Variable-temperature
1H NMR measurements for the [Ag3(Ls)
2ACHTNUTRGNEG(UN NO3)3] complex
showed that the signals of the imidazoline rings did not split
into two sets that would arise from the mixture of the P and
M isomers between 293 and 193 K, which reveals the exis-
tence of a single stable helical form of the complex in solu-
tion (Figure 6).[9b,f]
For [Pd3(Ls)2Cl6], the coordination environment of the di-
valent Pd2+ center is different from that of the monovalent
Ag+ ion in (M)-8 (Figure 7). Each palladium ion is coordi-
nated with a nearly square-planar geometry by the two Clꢀ
ions and the two imidazoline units. The distance between
the two central aromatic rings is approximately 5.11 ꢁ,
which is the longest among the three crystals. Interestingly,
the dihedral angle between the imidazoline ring and the
central phenyl ring is 51.378, which is also the largest among
Figure 7. Crystal structure of (M)-[Pd3(Ls)2Cl6] ((M)-9). Color code: C
(light gray), N (black), Pd (dark gray), and Cl (gray). All hydrogen
atoms have been omitted for clarity.
rings arranged face-to-face is about 4.09 ꢁ, which is the
shortest among the three crystals. The tilting angle between
the central benzene ring and the imidazoline ring is 47.188,
which is also the smallest among the three self-assemblies.
ꢀ
The Cu Cu distance and the N-Cu-N angle are 6.68 ꢁ and
ꢀ
ꢀ
the three crystals. The Pd Pd distance and the N-Pd-N
157.638, respectively. The N Cu bond lengths range from
ꢀ
angle are 6.19 ꢁ and 177.718, respectively. The N Pd bond
1.949 to 1.968 ꢁ. In the crystal structure of trinuclear
[Cu3(Ls)2Cl6], the self-assembly has to adopt the M helicity
to relax the van der Waals repulsions of the phenyl and iso-
propyl groups on the imidazoline rings, in which all bulky
substituents are away from the Cu2+ ions, completely pre-
venting the formation of the P form. As expected, the use
lengths are about 2.01 ꢁ.
In contrast to [Ag3(Ls)2ACTHNURGTNE(UNG NO3)3] and [Pd3(Ls)2Cl6], each
Cu2+ ion in [Cu3(Ls)2Cl6] ((M)-10) is tetrahedrally coordi-
nated with two Clꢀ ions and two imidazoline moieties (Fig-
ure 8a). The distance between the two central aromatic
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11605

J. You, C.-T. Chen et al.
lowed by a negative Cotton effect at higher wavelength. The
signal passes through zero around 270 nm, indicating the for-
mation of a helical superstructure with a preferred handed-
ness.[4] These observations clarify that the supramolecular
chirality is realized by the chiral translation of the ligand.
Powder X-ray diffraction analysis: We examined the struc-
tural homogeneity of bulk powder samples of (P)-10 by a
comparison of experimental and simulated powder XRD
(PXRD) patterns. As shown in Figure 10, the experimental
pattern favorably correlates with the simulated one generat-
ed from the single-crystal X-ray diffraction data.
Figure 8. Comparison of crystal structures of a) (M)-[Cu3(LS)2Cl6] ((M)-
10) and b) (P)-[Cu3(LR)2Cl6] ((P)-10). Color code: C (light gray), N
(black), Cu (dark gray), and Cl (gray). All hydrogen atoms have been
omitted for clarity.
of the ligand with R configuration exclusively generates the
[Cu3(LR)2Cl6] complex ((P)-10) with right-handed P helicity
(Figure 8b).
All attempts to obtain single crystals of the trinuclear
[Cd3(LS)2Cl6] complex suitable for X-ray analysis were not
successful. Despite the lack of X-ray crystallographic analy-
sis, we could conclude that a single [Cd3(LS)2Cl6] structure
1
was formed quantitatively in solution from the H NMR and
the ESI-TOF mass measurements. The 1H NMR spectrum
of a mixture of (S,S,S)-4 and CdCl2 (2:3) in CD3OD at
293 K is highly symmetrical. Interestingly, the colorless crys-
tals, which were obtained by slow vapor diffusion of diethyl
ether into a DMF solution of the complex prepared from
CdCl2·5H2O and (S,S,S)-4 in methanol, was confirmed to be
the coordination polymer of CdCl2·2DMF by X-ray analysis
Figure 10. Comparison of powder X-ray diffraction patterns: the top and
bottom patterns correspond to the simulated and experimental results,
respectively.
