Enantioselective assembly structures of copper(II) and zinc(ll) complexes with the 1:1 condensation products of lmidazole-4-carbaldehyde derivatives and dl-phenylalanine

Article abstract of DOI:10.1246/bcsj.82.458

Copper(II) complexes with tridentate ligands H2LR (R = H, 2-Me, and 5-Me) derived from 1:1 condensation products of DL-phenylalanine and imidazole-4-carbaldehyde derivatives (imidazole-4-carbaldehyde, 2-methylimidazole-4-carbaldehyde, and 5-methylimidazol

Full text of DOI:10.1246/bcsj.82.458

458
Bull. Chem. Soc. Jpn. Vol. 82, No. 4, 458–466 (2009)
© 2009 The Chemical Society of Japan
Enantioselective Assembly Structures of Copper(II) and Zinc(II)
Complexes with the 1:1 Condensation Products of
Imidazole-4-carbaldehyde Derivatives and DL-Phenylalanine
Tomotaka Iihoshi,1 Tetsuya Sato,1 Masaaki Towatari,1 Naohide Matsumoto,*1 and Masaaki Kojima2
1Department of Chemistry, Faculty of Science, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555
2Department of Chemistry, Faculty of Science, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530
Received November 17, 2008; E-mail: naohide@aster.sci.kumamoto-u.ac.jp
Copper(II) complexes with tridentate ligands H2LR (R = H, 2-Me, and 5-Me) derived from 1:1 condensation
products of DL-phenylalanine and imidazole-4-carbaldehyde derivatives (imidazole-4-carbaldehyde, 2-methylimidazole-
4-carbaldehyde, and 5-methylimidazole-4-carbaldehyde), [CuClHLH] (1), [CuClHL2-Me] (2), [CuClHL5-Me]¢MeOH (3),
[CuBrHLH] (4a and 4b), and [CuBrHL2-Me] (5), were prepared. Two types of crystals in the reaction vessel, 4a and 4b,
were obtained in the reaction between CuBr2 and H2LH. Their assembled structures were determined by single-crystal
analysis. Except for 4b, the complexes assumed a homochiral chain structure constructed by intrachain imidazole-
carboxylato hydrogen bonds between the adjacent two molecules, and the adjacent chains are linked by interchain Cu-X
(X = Cl or Br) interactions. 3 exhibited a shorter interchain Cu-X distance and crystallized in the acentrosymmetric space
group C2221 representing a spontaneous resolution (conglomerate), while 1, 2, 4a, and 5 gave a stacking of the adjacent
chains with opposite chiralities to give racemic crystals. 4b assumed a homochiral chain structure constructed by a
coordination bond between a copper(II) ion and an oxygen atom of the carboxyl group of the adjacent complex. A zinc(II)
complex, [ZnClHLH] (6), showed a similar racemic crystal to that of 4b.
The fields of crystal engineering and supramolecular
tions may arise from coordination bonds and/or hydrogen
bonds, which are quite strong, selective, and directional. In our
previous papers,6 we have reported the chiral aggregation
process for a copper(II) complex with methylbis{3-[(2-meth-
ylimidazol-4-yl)methyleneamino]propyl}amine [Cu(H2LA)]2+
and a cobalt(III) complex with tris{2-[(imidazol-4-yl)methyl-
eneamino}ethyl]ethyl}amine ([Co(H3LB)]3+). Both metal com-
plexes are chiral molecules with either a C (clockwise) or an
A (anticlockwise) configuration, resulting from the screw
coordination arrangement of the achiral ligand. Their depro-
tonated species, [Cu(HLA)]ClO4 and [Co(H1.5LB)]Cl1.5 (i.e.,
[CoIII(H3LB)][CoIII(LB)]Cl3), were prepared by controlled de-
protonation. In a crystal of [Cu(HLA)]ClO4,6a a homochiral 1D
chain is formed by intermolecular imidazole-imidazolato
hydrogen bonds. In a crystal of [CoIII(H3LB)][CoIII(LB)]Cl3,6f
two components, [CoIII(H3LB)]3+ and [CoIII(LB)]0, which have
the same absolute configuration, are linked by imidazole-
imidazolato hydrogen bonds to form a 2D puckered sheet
structure and the sheets with the same chirality are stacked to
give a homochiral crystal (conglomerate).
In this study, we have focused on the enantioselective
assembly process of copper(II) complexes with tridentate
Schiff-base ligands derived from the 1:1 condensation reaction
of imidazole-4-carbaldehyde derivatives and DL-phenylalanine
(H2LR), [MXHLR] (M = Cu and Zn; X = Cl and Br; R = H,
2-Me, and 5-Me) (Chart 1).7 Since the complex contains
imidazole and chiral amino acid moieties in the tridentate
ligand, the intermolecular interactions8 as well as the chiral
discriminations may arise from coordination bonds and/or
chemistry have attracted much attention in the past two
decades, where an assembly process of well-designed mole-
cules plays an important role.1 Chirality is positioned at the top
of the stereo-structural classes, and the chiral assembly process
is an important subject in chemistry.2 Numerous studies have
been devoted to elucidating the detailed mechanism of
enantioselectivity. When a racemate aggregates and condenses,
it can form the following: (1) a racemic crystal (also called a
racemic compound or a true racemate, in which the two
enantiomers are present in equal quantities in a well-defined
arrangement within the crystal lattice); (2) a conglomerate (a
mechanical mixture of crystals of the pure enantiomers); or (3)
a racemic solid solution (a pseudoracemate).3 The formation of
a conglomerate is rare and most racemates (>90%) crystallize
as racemic crystals. The factors governing conglomerate crys-
tallization have been discussed for chiral organic compounds
and metal complexes.4 The molecular design of a conglomerate
is very simple. If one can find a homochiral discrimination
between neighboring chiral units and then the homochiral
interaction can be extended to the three-dimensional lattice, the
compound will be a conglomerate.5 It can be a reasonable
approach toward forming a conglomerate to begin by studying
the chiral discrimination between the two neighboring units,
and then to extend this to chiral one-dimensional, two-
dimensional, and finally three-dimensional aggregation.
