Coordination Lamellar Nanofibers Consisting of N-(2-Hydroxydodecyl)-substituted
Amino Acid and Divalent Copper Cation
Teppei Yamada,*1,2 Yusuke Minami,1 and Nobuo Kimizuka1,2Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395
2
Center for Molecular Systems (CMS), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395
1(E-mail: teppei@mail.cstm.kyushu-u.ac.jp)

Lamellar nanoribbons consisting of 1:2 complexes of divalentcopper and N-(2-hydroxydodecyl)-substituted amino acid were
newly developed. When racemic ligand was used, the lamellar size
showed shrinkage and the stability of the ﬁbrous nanostructures
were enhanced compared to that consisting of pure enantiomer.

REPRINTED FROM

Vol.43 No.7

2014 p.1031–1033
CMLTAG
July 5, 2014

The Chemical Society of Japan

Received: March 23, 2014 | Accepted: April 1, 2014 | Web Released: April 8, 2014

CL-140271

Coordination Lamellar Nanofibers Consisting of N-(2-Hydroxydodecyl)-substituted
Amino Acid and Divalent Copper Cation
Teppei Yamada,*1,2 Yusuke Minami,1 and Nobuo Kimizuka1,2Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395
2Center for Molecular Systems (CMS), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395
1(E-mail: teppei@mail.cstm.kyushu-u.ac.jp)
Lamellar nanoribbons consisting of 1:2 complexes of divalent copper and N-(2-hydroxydodecyl)-L-alanine (L-AlaC12OH)
and related N-(2-hydroxydodecyl)-substituted amino acid were
newly developed. When racemic ligand was used, the lamellar
size showed shrinkage and the stability of the ﬁbrous nanostructures were enhanced, reﬂecting the spontaneous eﬀective
mixing of L and D complexes to form supramolecular racemiccompounds.

Coordination compounds show a wide variety of uniqueproperties such as magnetism, optical, redox, and catalytic
activities. These properties originate from the arrangement of
the metal center and ligands, whereas their functionalities areoften determined in the nanometer or subnanometer scale. Thatis, when these coordination compounds are fabricated forapplication, structure of some tenth nanometers or submicronscale critically aﬀects these properties. For example, the total
catalytic activity is not only aﬀected by the reactivity of theactive site but also by the diﬀusion of the reactant for accessing
to and eliminating from these active sites. Thus, the catalytic
function requires a design both for the active site and themesoscale structure that direct the mass transport of thesubstrates and reactants. In contrast to the brightful success indeveloping the structures of nanometer or subnanometer scaleby suitably programming covalent or coordination bonds,1
control of coordination architectures and their functions in themesoscale is in the developing world.2
We focused here on the programming of mesostructuresby using weak interactions between amphiphilic chiral metalcomplexes. Although the formation of chiral nanostructuresfrom chiral organic molecules has been widely investigated,3
the self-assembly of chiral and racemate metal complexes as atool to regulate their molecular alignment has been largely
unexplored.
In this study, a family of chiral lipophilic ligands, alanineand valine modiﬁed with hydroxydodecyl groups, N-(2-hydroxydodecyl)-L-alanine (L-AlaC12OH), N-(2-hydroxydodecyl)D-alanine (D-AlaC12OH), and N-(2-hydroxydodecyl)-L-valine(L-ValC12OH), were synthesized. They are known to form
twisted ﬁber structures with sodium or magnesium salt;4,5
however, the complex with copper has not been investigated.
The ligands D/L-AlaC12OH and L-ValC12OH were synthesized according to the previous report.4 NMR and elemental
analyses showed a good purity of the obtained compounds. They
were easily soluble in methanol but not in water, and they
become soluble in water when excess amount of sodiumhydroxide was added. These alkaline aqueous solutions aﬀorded
precipitates when mixed with aqueous solution of copper nitrate.
Similar ligands construct 1:2 complex with copper ion where
Chem. Lett. 2014, 43, 1031–1033 | doi:10.1246/cl.140271