1
(see the Supporting Information).[15] The H NMR study il-
lustrated that the [Cd3(LS)2] complex is unstable in
[D7]DMF, affording signals identical to those of the metal-
free ligand.
Conclusion
In summary, this study demonstrates that a novel kind of
chiral tris-monodentate imidazolinyl ligand facilitates the
formation of sandwich-shaped trinuclear M3L2 architectures
with programmed chiral helicity with a series of d3–d10 tran-
sition-metal ions involving monovalent metal ions (e.g., Ag+),
CD analysis: To further verify the formation of chiral helical
topologies in solution, the circular dichroism (CD) spectra
of (M)-10 and (P)-10 were studied, and the mirroring
Cotton effects were observed in CH3CN (Figure 9). The CD
spectra of (M)-10 showed first a positive Cotton effect fol-
divalent metal ions (e.g., Pd2+, Cu2+, Cd2+, Zn2+, Co2+
,
Mn2+, and Ni2+), and trivalent metal ions (e.g., Fe3+ and
Cr3+). Installation of stereocenters fused onto the imidazo-
line rings leads to favored handedness of the self-assemblies
through the expression of molecular chirality into supra-
molecular helicity. These results could give important clues
to the design of chiral helical capsule-like self-assemblies.
We foresee that these kinds of chiral metal complexes could
impart outstanding chemical and physical properties of tran-
sition-metal ions and could be applicable in asymmetric cat-
alysis.
Experimental Section
1
General remarks: H NMR spectra were obtained on a Varian Inova-400
Figure 9. CD spectra of (M)-10 (gray) and (P)-10 (black) were observed
in CH3CN (c=1.5ꢂ10ꢀ4 m).
(400 MHz) or a Varian Inova-600 (600 MHz) spectrometer, and 13C NMR
spectra were recorded on a Varian Inova-400 (100 MHz) or a Varian
11606
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 11601 – 11609

Homochiral Helical Sandwich Complexes
FULL PAPER
Inova-600 (150 MHz). Mass spectra were obtained on a BioTOF Q or a
Finnigan-LCQDECA instrument. The ESI-TOF mass spectra were record-
ed with a Waters Q-Tof premier instrument. The high-resolution fast
atom bombardment (FAB) mass spectra were obtained with a JEOL
JMS-SX/SX 102 A spectrometer, and the optical rotations were deter-
mined on a WZZ-2B polarimeter. Elemental analyses were performed
with a CARLO ERBA1106 instrument or a Heraeus CHN-O-RAPID in-
strument. Melting points were determined and are uncorrected. CD spec-
tra were recorded on a JASCO J-500C spectropolarimeter. Unless other-
wise noted, all reagents were obtained from commercial suppliers and
used without further purification. Unless otherwise indicated, all synthe-
ses and manipulations were carried out under a dry N2 atmosphere. An-
hydrous solvents were dried by using standard procedures. Benzene-
1,3,5-tricarbonyl trichloride was prepared by a literature procedure.[18]
1551.1 [Ag2(Ls)
[Ag3(Ls)2·2H2O]3+
(NO3)2]+; elemental analysis calcd (%) for C84H96Ag3N15O9: C 56.57, H
G
(NO3)·2H2O]+, 1632.0
1721.9 [Ag3(Ls)2-
(NO3)·H2O]2+
,
AHCTUNGTRENNUNG
5.43, N 11.78; found: C 55.73, H 5.27, N 11.54.
Synthesis of the [Pd3(LS)2Cl6] complex ((M)-9): A solution of (S,S,S)-4
(50 mg, 0.0078 mmol) in methanol (2 mL) was carefully layered over a
solution of PdCl2 (21 mg, 0.0119 mmol) in dimethyl sulfoxide (DMSO).