For the purpose of the molecular design of conglomerates, a
self-assembly process involving a metal ion is especially
attractive and useful, where the chiral discriminative interac-
Published on the web April 11, 2009; doi:10.1246/bcsj.82.458

T. Iihoshi et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 4 (2009)
459
hydrogen bonds. Here we report enantioselective assembly
structures of [CuClHLH] (1), [CuClHL2-Me] (2), [CuClHL5-Me]¢
The yields are around 30%, whose relatively small values are
probably due to the high solubilities of the complexes in
crystallization solvent. The yield was not changed by increas-
ing the reaction scale (ca. 2 times large scale). The syntheses of
bromo copper(II) complexes with the three ligands were
attempted. In the reaction of CuBr2 and H2LH, two types of
crystals, [CuBrHLH] (4a) and [CuBrHLH] (4b), were obtained
as blue rhombic and green plate-like crystals in the reaction
vessel, and they were separated manually. [CuBrHL2-Me] (5)
was obtained as blue crystals, but [CuBrHL5-Me] was obtained
as thin needle crystals that were not suitable for X-ray analysis.
A zinc(II) complex, [ZnClHLH] (6) was prepared using zinc(II)
chloride, in order to see the effect of the metal ion. The seven
metal complexes obtained in this study, 1, 2, 3, 4a, 4b, 5, and 6,
showed satisfactory agreement of the C, H, and N elemental
analytical data. Reflectance spectra were measured. The maxima
in the visible region are given in the experimental section. On
the basis of the characteristic IR bands, the seven complexes are
MeOH (3), [CuBrHLH] (4a), [CuBrHLH] (4b), [CuBrHL2-Me
(5), and [ZnClHLH] (6).
]
Results and Discussion
Synthesis and Characterization of Copper(II) and
Zinc(II) Complexes. A tridentate ligand H2LH was prepared
by the 1:1 condensation reaction of imidazole-4-carbaldehyde
and DL-phenylalanine in a mixed solution of methanol and H2O
(2/1 by volume), and the resulting ligand solution was used for
the synthesis of the metal complex without ligands isolation.
Two other ligands, H2L2-Me and H2L5-Me, were prepared by
a similar synthetic method as for H2LH by the use of 2-
methylimidazole-4-carbaldehyde and 5-methylimidazole-4-
carbaldehyde, respectively, instead of imidazole-4-carbalde-
hyde. The chloro copper(II) complexes, [CuClHLH] (1),
[CuClHL2-Me] (2), and [CuClHL5-Me]¢MeOH (3), were prepared
by mixing each of the tridentate ligands and CuCl2¢2H2O in a
mixed solution of methanol and H2O (2/1 by volume). On slow
crystallization at room temperature, 1 and 2 were obtained as
blue plate crystals, and 3 was obtained as blue needle crystals.
divided into two groups. The first group of 1, 2, 3, 4a, and 5
¹1
showed two intense bands at ca. 1635 and 1670 cm
,
assignable to the C=N stretching vibration of the Schiff-base
ligand and the C=O stretching vibration of the carboxylato
group of the DL-phenylalanine moiety, respectively.9 On the
other hand, the second group of 4b and 6 showed bands
at ca. 1640 and 1560 cm¹1, where the band assigned to the
R1
X
M = Cu2+, Zn2+
X = Cl-, Br-
¹1
C=O stretch vibration around 1560 cm was shifted to lower
wavenumber because of the bridging structure.9
HN
N
M
N
O
Structure of [CuIIClHLH] (1). Tables 1 and 2 show the
crystallographic data and relevant bond distances of chloro
copper(II) complexes 1, 2, and 3, respectively. Complex 1
crystallized in the centrosymmetric space group P21/a
(No. 14), indicating that two enantiomers of copper(II) com-
plexes of the tridentate ligands consisting of the D- and L-
phenylalanine moieties are contained in the unit cell. The
molecular structure of 1 with the atom numbering scheme is
shown in Figure 1. The copper(II) ion is coordinated by the
( R1 , R2 )
= ( H , H )
= ( Me , H )
HLH
HL2-Me
O
R2
*
HL5-Me = ( H , Me )
Ph
Chart 1.
Table 1. Crystallographic Data for [CuClHLH] (1), [CuClHL2-Me] (2), and [CuClHL5-Me]¢MeOH (3)
[CuIIClHLH] (1)
[CuIIClHL2-Me] (2)
[CuIIClHL5-Me]¢MeOH (3)
Formula
Fw
C13H12N3O2ClCu
341.26
C14H14N3O2ClCu
355.28
C15H18N3O3ClCu
387.32
Crystal system
Space group
a/¡
b/¡
c/¡
¢/°
V/¡3
T/K
Monoclinic
P21/a (No. 14)
6.992(3)
16.715(7)
11.970(4)
94.17(1)
1395.2(9)
296
Monoclinic
P21/a (No. 14)
6.843(3)
16.727(7)
12.671(5)
90.13(2)
1450(1)
Orthorhombic
C2221 (No. 20)
7.254(3)
17.409(5)
26.921(9)
3399(2)
296
296
Z
D
4
4
8
¹3
calcd/Mg m
1.624
1.759
0.33 © 0.13 © 0.13
0.026
0.070
1.627
1.696
0.31 © 0.22 © 0.18
0.028
0.074
1.513
1.458
0.22 © 0.22 © 0.12
0.056
0.141
1.012
1.34, ¹0.85
0.03(2)
¹1
®(Mo Kα)/cm
Crystal size/mm3
Ra)
wRb)
S
1.012
0.72, ¹0.33
1.041
0.41, ¹0.39
¹3
(¦/µ)max,min/e ¡
Flack parameter
2
2
2 2 1/2
a) R = -««Fo« ¹ «Fc««/-«Fo«. b) wR = [-w(«Fo « ¹ «Fc «)2/-w«Fo « ]
.