Figure 1. Suggested coordination geometry (top) and optimized structure (bottom) of [Cu(L-AlaC12OH)2].
two nitrogen atoms and two oxygen atoms of carboxylate groups
coordinate in the trans-fashion and construct square-planarcopper center coordination geometry.6 Two oxygen atoms of thehydroxy groups, on the other hand, weakly coordinate to theaxial site of the copper, as shown in Figure S1.6­8 A presumablestructure for the obtained complex is shown in Figure 1. This
structure is also supported by the Job’s plot of L-ValC12OH andcopper, as shown in Figure S2. A peak is observed at the metal
to ligand ratio of approximately 0.37:0.73, and the complexation
ratio of 1:2 was also supported by the result of elemental
analysis. This result is reasonable because no nitrate ion wasdetected in the IR spectrum. From MM2 optimization, themolecule has two alkyl chains elongated to the opposite
direction and the C­C distance of both ends is ca. 30 ¡, asshown in Figure 1.
When the reaction was executed in a mixture of water andmethanol (1:4 vol/vol), the resultant complexes were stably
dispersed. Interestingly, in the circular dichroism (CD) spectraof CT absorption band at 250 nm, the L and D forms showed
positive and negative Cotton eﬀects, respectively, whereas theequimolar racemic mixture was CD silent (Figure S3). Because
the complex [Cu(L-AlaC12OH)2] is hydrophobic, it is expected
to self-assemble even in a water/methanol solution.9 The CD
signals observed for pure enantiomer complexes seem to reﬂect
the interactions between the adjacent molecules. When theracemic ligand was reacted with copper, naturally both typesof stacking averaged the CD signals and thus no peaks wereobserved.
The coordination structure of [Cu(L-AlaC12OH)2] wasfurther investigated by IR spectra (Figure S4). The IR spectraof L-AlaC12OH ligand showed N­H stretching at 3075 cm¹1,NH2 rocking at 1595 cm¹1 and scissoring at 1466 cm¹1, and© 2014 The Chemical Society of Japan | 1031

(002) (003)

Figure 2. SEM images of [Cu(L-AlaC12OH)2] (left), [Cu(DAlaC12OH)2] (center), and [Cu(D,L-AlaC12OH)2] (right).
C=O asymmetric and symmetric stretching of carboxylate at
1567 and 1400 cm¹1, respectively. These peaks are almost thesame as those in pristine alanine, along with the O­H stretchingat 3381 cm¹1 and indicative of the zwitterionic structure.
Meanwhile, when the ligand coordinates to copper, the O­H
stretching mode showed a shift to 3168 cm¹1. It clearly
demonstrates that the hydroxy group coordinates to copper ion
without the loss of proton. The peaks at 1634 cm¹1 corresponds
to C=O asymmetric stretching peak of carboxylate that coordinate monodentately to copper ions. These IR peaks clearlysupport the coordination geometry of the copper center, as
shown in Figure 1.
Scanning electron microscopic (SEM) images are shownin Figure 2. [Cu(L-AlaC12OH)2] showed nanoﬁbrous structureswith ca. 150 nm in width and 50 nm in thickness. The lengths ofthese nanoﬁbers are quite long. The nanoﬁber structures aredirected by the solvophobic self-assembly of the lipophiliccomplexes, which would be stabilized by the intermolecularhydrogen bonds between the carboxylate and hydroxy groups.
The X-ray diﬀraction measurement revealed that [Cu(LAlaC12OH)2] has lamellar basic structure, as shown in Figure 3.
The (001) peak can be observed at 2.70° that corresponds to ad value of 32.7 ¡, which is in good agreement with the sizeof the molecule that stacks to the lamellar structure without
interdigitation. The ligand alone aﬀorded ﬁbrous lamellar
structures; however, it aﬀorded XRD peaks at 4.61 and 5.15°,
which corresponded to the long periods of 19.2 and 17.1 ¡,respectively (Figure S5). These data are in good agreement with
the interdigitated lamellar structures. It should be noted thatthe ligand has two optically active centers at the alanine part(Cα) and a carbon atom adjacent to the hydroxy group (Cβ),
as schematically shown in Scheme S1. The chirality of Cα isdetermined by the amino acid used for the preparation, whileboth the conﬁgurations are possible for Cβ. Thus, it is reasonable
that the XRD pattern of the ligands revealed two types oflamellar structure d-spacings. It is notable that the XRD patternsof copper complexes do not have such side peaks, indicating that
regular lamellar structures are formed.
When the copper ion was reacted with a mixture of DAlaC12OH and L-AlaC12OH, similar ribbon-like nanoﬁbers canbe observed with the width and thickness of ca. 250 and 50 nm,
respectively (Figure 2). The obtained ribbon for the racemic
complex is slightly broader than that obtained for the chiralcomplex. The XRD pattern of racemic [Cu(D,L-AlaC12OH)2]
was found to be similar to that observed for chiral [Cu(LAlaC12OH)2], with a slight shift to higher angle ((001) peak at
2.78° and d-spacing of 31.9 ¡) (Figure 3). This shrinkage isprobably due to the tight packing of the D and L compounds
compared to the chiral complexes. The tight packing is also1032 | Chem. Lett. 2014, 43, 1031–1033 | doi:10.1246/cl.140271