Orange crystals were isolated after several days in 71% yield. 1H NMR
(600 MHz, CD2Cl2): d=0.86 (d, J=7.2 Hz, 9H), 0.82 (d, J=7.2 Hz, 9H),
2.49–2.55 (m, 3H), 3.15 (dd, J=6.6 Hz, 10.2 Hz, 3H), 3.17 (dd, J=6.6 Hz,
9.6 Hz, 3H), 4.41–4.44 (m, 3H), 6.31 (d, J=7.2 Hz, 6H), 7.11–7.16 (m,
9H), 8.52 (s, 3H) ppm; 13C NMR (150 MHz, CD2Cl2): d=16.0, 17.6, 31.3,
41.0, 69.0, 126.3, 126.5, 127.0, 129.1, 137.1, 140.5, 162.8 ppm; MS (ESI-
TOF) (CH3OH): m/z: 1625.5 [Pd3(Ls)2Cl]5+
,
1734.4 [Pd3(Ls)2Cl4]2+
,
:
1769.4 [Pd3(Ls)2Cl5]+; elemental analysis calcd (%) for C84H96Cl6Pd3N12
C 55.87, H 5.36, N 9.31; found: C 55.76, H 5.39, N 9.12.
Synthesis of 1,3,5-tris((S)-4-isopropyl-1-phenyl-4,5-dihydro-1H-imidazol-
2-yl)benzene ((S,S,S)-4): A solution of benzene-1,3,5-tricarbonyl trichlor-
ide 5 (4.8 g, 18 mmol) in CH2Cl2 (80 mL) was added dropwise to a stirred
solution of (S)-valinol (5.8 g, 56 mmol) and triethylamine (9.4 mL,
68 mmol) in CH2Cl2 (80 mL) at 08C. The reaction mixture was then al-
lowed to warm to room temperature, and stirring was continued for 12 h,
followed by the addition of water (100 mL). The mixture was filtered to
give the corresponding tris-amido alcohol 6 as a white solid (8.0 g, 96%).
A solution of 6 (5.0 g, 10.8 mmol) in SOCl2 (20 mL) was stirred at reflux
for 10 h, and volatiles were then removed under reduced pressure to
afford compound 7. CH2Cl2 (60 mL), Et3N (14.0 mL, 100 mmol), and ani-
line (3.25 mL, 35.5 mmol) were added to the residue at 08C. The result-
ing mixture was allowed to warm to room temperature and stirred for
24 h. The solution was then washed with NaOH (10%, 50 mL), and the
aqueous layer was extracted with CH2Cl2 (3ꢂ60 mL). The combined or-
ganic layers were dried over MgSO4 and the solvent was removed in
vacuo to give a yellow solid, which could be purified by column chroma-
tography on silica gel with elution with ethyl acetate/methanol (3:1) or
by recrystallization from EtOAc to afford (S,S,S)-4 as a white solid (6.1 g,
89%). The single crystals suitable for X-ray analysis were achieved by re-
crystallization from EtOAc. M.p.: 232–2348C; [a]2D5 =349.0 (c=0.5 in
CH3OH); 1H NMR (600 MHz, CDCl3): d=0.83 (d, J=6.6 Hz, 9H), 0.91
(d, J=7.2 Hz, 9H), 1.78–1.84 (m, 3H), 3.47 (dd, J=7.2 Hz, 9.6 Hz, 3H),
3.94 (dd, J=7.2 Hz, 9.6 Hz, 3H), 3.96–3.99 (m, 3H), 6.59 (d, J=7.8 Hz,
6H), 6.94 (t, J=7.2 Hz, 3H), 7.11 (t, J=7.2 Hz, 6H), 7.60 (s, 3H) ppm;
13C NMR (150 MHz, CDCl3): d=17.7, 18.4, 32.8, 56.0, 69.9, 122.8, 123.2,
128.4, 130.1, 131.6, 142.8, 160.0 ppm; HRMS (FAB) calcd for [C42H48N6]+
: 636.3940; found: 636.3935; elemental analysis calcd (%) for C42H48N6: C
79.21, H 7.60, N 13.20; found: C 79.02, H 7.83, N 12.97.