460
Bull. Chem. Soc. Jpn. Vol. 82, No. 4 (2009)
Enantioselective Assembly of CuII Complexes
Table 2. Relevant Coordination Bond Lengths/¡, Angles/° for [CuClHLH] (1), [CuClHL2-Me] (2), and
[CuClHL5-Me]¢MeOH (3) at 296 K
[CuIIClHLH] (1)
[CuIIClHL2-Me] (2)
[CuIIClHL5-Me]¢MeOH (3)
Cu-N(1)
Cu-N(3)
Cu-O(1)
Cu-Cl(1)
N(1)-Cu-O(1)
N(1)-Cu-N(3)
N(3)-Cu-O(1)
N(1)-Cu-Cl(1)
N(3)-Cu-Cl(1)
O(1)-Cu-Cl(1)
1.998(1)
1.962(1)
1.946(1)
2.2332(5)
157.41(6)
80.89(6)
80.79(6)
101.47(4)
176.65(4)
96.36(4)
2.007(2)
1.960(2)
1.963(2)
2.2326(6)
158.98(7)
81.23(7)
81.37(7)
102.87(5)
175.15(5)
94.08(5)
1.981(4)
1.942(4)
1.965(4)
2.244(2)
158.2(2)
81.6(2)
80.0(2)
98.0(1)
172.9(2)
98.8(1)
Intermolecular distances
N(2)£O(1)
Cu(1)£Cl(1)
2.723(2)a)
2.8949(5)b)
2.810(2)a)
2.8936(7)b)
2.903(5)c)
2.737(2)d)
D-H£A distances
N(2)-H£O(1)a) D-H
H£A
N(2)-H£O(1)c) D-H
H£A
0.950
1.784
0.950
1.870
0.950
1.957
D-H£A angles
169.1(2)
169.8(2)
173.1(5)
Symmetry operation to the latter atom. a) 1/2 ¹ x, 1/2 + y, 2 ¹ z. b) ¹x, ¹y, 2 ¹ z. c) 3/2 ¹ x, ¹1/2 + y,
3/2 ¹ z. d) 2 ¹ x, y, 3/2 ¹ z.
complex, that is related by a two-fold screw axis along the b
axis (*1 represents a symmetry operation; 1/2 ¹ x, 1/2 + y,
2 ¹ z) with the hydrogen-bond distance of N(2)£O(1)*1 =
2.723(2) ¡. Because of the repeating hydrogen bonds, the
complex forms a chain structure running along the b axis. It
should be noted that the 1D chain is a homochiral chain
consisting of one enantiomer of the D- or L-phenylalanine
moiety, since within a chain, the adjacent copper(II) complexes
are related by a two-fold screw axis. However, the crystal
consists of chains with opposite chiralities, where the adjacent
chains with opposite chiralities are doubly bridged by two
chloride ions due to the core of Cu2Cl2. The Cu2Cl2 core has an
inversion center with the dimensions Cu-Cl = 2.2332(5) ¡ and
Cu-Cl*2 = 2.8949(5) ¡ (*2 represents a symmetry operation;
¹x, ¹y, 2 ¹ z). As shown in Figure 2, the heterochiral 2D
network structure is constructed by two kinds of interaction,
i.e., an intrachain imidazole-carboxylato hydrogen bond and a
very weak interchain Cu-Cl*2 coordination bond, where the
former interaction gives a homochiral chain and the latter gives
a heterochiral stacking.
Substituent Effect on Chiral Aggregation: Structures of
[CuClHL2-Me] (2) and [CuClHL5-Me]¢MeOH (3). In order to
answer the question “How does the methyl substituent of the
imidazole moiety affect the structure assembly?,” two com-
plexes with the methyl group attached at different positions of
the imidazole moiety, [CuClHL2-Me] (2) and [CuClHL5-Me]¢
MeOH (3), were subjected to X-ray structural analyses.
Complex 2 crystallized in the monoclinic centrosymmetric
space group P21/a (No. 14) with similar cell parameters to 1
and assumes a similar crystal structure to 1; that is, 2 assumes a
Figure 1. ORTEP drawing of [CuClHLH] (1) with the
selected numbering scheme at 296 K, where the thermal
ellipsoids are represented at 50% probability level.
ClN2O donor atoms of a chloride ion and an electronically
mono-negative planar tridentate ligand derived from the 1:1
condensation reaction of imidazole-4-carbaldehyde and D- or
L-phenylalanine (HLH), where the carboxylato group of the DL-
phenylalanine moiety is deprotonated but the imidazole group
is not deprotonated. The coordination geometry around the
copper(II) ion is described as square planar, with Cu-N(1)
(imidazole) = 1.998(1) ¡, Cu-N(3) (imine) = 1.962(1) ¡, Cu-
O(1) (carboxylato) = 1.946(1) ¡, and Cu-Cl = 2.2332(5) ¡.
Figure 2 shows the crystal structure of 1, representing the
homochiral 1D assembled structure. The imidazole nitrogen
atom N(2) of the copper(II) complex is hydrogen bonded to the
carboxylato oxygen atom O(1)*1 of the adjacent copper(II)

T. Iihoshi et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 4 (2009)
461
Figure 3. ORTEP drawing of [CuClHL5-Me]¢MeOH (3)
with the selected atom-numbering scheme at 296 K, where
the atom ellipsoids are 50% probability level.
Figure 2. Heterochiral 2D network structure of [CuClHLH]
(1), where red- and green-colored molecules represent two
enantiomers consisting of D- and L-phenylalanine moieties,
respectively. The homochiral chain is constructed by
intermolecular imidazole-carboxylato hydrogen bonds
(blue dots). The adjacent chains with opposite chiralities
are linked by interchain Cu-Cl*2 coordination bond
(yellow, *2 represents a symmetry operation of ¹x, ¹y,
2 ¹ z). (a) Side view of the 2D structure and (b) top view
of the 2D structure.
2D network structure consisting of homo-chiral 1D chains
resulting from the imidazole-carboxylato hydrogen bonds and
heterochiral stacking of the 1D chain with opposite chiralities
because of the weak interchain Cu-Cl bridge.
On the other hand, complex 3 shows a different crystal
structure from 1 and 2. Complex 3 crystallized in the non-
centrosymmetric space group C2221 (No. 20), suggesting that a
spontaneous resolution occurred. The molecular structure with
the atom-numbering scheme is shown in Figure 3. The
copper(II) ion assumes a square-planar coordination geometry
with ClN2O donor atoms and the coordination bond distances
of Cu-N(1) (imidazole) = 1.981(4) ¡, Cu-N(3) (imine) =
1.942(4) ¡, Cu-O(1) (carboxylato) = 1.965(4) ¡, and Cu-
Cl = 2.244(2) ¡, whose geometry is similar to that of 1 and
2. Figure 4 shows the 2D network structure of 3, in which the
imidazole group of a copper(II) complex is hydrogen bonded to
the carboxylato oxygen atom O(1)*3 (*3 represents a symmetry
operation; 3/2 ¹ x, ¹1/2 + y, 3/2 ¹ z) of the adjacent
copper(II) complex with the distance of N(2)£O(1)*3 =
2.903(5) ¡. Because of the repeating NH£O hydrogen bonds,
the molecules with the same chirality produce a homochiral
chain structure, the same as 1 and 2. In the crystal, the adjacent
chains with the same chirality (D-form) are stacked and linked
Figure 4. Homochiral
2D
network
structure
of
[CuClHL5-Me]¢MeOH (3), where the red-colored molecule
represents one enantiomer consisting of the D-phenyl-
alanine moiety. The homochiral chain is constructed by
intrachain imidazole-carboxylato hydrogen bonds (blue
dots). Adjacent chains with the same chirality are linked by
the interchain coordination bond (yellow). (a) Side view of
the 2D structure and (b) top view showing the stacking
manner of the two adjacent chains.