2.5
3.0
2θ / degree

3.5

5(004)(005) (006)1015
2θ / degree

20

Figure 3. XRD patterns of [Cu(L-AlaC12OH)2] (red), [Cu(DAlaC12OH)2] (blue), and [Cu(D,L-AlaC12OH)2] (purple).

Figure 4. Photograph of [Cu(L-AlaC12OH)2] (left), [Cu(DAlaC12OH)2] (right), and [Cu(D,L-AlaC12OH)2] (center).
supported by the fact that the racemic complex is much morestable against the damage caused by the high-voltage electron
beam in the SEM measurement. Interestingly, when the water/
methanol (1:4) solutions of [Cu(L-AlaC12OH)2] and [Cu(DAlaC12OH)2] were mixed, the solution quickly became cloudy,
as shown in the center of Figure 4. It also reﬂects the interactionof enantiomer complexes to self-assemble into racemic compounds. This result can be attributed to the so-called “Wallach’srule,” which states that racemic compounds tend to constructdenser crystals than their chiral counterparts.10 Here, the lamellar
nanoribbons also show a similar behavior with the previouscompounds.
When alanine was replaced with valine, rigid nanoﬁbers
were obtained with a reaction of copper ions (Figure S6). From
the IR spectra, the copper center was isolated compared to thatof alanine, and a free O­H stretching mode can be observed at3520 cm¹1 (Figure S7). The other peaks are quite similar to
those with alanine derivatives, and similar stacking was alsoachieved by the valine derivative. The XRD patterns of thevaline derivative were also quite similar to those of the alanine
derivative and have typical lamellar patterns from 2.72°,
corresponding to the periodic structure of 32.5 ¡. These results
indicate that the lamellar structures are obtained regardless of theamino acid structure, indicating that the hydrophobic interactions among the alkyl chains determine the molecular alignment
in the crystalline structures.

© 2014 The Chemical Society of JapanIn summary, we have demonstrated lipophilic chiral coppercomplexes self-assemble to form nanoribbon structures in thewater/methanol mixtures. Upon mixing pure enantiomer complexes, they self-assemble into racemic compounds. It indicatesthat interactions between chiral groups can be employed as atool to regulate the molecular organization of metal complexes.
The authors gratefully acknowledge the Grant-in-Aid for
Scientiﬁc Research on Innovative Areas (“Coordination Programming” Area 2107, No. 24108718) from the Ministry ofEducation, Culture, Sports, Science and Technology, Japan.
References and Notes
1 H. Nishihara, Chem. Lett. 2014, 43, 388.
2 N. Kimizuka, in Advances in Polymer Science, ed. by T.
Shimizu, Springer, 2008, Vol. 219, pp. 1-26. doi:10.1007/12_2008_170.

Chem. Lett. 2014, 43, 1031–1033 | doi:10.1246/cl.140271

A. Brizard, R. Oda, I. Huc, in Low Molecular Mass Gelatorin Topics in Current Chemistry, Springer, 2005, Vol. 256,
pp. 167­218. doi:10.1007/b107174.
4 H. Hidaka, M. Murata, T. Onai, J. Chem. Soc., Chem.
Commun. 1984, 562.
5 A. Ghosh, J. Dey, Langmuir 2009, 25, 8466.
6 A. V. Pestov, P. A. Slepukhin, Y. G. Yatluk, Russ. J. Coord.
Chem. 2012, 38, 738.
7 B. Gyurcsik, T. Gajdab, L. Nagyb, K. Burger, A.
Rockenbauer, L. Korecz, Jr., Inorg. Chim. Acta 1993, 214,57.
8 Supporting Information is available electronically on JSTAGE.
9 M.-a. Morikawa, S. Tsunofuri, N. Kimizuka, Langmuir
2013, 29, 12930.
10 C. P. Brock, W. B. Schweizer, J. D. Dunitz, J. Am. Chem.
Soc. 1991, 113, 9811.
3© 2014 The Chemical Society of Japan | 1033