Synthesis of the [Cu3(LS)2Cl6] complex ((M)-10): Reaction of (S,S,S)-4
(50 mg, 0.0078 mmol) with CuCl2·2H2O (20 mg, 0.0117 mmol) gave im-
mediate precipitation of a brown product. Olive-drab crystals of (M)-10
were isolated by liquid diffusion of CH3OH into a DMF solution of the
solid after several days (61% yield). 1H NMR (600 MHz, CD3OD): d=
0.94 (d, J=5.4 Hz, 9H), 0.97(d, J=7.2 Hz, 9H), 1.94–1.94 (m, 3H), 3.91
(br, 3H), 4.28 (br, 3H), 4.47 (br, 3H), 6.80 (d, J=5.4 Hz, 6H), 7.28–7.37
(m, 9H), 7.67 (br, 3H) ppm; 13C NMR (150 MHz, CD3OD): d=17.9,
33.8, 57.8, 64.6, 66.8, 126.3, 128.8, 129.5, 130.9, 133.5, 139.7, 163.2 ppm;
MS (ESI-TOF) (CH3OH): m/z: 1605.8 [Cu3(LS)2Cl4]2+
,
1787.7
[Cu3(Ls)2Cl6·6H2O]; elemental analysis calcd (%) for C84H96Cl6Cu3N12: C
60.16, H 5.77, N 10.02; found: C 59.29, H 5.55, N 9.92.
General synthetic procedure for other [M3(LS)2] complexes: The metal
salt (0.047 mmol, 1.5 equiv) was added to a solution of (S,S,S)-4 (20 mg,
0.031 mmol) in CH3OH (3 mL). After the mixture had been stirred for
4 h at room temperature, the solvent was evaporated to afford the
[M3(Ls)2] complex, which could be purified by recrystallization.
AHCTUNGTRENNUNG
[Cd3(LS)2Cl6] complex: 1H NMR (600 MHz, CD3OD): d=0.83 (d, J=
6.6 Hz, 9H), 0.89 (d, J=6.6 Hz, 9H), 1.94–1.97 (m, 3H), 3.62 (t, J=
7.2 Hz, 3H), 4.13–4.14 (m, 3H), 4.34 (t, J=10.8 Hz, 3H), 6.73 (d, J=
8.4 Hz, 6H), 7.16 (t, J=7.2 Hz, 3H), 7.30 (t, J=8.4 Hz, 6H), 7.63 (s,
3H) ppm; 13C NMR (150 MHz, CD3OD): d=15.9, 18.1, 31.8, 50.5, 54.6,
66.4, 124.8, 126.1, 129.0, 133.0, 139.0, 161.9 ppm; MS (ESI-TOF)
(CH3OH): m/z: 1633.5 [Cd3(Ls)2·H2O]6+
,
1663.5 [Cd3(Ls)2Cl·H2O]5+
,
1716.4
[Cd3(Ls)2Cl3]3+
,
1733.4
[Cd3(Ls)2Cl3·H2O]3+
,
1773.4
[Cd3(Ls)2Cl4·H2O]2+, 1821.4 [Cd3(Ls)2Cl6]; elemental analysis calcd (%)
for C84H96Cl6Cd3N12: C 55.32, H 5.31, N 9.22; found: C 54.76, H 5.71, N
Synthesis of 1,3,5-tris((R)-4-isopropyl-1-phenyl-4,5-dihydro-1H-imidazol-
2-yl)benzene ((R,R,R)-4): Starting from (R)-valinol, the ligand (R,R,R)-4
was prepared following the same procedure as described above for
(S,S,S)-4. Yield: 90%; m.p.: 232–2348C; [a]2D5 =ꢀ349.0 (c=0.5 in
CH3OH); 1H NMR (400 MHz, CDCl3): d=0.85 (d, J=6.4 Hz, 9H), 0.92
(d, J=6.8 Hz, 9H), 1.78–1.85 (m, 3H), 3.48 (dd, J=6.8 Hz, 10.4 Hz, 3H),
3.50 (dd, J=6.8 Hz, 10.4 Hz, 3H), 3.96–4.00 (m, 3H), 6.60 (d, J=8.4 Hz,
6H), 6.95 (t, J=7.2 Hz, 3H), 7.12 (t, J=8.0 Hz, 6H), 7.61 (s, 3H) ppm;
13C NMR (100 MHz, CDCl3): d=17.8, 18.5, 32.9, 56.2, 70.1, 123.0, 123.4,
128.5, 130.3, 131.6, 143.0, 160.1 ppm; HRMS (FAB) calcd for [C42H48N6]+
: 636.3940; found: 636.3931; elemental analysis calcd (%) for C42H48N6: C
79.21, H 7.60, N 13.20; found: C 79.13, H 7.72, N 13.01.