by interchain Cu-Cl*4 coordination bonds with Cu-Cl*4 =
2.737(2) ¡ (*4 represents the symmetry operation; 2 ¹ x, y,
3/2 ¹ z) to produce a homochiral 2D network structure. It
should be noted that the interchain Cu-Cl*4 distance of 3 is
shorter than that of 1 (2.8949(5) ¡) and 2 (2.8936(7) ¡). It is
also noteworthy that the crystal involves a methanol per the
chemical formula as the crystal solvent and the methanol is

462
Bull. Chem. Soc. Jpn. Vol. 82, No. 4 (2009)
Enantioselective Assembly of CuII Complexes
Table 3. Crystallographic Data for [CuBrHLH] (4a), [CuBrHLH] (4b), [CuBrHL2-Me] (5), and [ZnClHLH] (6)
[CuBrHLH] (4a)
[CuBrHLH] (4b)
[CuBrHL2-Me] (5)
[ZnClHLH] (6)
Formula
Fw
C13H12N3O2BrCu
385.71
C13H12N3O2BrCu
385.71
C14H14N3O2BrCu
399.73
C13H12N3O2ClZn
343.09
Crystal system
Space group
a/¡
b/¡
c/¡
¢/°
V/¡3
T/K
Monoclinic
P21/a (No. 14)
7.222(3)
16.697(5)
11.949(6)
95.64(2)
1434.0(10)
296
Monoclinic
P21/c (No. 14)
12.103(6)
9.381(4)
13.035(7)
106.81(2)
1416.7(12)
180
Monoclinic
P21/a (No. 14)
7.023(2)
16.856(5)
12.626(6)
91.42(2)
1494.2(9)
296
Monoclinic
P21/c (No. 14)
12.051(7)
9.494(4)
12.877(6)
105.15(2)
1422.1(11)
296
Z
D
4
4
4
4
¹3
calcd/Mg m
1.786
4.3213
0.45 © 0.37 © 0.18
0.026
0.061
1.808
4.3740
0.22 © 0.13 © 0.07
0.048
0.081
1.777
4.1504
0.24 © 0.16 © 0.14
0.028
0.048
1.602
1.9181
0.17 © 0.13 © 0.05
0.039
0.047
¹1
®(Mo Kα)/cm
Crystal size/mm3
Ra)
wRb)
S
1.034
0.66, ¹0.52
0.907
1.42, ¹1.28
1.022
0.67, ¹0.81
1.007
0.96, ¹1.00
¹3
(¦/µ)max,min/e ¡
2
2
2 2 1/2
a) R = -««Fo« ¹ «Fc««/-«Fo«. b) wR = [-w(«Fo « ¹ «Fc «)2/-w«Fo « ]
.
Figure 5. ORTEP drawing of [CuBrHLH] (4b) with the
atom-numbering scheme at 180 K, where the thermal
ellipsoids are presented at 50% probability level.
hydrogen bonded to O(2) of the amino acid moiety with a
distance of 3.00(2) ¡. Although the elemental analyses of 3 and
the large thermal parameters of the methanol molecule
suggested the easy elimination of the crystal solvent, it is
certain that the crystal solvent plays a role for the spontaneous
resolution.
Effect of the Bridging Halogen Ion on the Assembly
Structure: Structures of [CuBrHLH] (4a), [CuBrHLH] (4b),
and [CuBrHL2-Me] (5). In order to answer the question “How
does the bridging halogen ion affect the structure assembly?,”
bromo complexes were prepared and studied. The complexes,
[CuBrHLH] (4a), [CuBrHLH] (4b), and [CuBrHL2-Me] (5),
were subjected to X-ray structural analyses. Unfortunately, no
crystals of [CuBrHL5-Me] suitable for X-ray analysis were
obtained. Tables 3 and 4 show the crystallographic data and
relevant bond distances of bromo copper(II) complexes 4a, 4b,
and 5, respectively.
Figure 6. (a) Homochiral 1D chain structure of 4b
constructed from intermolecular Cu-O*5 coordination
bonds (*5 represents the symmetry operation; ¹x,
¹1/2 + y, 3/2 ¹ z). (b) The heterochiral 2D network
structure is constructed by interchain imidazole-Br
hydrogen bonds.
¹
and 2, and assumes a similar crystal structure to 1 and 2. That
is, 4a assumes a square-planar coordination geometry with
ClN2O donor atoms and a 2D network structure consisting of a
homochiral 1D chain resulting from the imidazole-carboxylato
hydrogen bonds and heterochiral stacking of the 1D chains
Complex 4a crystallized in the monoclinic centrosymmetric
space group P21/a (No. 14) with similar cell parameters to 1

T. Iihoshi et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 4 (2009)
463
Table 4. Relevant Coordination Bond Lengths/¡, Angles/° for [CuBrHLH] (4a), [CuBrHLH] (4b), [CuBrHL2-Me] (5),
and [ZnClHLH] (6) at 296 K
[CuBrHLH] (4a) [CuBrHLH] (4b) [CuBrHL2-Me] (5)
[ZnClHLH] (6)
Cu-N(1)
Cu-N(3)
Cu-O(1)
Cu-Br(1)
Cu-O(2)a)
1.9917(16)
1.9581(19)
1.9390(15)
2.3721(3)
2.018(3)
1.957(3)
2.040(2)
2.6043(9)
1.929(2)
2.0131(18)
1.958(2)
1.9581(16)
2.3678(4)
Zn(1)-N(1)
Zn(1)-N(3)
Zn(1)-O(1)
Zn(1)-Cl(1)
Zn(1)-O(2)a)
2.101(2)
2.091(2)
2.2082(19)
2.2607(12)
1.993(2)
N(1)-Cu-O(1)
N(1)-Cu-N(3)
N(3)-Cu-O(1)
N(1)-Cu-Br(1)
N(3)-Cu-Br(1)
O(1)-Cu-Br(1)
N(1)-Cu-O(2)a)
N(3)-Cu-O(2)a)
O(1)-Cu-O(2)a)
Br(1)-Cu-O(2)a)
158.19(8)
81.17(8)
81.00(7)
100.29(6)
175.84(5)
96.70(5)
149.58(14)
80.90(14)
78.73(12)
110.51(12)
99.76(13)
95.08(10)
105.26(13)
162.61(15)
88.80(11)
93.32(10)
160.13(8)
81.49(8)
81.71(8)
102.56(6)
174.23(5)
93.52(5)
N(1)-Zn(1)-O(1)
N(1)-Zn(1)-O(2)a)
N(1)-Zn(1)-N(3)
N(1)-Zn(1)-Cl(1)
N(3)-Zn(1)-O(1)
N(3)-Zn(1)-O(2)a)
N(3)-Zn(1)-Cl(1)
O(1)-Zn(1)-Cl(1)
O(1)-Zn(1)-O(2)a)
139.01(9)
108.05(9)
77.50(9)
113.58(8)
72.80(7)
146.65(10)
107.04(8)
101.84(6)
83.81(7)
Cl(1)-Zn(1)-O(2)a) 100.69(7)
Intermolecular distances
N(2)£O(1)b)
Cu(1)£Br(1)c)
N(2)£Br(1)d)
2.724(2)
3.0045(3)
2.875(2)
3.0363(3)
3.304(3)
N(2)£Cl(1)d)
3.214(2)
D-H£A distances
N(2)-H£O(1)b) D-H 0.950
H£A 1.780
0.950
1.929
N(2)£Br(1)d)
D-H
H£A
0.950
2.367
0.950
2.279
D-H£A angles
172.0(2)
168.6(4)
173.4(2)
167.7(3)
Symmetry operation to the a) ¹x, ¹1/2 + y, 3/2 ¹ z. b) 1/2 ¹ x, 1/2 + y, 2 ¹ z. c) ¹x, ¹y, 2 ¹ z. d) x, ¹1/2 ¹ y, 1/2 + z.
with the opposite chiralities because of the weak interchain
Cu£Br bridge (3.0045(3) ¡).
axis so that the complexes within a chain involve only one
enantiomer of the D- or L-phenylalanine residue. However, the
chains with opposite chiralities are stacked alternately to give a
racemic crystal, where an interchain hydrogen bond with
N(2)£Br*6 = 3.304(3) ¡ (*6 represents the symmetry opera-
tion; x, ¹1/2 ¹ y, 1/2 + z) is operating to produce a hetero-
chiral 2D network structure.
On the other hand, 4b assumes a different coordination
geometry and a different assembly structure. The molecular
structure of 4b with the selected atom-numbering scheme and
crystal structure are shown in Figures 5 and 6, respectively. As
¹
shown in Figure 5, a Br ion coordinates to a copper(II) ion,
but it is not in the plane of the tridentate ligand. An oxygen
atom O(2)*5 (*5 represents the symmetry operation; ¹x,
¹1/2 + y, 3/2 ¹ z) of the amino acid moiety of the adjacent
copper(II) complex coordinates to the copper(II) ion with a
distance of O(2)*5-Cu(1) = 1.929(2) ¡ and occupies one of the
four equatorial coordination sites. The copper(II) ion has a
square-pyramidal coordination geometry with BrN2O2 donor
Effect of the Central Metal Ion on the Assembly
Structure: Structure of [ZnClHLH] (6). In order to answer
the question “How does the central metal ion affect the
structure assembly?,” a zinc(II) complex, [ZnClHLH] (6) was
subjected to an X-ray analysis. Complex 6 exhibited the same
features in the IR bands and crystallized in the monoclinic
centrosymmetric space group P21/c (No. 14) with similar cell
parameters to those of 4b. The molecular structure and the
crystal structure are shown in Figures 7 and 8, respectively.
Zinc(II) ion has a pentacoordinated geometry with a tridentate
ligand and chloride ion, and an amino-carboxylato oxygen
atom of the adjacent molecule. The coordination geometry is
better described as a trigonal bipyramid than a square pyramid,
in comparison with that of 4b. The complex assumes a
polynuclear chain structure constructed by the intermolecular
coordination bonds running along the b axis, where within a
chain the two adjacent zinc(II) complexes are related by a two-
¹
atoms, where the axial site is occupied by a Br ion and the
equatorial sites are occupied by a tridentate ligand and an
oxygen atom of the adjacent molecule. For that reason, the IR
band of C=O stretching vibration appeared at the lower
wavenumber. As a result of the repeating coordination bonds
of O(2)*5-Cu(1), the complex produces a polynuclear chain
structure running along the b axis, where the bridging
conformation of the carboxylato group of the amino acid
moiety is a triatomic syn-anti type. Within a chain the two
adjacent copper(II) complexes are related by a two-fold screw

464
Bull. Chem. Soc. Jpn. Vol. 82, No. 4 (2009)
Enantioselective Assembly of CuII Complexes
Figure 7. ORTEP drawing of [ZnClHLH] (6) with the
selected atom numbering scheme at 296 K, where the atom
ellipsoids are 50% probability level.
Figure 9. Field dependence of the magnetization of M at
2 K as a function of the applied magnetic field H for 3 ( )
and 4b ( ). The broken dotted line represents the
theoretical curves for the Brillouin functions for CuII ion
S = 1/2 and g = 2.00.
can be described as that di-®-chloro dinuclear copper(II)
species are bridged by hydrogen bonds of inter-dimer imida-
zole-amino hydrogen bonds to form a 2D network structure.
The magnetic data suggest that a weak antiferromagnetic
interaction at di-®-chloro dinuclear copper(II) species is
predominant at 2 K. Compound 4b is described as a carbox-
ylato-bridged copper(II) 1D chain with tri-atomic syn-anti
bridging conformation, whose chains are linked by interchain
Figure 8. Homochiral 1D chain structure of 6 constructed
by Zn-O*5 coordination bond and racemic 2D network
structure. The heterochiral 2D network structure is
¹
imidazole-Br hydrogen bonds to form 2D network structure.