8.99.
1
A
2.02 (br, 3H), 3.64 (br, 3H), 4.24 (br, 3H), 4.44 (br, 3H), 6.77 (br, 6H),
7.20 (br, 3H), 7.34 (s, 6H), 7.74 (br, 3H) ppm; 13C NMR (150 MHz,
CD3OD): d=16.1, 18.3, 33.0, 55.2, 67.7, 126.2, 127.8, 129.5, 130.7, 135.2,
140.5, 163.9 ppm; MS (ESI-TOF) (CH3OH): m/z:1505.5 [Zn3(Ls)2Cl]5+
,
1647.3 [Zn3(Ls)2Cl5]+, 1771.2 [Zn3(Ls)2Cl6·5H2O]; elemental analysis
calcd (%) for C84H96Cl6Zn3N12: C 59.96, H 5.75, N 9.99; found: C 59.12,
H 6.01, N 9.54.
AHCTUNGTRENNUNG
[Co3(LS)2Cl6] complex: 1H NMR (600 MHz, CD3OD): d=2.29 (br), 4.65
(br), 5.63 (br), 6.55 (br), 7.89 (br), 8.52 (br), 8.64 (br) ppm; MS (ESI-
TOF) (CH3OH): m/z: 1626.4 [Co3(Ls)2Cl5]+, 1755.3 [Co3(Ls)2Cl6·5H2O];
elemental analysis calcd (%) for C84H96Cl6Co3N12: C 60.66, H 5.82, N
10.11; found: C 59.98, H 6.11, N 9.87.
Synthesis of the [Ag3(LS)2ACTHNUTRGNE(UNG NO3)3] complex ((M)-8): A solution of (S,S,S)-
4 (50 mg, 0.0078 mmol) in methanol (2 mL) and an aqueous solution of
AgNO3 (1.5 equiv, 1 mL) were added to a flask wrapped with aluminum
foil. The reaction mixture was stirred for 30 min and was then concentrat-
ed to dryness. Single crystals suitable for X-ray analysis were obtained by
slow diffusion of diethyl ether into a solution of the corresponding com-
pound in acetone at ambient temperatures after several days (yield
75%). 1H NMR (600 MHz, CD2Cl2): d=0.80 (d, J=6.6 Hz, 9H), 0.82 (d,
J=6.6 Hz, 9H), 1.82–1.85 (m, 3H), 3.41 (dd, J=6.0 Hz, 10.2 Hz, 3H),
3.42 (dd, J=6.6 Hz, 9.6 Hz, 3H), 4.18–4.22 (m, 3H), 6.49 (d, J=7.2 Hz,
6H), 7.15 (t, J=7.8 Hz, 3H), 7.23 (t, J=8.4 Hz, 6H), 7.49 (s, 3H) ppm;
13C NMR (150 MHz, CD2Cl2): d=16.6, 17.0, 33.0, 55.3, 68.8, 125.7, 126.4,
129.1, 130.6, 131.5, 139.9, 163.0 ppm; MS (ESI-TOF) (CH3OH): m/z:
AHCTUNGTRENNUNG
[Mn3(LS)2Cl6] complex: 1H NMR (600 MHz, CD3OD): d=2.34 (br,
21H), 5.57 (br, 3H), 5.79 (br, 6H), 8.15 (br, 6H), 8.63 (br, 3H), 8.76 (br,
6H), 8.96 (br, 3H) ppm; 13C NMR (150 MHz, CD3OD): d=17.0, 17.2,
32.9, 56.7, 125.0, 126.4, 129.6, 130.3, 132.0, 141.0 ppm; MS (ESI-TOF)
(CH3OH): m/z: 1507.6 [Mn3(Ls)2Cl2]4+, 1614.4 [Mn3(Ls)2Cl5]+, 1740.2
[Mn3(Ls)2Cl6·5H2O]; elemental analysis calcd (%) for C84H96Cl6Mn3N12
C 61.10, H 5.86, N 10.18; found: C 60.78, H 5.98, N 9.77.