A
number of one-dimensional (1D) carboxylato-bridged
copper(II) complexes has been synthesized and their magnetic
properties have been extensively studied.10 In these complexes,
a carboxylato group can assume many types of bridging
conformations, such as triatomic syn-syn, anti-anti, and syn-
anti types. It is known that the 1D copper(II) complexes with
syn-anti bridging conformation assume weak ferro- or anti-
ferro-magnetic interaction between the adjacent CuII centers
to give a ferromagnetic or antiferromagnetic chain. The
present complex 4b did not show apparent character of a
1D ferromagnetic chain, probably due to the weak magnetic
interaction within a chain and co-existence of weak inter- and
intra-chain magnetic interactions, demonstrating that the
magnetic interaction is too weak to detect the character of the
1D chain. As a whole, the magnetic data are not so informative
to distinguish the difference of the network structures.
¹
constructed by interchain imidazole-Cl hydrogen bonds.
fold screw axis and have the same chirality. However, the
adjacent two chains with opposite chiralities are linked by the
interchain N(2)£Cl*6 (*6 represents the symmetry operation;
x, ¹1/2 ¹ y, 1/2 + z) hydrogen bond with N(2)£Cl*6 =
3.214(2) ¡ to produce a racemic 2D network structure.
Magnetic Properties of 3 and 4b. Compounds 3 and 4b
were subjected to the magnetic studies, as the representative
compounds. The temperature dependences of the magnetic
susceptibilities in the temperature range of 2.0-300.0 K under
the external magnetic field of 0.5 T and the field dependence of
the magnetization at 2.0 K from 0 to 5 T were measured. The
temperature dependences of »AT vs. T for 3 and 4b are
essentially similar to each other. The »AT value at the higher
temperature region (>ca. 20 K) is close to the calculated value
Concluding Remarks
¹1
of 0.375 cm3 K mol expected for one CuII (S = 1/2) ion
Copper(II) and zinc(II) complexes with the tridentate ligands
H2LR of 1:1 condensation products of DL-phenylalanine and
imidazole-4-carbaldehyde derivatives (imidazole-4-carbalde-
hyde, 2-methylimidazole-4-carbaldehyde, and 5-methylimida-
and decreases abruptly to at the lowest temperature (0.17
cm3 K mol¹1 at 2 K for 3; 0.22 cm3 K mol¹1 at 2 K for 4b). The
field dependence of the magnetization at 2.0 K was plotted in
Figure 9 in the form of M/N¢ vs. H and compared with
the Brillouin functions calculated for the isolated copper(II)
molecule (SCu = 1/2). Both magnetization data are lower than
the calculated Brillouin function of g = 2.00 and S = 1/2,
suggesting an operation of antiferromagnetic coupling. The
magnetization of 3 is much lower than that of 4b. Compound 3
zole-4-carbaldehyde), [CuClHLH] (1), [CuClHL2-Me
] (2),
[CuClHL5-Me]¢MeOH (3), [CuBrHLH] (4a and 4b),
[CuBrHL2-Me] (5), and [ZnClHLH] (6), were prepared, since
the imidazole and chiral amino-acid moieties of the complexes
can act as functional groups to construct an intermolecular
network structure. 1, 2, 3, 4a, and 5 assumed a homochiral

T. Iihoshi et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 4 (2009)
1670.2 cm¹1. Yield 34%
465
chain structure constructed by intrachain imidazole-carboxyl-
ato hydrogen bonds, and 4b and 6 assumed a homochiral chain
constructed by intermolecular coordination bonds. Among
seven complexes, only 3 crystallized in the acentrosymmetric
space group C2221 representing a spontaneous resolution
(conglomerate). Spontaneous resolution is achieved by three-
dimensional homo-chiral assembling of chiral molecules. In
order to realize a spontaneous resolution on the basis of a
rational molecular design, we have examined step-wise
processes toward 3D homo chiral assembly,5a,6c,7 that is, (1)
intermolecular homo-chiral recognition between adjacent two
chiral molecules, (2) 1D homo-chiral chain extended by the
homochiral interaction between the two molecules, (3) 2D
homo-chiral layer constructed by homo-chiral assembly be-
tween the chains, and (4) 3D homo-chiral assembly in the
crystal lattice. Although only one of the seven complexes 3 is
successfully displayed spontaneous resolution, all complexes
showed 1D homo-chiral aggregations, where one type of
complexes 1, 2, 3, 4a, 5, and 6 is connected by the hydrogen
bonds and another type of complex 4b is connected by
coordination bonds. It is noted that this molecular system is a
good system to reach a spontaneous resolution by step-wise
process and to know the detailed structural factors toward
spontaneous resolution.
[CuClHL5-Me] (3): Crystal solvent MeOH is eliminated in air
to give the desolvated complex. Anal. Found: C, 47.35; H, 4.17; N,
11.72%. Calcd for [CuClHL5-Me] = C14H14N3O2ClCu: C, 47.33;
H, 3.97; N, 11.83%. IR (KBr disk): ¯C=N 1638.4 cm¹1; ¯C=O
1665.2 cm¹1. Yield 22%
[CuBrHLH] (4a): Anal. Found: C, 40.69; H, 3.20; N, 10.93%.
Calcd for [CuBrHLH] = C13H12N3O2BrCu: C, 40.48; H, 3.14; N,
¹1
10.89%. IR (KBr disk): ¯C=N 1636.8 cm¹1; ¯C=O 1676.0 cm
.
-
max(solid) 474 nm.
[CuBrHLH] (4b): Anal. Found: C, 40.52; H, 3.27; N, 10.96%.
Calcd for [CuBrHLH] = C13H12N3O2BrCu: C, 40.48; H, 3.14; N,
10.89%. IR (KBr disk): ¯C=N 1639.5 cm¹1; ¯C=O 1559.2 cm
¹1
.
-
max(solid) 484 nm.
[CuBrHL2-Me] (5):
Anal. Found: C, 41.68; H, 3.49; N,
10.29%. Calcd for [CuBrHL2-Me] = C14H14N3O2BrCu: C, 42.07;
H, 3.53; N, 10.51%. IR (KBr disk): ¯C=N 1635.8 cm¹1; ¯C=O
1668.6 cm¹1. -max(solid) 487 nm.
[ZnClHLH] (6). To a solution of the ligand (0.5 mmol) was
added a solution of ZnCl2 (0.068 g, 0.5 mmol) in methanol (5 mL).
The mixture was warmed and stirred for 30 min and then filtered.