[Ni3(LS)2Cl6] complex: 1H NMR (600 MHz, CD3OD): d=0.88–0.94 (m,
AHCTUNGTRENNUNG
18H), 1.82 (br, 3H), 3.77 (m, 3H), 4.15–4.47 (m, 3H), 4.36–4.40 (m, 3H),
6.72–6.76 (m, 6H), 7.23–7.25 (m, 3H), 7.22–7.35 (m, 6H), 7.53–7.55 (m,
:
Chem. Eur. J. 2008, 14, 11601 – 11609
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
11607

J. You, C.-T. Chen et al.
3H) ppm; 13C NMR (150 MHz, CD3OD): d=17.8, 17.9, 33.8, 57.4, 67.8,
125.9, 128.0, 130.7, 133.0, 140.7, 162.8 ppm; MS (ESI-TOF) (CH3OH):
m/z: 1625.3 [Ni3(Ls)2Cl5]+, 1751.2 [Ni3(Ls)2Cl6·5H2O]; elemental analysis
calcd (%) for C84H96Cl6Ni3N12: C 60.68, H 5.82, N 10.11; found: C 60.11,
H 6.11, N, 9.68.
Y. Okamoto, J. Am. Chem. Soc. 2005, 127, 5740–5741; f) M. Seitz, S.
Stempfhuber, M. Zabel, M. Schꢃtz, O. Reiser, Angew. Chem. 2005,
117, 246–249; Angew. Chem. Int. Ed. 2005, 44, 242–245; g) E. V.
Anokhina, Y. B. Go, Y. Lee, T. Vogt, A. J. Jacobson, J. Am. Chem.
Soc. 2006, 128, 9957–9962; h) H.-Y. An, E.-B. Wang, D.-R. Xiao, Y.-
G. Li, Z.-M. Su, L. Xu, Angew. Chem. 2006, 118, 918–922; Angew.
Chem. Int. Ed. 2006, 45, 904–908; i) K. S. Jeong, Y. S. Kim, Y. J.
Kim, E. Lee, J. H. Yoon, W. H. Park, Y. W. Park, S.-J. Jeon, Z. H.
Kim, J. Kim, N. Jeong, Angew. Chem. 2006, 118, 8314–8318; Angew.
ACHTUNGTRENNUNG
[Cr3(LS)2Cl9] complex: 1H NMR (600 MHz, CD3OD): d=1.05 (d, J=
6.6 Hz, 9H), 1.08 (d, J=6.6 Hz, 9H), 2.05 (s, 3H), 4.26 (t, J=8.4 Hz,
3H), 4.40–4.41 (m, 3H), 4.64 (t, J=11.4 Hz, 3H), 6.98 (d, J=7.2 Hz,
6H), 7.41–7.44 (m, 9H), 8.00 (s, 3H) ppm; 13C NMR (150 MHz,
CD3OD): d=17.8, 17.9, 33.6, 58.6, 64.3, 126.8, 130.5, 131.3, 135.3, 136.7,
163.3 ppm; MS (ESI-TOF) (CH3OH): m/z: 1569.4 [Cr3(Ls)2Cl4]5+; ele-
mental analysis calcd (%) for C84H96Cl9Cr3N12: C 57.69, H 5.53, N 9.61;
found: C 57.22, H 5.71, N 9.45.