The filtrate was allowed to stand for several days, during which
time crystals precipitated. They were collected by suction filtration,
washed with methanol and dried. Anal. Found: C, 45.84; H, 3.86;
N, 12.39%. Calcd for [ZnClHLH] = C13H12N3O2ClZn: C, 45.51;
H, 3.53; N, 12.25%. IR (KBr disk): ¯C=N 1647.8 cm¹1; ¯C=O
Experimental
¹1
1673.1 cm
.
General and Materials. All chemicals and solvents were
obtained from Tokyo Kasei Co., Ltd., and Wako Pure Chemical
Industries, Ltd. These were of reagent grade and were used for the
syntheses without further purification. All the synthetic procedures
were carried out in an open atmosphere.
Physical Measurements. C, H, and N elemental analyses were
performed by Miss. Kikue Nishiyama at the Center for Instru-
mental Analysis of Kumamoto University. Infrared spectra were
recorded at room temperature using a Nicolet Avatar 370 DTGS
(Thermo Electron Corporation) spectrometer with samples in KBr
disks. Magnetic data were measured by a MPMS5 SQUID
susceptometer (Quantum Design). Magnetic susceptibilities were
measured in a 2-300 K temperature range under an applied
magnetic field of 0.5 T. Magnetization at 2 K was measured from 0
to 5 T. The apparatus was calibrated with palladium metal.
Corrections for diamagnetism were applied by using Pascal’s
constants. Reflectance spectra were measured on a Shimadzu UV-
2450 UV-visible spectrophotometer.
Crystallographic Data Collection and Structure Determina-
tion for [MXHLR] (M = Cu and Zn; X = Cl and Br; R = H,
2-Me, and 5-Me) 1-6. All measurements were made on a Rigaku
RAXIS RAPID-F imaging plate area detector with graphite
monochromated Mo K¡ radiation (- = 0.71075 ¡). The data were
collected to a maximum 2ª value of 55°. The data except for 4b
were collected at 296 K, while the data of 4b were collected at
180 K. Absorption corrections were applied. The data were
corrected for Lorentz and polarization effects. The structures were
solved by direct methods and expanded using the Fourier
technique.11 The structures were refined on F2 full-matrix least-
squares method with anisotropic displacement parameters for all
non-hydrogen atoms. Hydrogen atoms were fixed in their
calculated positions and refined by using a riding model. The
atomic scattering factors and anomalous dispersion terms were
taken from the standard compilation.12 All calculations were
performed by using the Crystal Structure crystallographic software
package.13 The crystal data collection and refinement parameters
are given in Tables 1 and 3. X-ray crystallographic files in CIF
format for seven compounds are deposited at the Cambridge
Crystallographic Data Centre at the deposit numbers CCDC
299424, 604561, and 714836-714840.
Tridentate Ligands H2LR (R = H, 2-Me, and 5-Me). The
tridentate ligands H2LR (R = H, 2-Me, and 5-Me) were prepared
by the 1:1 condensation reactions of DL-phenylalanine and either
imidazole-4-carbaldehyde,
2-methylimidazole-4-carbaldehyde,
and 5-methylimidazole-4-carbaldehyde, respectively, in a mixed
solution of methanol and water (2/1 by volume) on a hot plate for
30 min.
[CuXHLR] (X = Cl and Br; R = H, 2-Me, and 5-Me) 1, 2, 3,
4a, 4b, and 5. The synthetic procedures for all of the copper(II)
complexes with the tridentate ligands are very similar to each other
and the synthesis of [CuClHLH] (1) is exemplified in detail. The
resulting ligand solutions were used for the syntheses of the
copper(II) complexes without the isolation of the ligands. The
tridentate ligand H2LH was prepared by the 1:1 condensation
reaction of DL-phenylalanine (0.083 g, 0.5 mmol) and imidazole-4-
carbaldehyde (0.048 g, 0.5 mmol), in a mixed solution of methanol
(10 mL) and water (5 mL) on a hot plate for 30 min. To a solution
of the ligand (0.5 mmol) was added a solution of CuCl2¢2H2O
(0.085 g, 0.5 mmol) in methanol (5 mL). The mixture was warmed
and stirred for 30 min and then filtered. The filtrate was allowed to
stand for several days, during which time crystals precipitated.
They were collected by suction filtration, washed with methanol
and dried.
[CuClHLH] (1): Anal. Found: C, 45.99; H, 3.56; N, 12.30%.
Calcd for [CuClHLH] = C13H12N3O2ClCu: C, 45.75; H, 3.55; N,
¹1
12.31%. IR (KBr disk): ¯C=N 1635.8 cm¹1; ¯C=O 1674.6 cm
.
-
max(solid) 469 nm.
[CuClHL2-Me] (2):
Anal. Found: C, 47.47; H, 4.19; N,
11.80%. Calcd for [CuClHL2-Me] = C14H14N3O2ClCu: C, 47.33;
H, 3.97; N, 11.83%. IR (KBr disk): ¯C=N 1632.3 cm¹1; ¯C=O

466
Bull. Chem. Soc. Jpn. Vol. 82, No. 4 (2009)
Enantioselective Assembly of CuII Complexes
b) M. Mimura, T. Matsuo, Y. Motoda, N. Matsumoto, T.
Nakashima, M. Kojima, Chem. Lett. 1998, 691. c) I. Katsuki, N.
Matsumoto, M. Kojima, Inorg. Chem. 2000, 39, 3350. d) M.
Mimura, T. Matsuo, T. Nakashima, N. Matsumoto, Inorg. Chem.
1998, 37, 3553. e) Y. Sunatsuki, Y. Motoda, N. Matsumoto, Coord.
Chem. Rev. 2002, 226, 199. f) H. Nakamura, Y. Sunatsuki, M.
Kojima, N. Matsumoto, Inorg. Chem. 2007, 46, 8170.
This work was supported in part by a Grant-in-Aid for
Science Research (No. 16205010) from the Ministry of
Education, Culture, Sports, Science and Technology, Japan.
References
1
a) J.-M. Lehn, Supramolecular Chemistry, VCH,
Weinheim, Germany, 1995. b) L. Fabbrizzi, A. Poggi, Transition
Metals in Supramolecular Chemistry, ASI Kluwer Academic
Publisheres, Dordrecht, The Netherlands, 1994. c) P. J. Stang, B.
Olenyuk, Angew. Chem., Int. Ed. Engl. 1996, 35, 732.
7
T. Iihoshi, S. Imatomi, T. Hamamatsu, R. Kitashima, N.
Matsumoto, Chem. Lett. 2006, 35, 792.
a) G. Kolks, C. R. Frihart, H. N. Rabinowitz, S. J. Lippard,
8
J. Am. Chem. Soc. 1976, 98, 5720. b) N. Matsumoto, Y. Motoda, T.
Matsuo, T. Nakashima, N. Re, F. Dahan, J.-P. Tuchagues, Inorg.
Chem. 1999, 38, 1165. c) M. A. M. Lorente, F. Dahan, Y. Sanakis,
V. Petrouleas, A. Bousseksou, J.-P. Tuchagues, Inorg. Chem. 1995,
34, 5346. d) C. T. Brewer, G. Brewer, M. Shang, W. R. Scheidt, I.
Muller, Inorg. Chim. Acta 1998, 278, 197. e) N. Ohata, H. Masuda,
O. Yamauchi, Angew. Chem., Int. Ed. Engl. 1996, 35, 531. f) Y.
Sunatsuki, H. Ohta, M. Kojima, Y. Ikuta, Y. Goto, N. Matsumoto,
S. Iijima, H. Akashi, S. Kaizaki, F. Dahan, J.-P. Tuchagues, Inorg.
Chem. 2004, 43, 4154.
2
a) M. Freemantle, Chem. Eng. News 2003, 81, 27. b) L.
Pasteur, Ann. Chim. Phys. 1848, 24, 442. c) G. B. Kauffman, I.
Bernal, H.-W. Schütt, Enantiomer 1999, 4, 33. d) L. Perez-Garcia,
D. B. Amabilino, Chem. Soc. Rev. 2002, 31, 342. e) E. Eliel, S. H.
Wilen, Stereochemistry of Organic Compounds, Wiley, New York,
1994. f) T. Konno, T. Yoshimura, G. Masuyama, M. Hirotsu, Bull.
Chem. Soc. Jpn. 2002, 75, 2185.
3
a) J. Jacques, A. Collet, S. H. Wilen, Enantiomers,
Racemates and Resolutions, Krieger Publishing Co., Malabar,
Florida, 1991. b) A. Collet, M.-J. Brienne, J. Jacques, Chem. Rev.
1980, 80, 215.
9
K. Nakamoto, Infrared and Raman Spectra of Inorganic
and Coordination Compounds, Part B, 5th ed., John Wiley and
Sons, New York, 1997.
4
a) K. Yamanari, J. Hidaka, Y. Shimura, Bull. Chem. Soc.
Jpn. 1973, 46, 3724. b) I. Bernal, J. Cetrullo, J. Coord. Chem.
1989, 20, 259. c) I. Bernal, J. Cetrullo, J. Myrczek, S. S. Massoud,
J. Coord. Chem. 1993, 30, 29. d) I. Bernal, J. Cetrullo, J. Myrczek,
Mater. Chem. Phys. 1993, 35, 290. e) I. Bernal, J. Cai, J. Myrczek,
Polyhedron 1993, 12, 1157. f) I. Bernal, J. Myrczek, J. Cai,
Polyhedron 1993, 12, 1149. g) M. Saha, R. Ramanujam, I. Bernal,
F. R. Fronczek, Cryst. Growth Des. 2002, 2, 205. h) S. G. Telfer, T.
Sato, T. Harada, R. Kuroda, J. Lefebvre, D. B. Leznoff, Inorg.
Chem. 2004, 43, 6168.
10 a) E. Colacio, M. Ghazi, R. Kivekäs, J. M. Moreno, Inorg.
Chem. 2000, 39, 2882. b) D. Schulz, T. Weyhermüller, K.
Wieghartdt, C. Butzlaff, A. X. Trautwein, Inorg. Chim. Acta 1996,
246, 387. c) S. Sen, M. K. Saha, T. Gupta, A. K. Karmakar, P.
Kundu, S. Mitra, M. B. Hursthouse, K. M. A. Malik, J. Chem.
Crystallogr. 1998, 28, 771.
11 a) SIR92: A. Altomare, G. Cascarano, C. Giacovazzo, A.
Guagliardi, M. Burla, G. Polidori, M. Camalli, J. Appl. Crystal-
logr. 1994, 27, 435. b) DIRDIF-99: P. T. Beurskens, G. Admiraal,
G. Beurskens, W. P. Bosman, R. de Gelder, R. Israel, J. M. M.
Smits, The DIRDIF-99 Program System, Technical Report of
the Crystallography Laboratory, University of Nijmegen, The
Netherlands, 1999.
12 International Tables for Crystallography, Kluwer Academ-
ic Publishers, Dordrecht, The Netherlands, 1992, Vol. C.
13 a) CrystalStructure 3.7.0, Crystal Structure Analysis Pack-
age, Rigaku and Rigaku/MSC, 9009 New Trails Dr. The
Woodlands TX 77381 U.S.A., 2000-2005 . b) D. J. Watkin,
C. K. Prout, J. R. Carruthers, P. W. Betteridge, CRYSTALS Issue
10, Chemical Crystallography Laboratory, Oxford, U.K.
5
a) I. Katsuki, Y. Motoda, Y. Sunatsuki, N. Matsumoto, T.
Nakashima, M. Kojima, J. Am. Chem. Soc. 2002, 124, 629. b) F.
Li, T. Li, X. Li, X. Li, Y. Wang, R. Cao, Cryst. Growth Des. 2006,
6, 1458. c) E.-Q. Gao, Y.-F. Yue, S.-Q. Bai, Z. He, C.-H. Yan,
J. Am. Chem. Soc. 2004, 126, 1419. d) F. M. Tabellion, S. R.
Seidel, A. M. Arif, P. J. Stang, Angew. Chem., Int. Ed. 2001, 40,
1529. e) T. Yamaguchi, F. Yamazaki, T. Ito, J. Am. Chem. Soc.
2001, 123, 743. f) J. Breu, H. Domel, A. Stoll, Eur. J. Inorg. Chem.
2000, 2401. g) R. Krämer, J.-M. Lehn, A. De Cian, J. Fischer,
Angew. Chem., Int. Ed. Engl. 1993, 32, 703.
6
a) Y. Shii, Y. Motoda, T. Matsuo, F. Kai, T. Nakashima,
J.-P. Tuchagues, N. Matsumoto, Inorg. Chem. 1999, 38, 3513.