´
Chem. Int. Ed. 2006, 45, 8134–8138; j) J. Gregolinski, J. Lisowski,
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45, 6122–6126; k) J. Heo, Y.-M. Jeon, C. A. Mirkin, J. Am. Chem.
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Kiehne, A. Lu1tzen, Org. Lett, 2007, 9, 5333–5336.
ACHTUNGTRENNUNG
[Fe3(LS)2Cl9] complex: 1H NMR (600 MHz, CD3OD): d=1.07 (br, 18H),
2.18 (br, 3H), 4.32 (br, 3H), 4.51 (br, 3H), 4.68 (br, 3H), 7.01 (br, 6H),
7.42 (br, 9H), 8.00 (br, 3H) ppm; 13C NMR (150 MHz, CD3OD): d=
19.3, 34.9, 62.1, 65.7, 126.7, 129.4, 131.1, 132.3, 136.5, 140.7, 163.4 ppm;
[5] Selected recent examples for selective ionophores, see: a) Z. Grote,
M.-L. Lehaire, R. Scopelliti, K. Severin, J. Am. Chem. Soc. 2003,
125, 13638–13639; b) A. Caballero, V. Lloveras, A. Tꢄrraga, A. Es-
pinosa, M. D. Velasco, J. Vidal-Gancedo, C. Rovira, K. Wurst, P.
Molina, J. Veciana, Angew. Chem. 2005, 117, 2013–2017; Angew.
Chem. Int. Ed. 2005, 44, 1977–1981; c) P. Bꢃhlmann, E. Pretsch, E.
Bakker, Chem. Rev. 1998, 98, 1593–1687.
MS (ESI-TOF) (CH3OH): m/z: 1617.3 [Fe3(Ls)2Cl5]4+
,
1652.3
[Fe3(Ls)2Cl6]3+
, 1780.1 [Fe3(Ls)2Cl9·H2O], 1816.1 [Fe3(Ls)2Cl9·3H2O],
1853.0 [Fe3(Ls)2Cl9·5H2O]; elemental analysis calcd (%) for
C84H96Cl9Fe3N12: C 57.31, H 5.50, N 9.55; found: C 56.45, H 5.71, N 9.13.
[6] Selected recent examples for memory, see: a) A. J. Terpin, M. Zie-
gler, D. W. Johnson, K. N. Raymond, Angew. Chem. 2001, 113, 161–
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Raudino, L. M. Scolaro, N. Micali, R. Purrello, J. Am. Chem. Soc.
2002, 124, 894–895; c) M. Ziegler, A. V. Davis, D. W. Johnson, K. N.
Raymond, Angew. Chem. 2003, 115, 689–692; Angew. Chem. Int.
Ed. 2003, 42, 665–668; d) G. A. Hembury, V. V. Borovkov, Y. Inoue,
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Z.-T. Li, Angew. Chem. 2006, 118, 810–814; Angew. Chem. Int. Ed.
2006, 45, 796–800.
[8] Selected examples for molecular capsulation, see: a) N. Takeda, K.
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796; b) F. Hof, S. L. Craig, C. Nuckolls, Jr., J. Rebek, Angew. Chem.
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7939; Angew. Chem. Int. Ed. 2005, 44, 7757–7761; f) S. Hiraoka, K.
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45, 6488–6491; h) K. Harano, S. Hiraoka, M. Shionoya, J. Am.
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Acknowledgements
This work was supported by grants from the National Natural Science
Foundation of China (Nos. 20572074, 20602027 and 20772086), the Pro-
gram for New Century Excellent Talents in University (NCET-04—0881),
and the Outstanding Young Scientist Award of Sichuan Provinces. We
thank the Centre of Testing and Analysis, Sichuan University for NMR,
CD, X-ray, and XRD measurements.
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Received: June 12, 2008
Revised: September 4, 2008
Published online: November 14, 2008
Chem. Eur. J. 2008, 14, 11601 – 11609
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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