From molecular wires to potential molecular cables

Article abstract of DOI:10.1016/j.inoche.2010.09.026

Triple-helices with additional functional groups for anchoring on metal electrodes are accessible via metal-coordination assisted self-assembling.

Full text of DOI:10.1016/j.inoche.2010.09.026

Inorganic Chemistry Communications 14 (2011) 42–45
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
From molecular wires to potential molecular cables
⁎
Alexandrina Stuparu, Matthias Fischer, Olaf Fuhr, Oliver Hampe, Christophe Stroh
Karlsruher Institut für Technologie (KIT), Institut für Nanotechnologie, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 June 2010
Accepted 17 September 2010
Available online 29 September 2010
Triple-helices with additional functional groups for anchoring on metal electrodes are accessible via metal-
coordination assisted self-assembling.
© 2010 Elsevier B.V. All rights reserved.
Keywords:
Helicate
Heterocycle
Coordination compound
Crystal structure
With the development of new techniques for the investigation of
molecules contacted to surfaces, and for instance to electrodes, electronic
properties of numerous chemical structures have been studied [1].
However, only a few hundreds of so called molecular wires have been
chemically contacted between two electrodes [2]. The investigation and
understanding of the influence of the chemical structure and the
molecule-electrode contact on the current-transport characteristics is
essential for the potential use of molecules in future commercial
electronic devices.
The use of metal-complexes integrated in nanosystems is particu-
larly promising for applications like information storage. Among plenty
of coordination compounds, polymetallic helical structures show
adequate structural features for contacting both sides to metal
electrodes [3]. However, to our knowledge, triple helicates functiona-
lized with anchoring groups for metal surface decoration have never
been reported. This is even more surprising since several examples of
double or triple bimetallic helicates have been reported in the literature
[4]. Among them, the 4,4'-methylenedianiline bridging motif combines
the simplicity of the synthesis and structural modification [5]. In
addition, various metals have already been complexed with derivatives
of this molecule, and recently an enantiomeric resolution has even been
reported with analogous compounds [6].
In this communication, we present the synthesis of two triple
helices made by the assembling of three organic ligands L1 around two
zinc(II) or iron(II) cations respectively. This preliminary work
emphasises the simplicity of the strategy to build hybrid scaffolds
combining rigidity and functionality of the organic structure, and
additional properties of metal complexes.
The ligand L1 was synthesized in three steps (Scheme 2). First, a 5-
bromo-pyridine-2-carboxaldehyde was prepared following a reported
procedure [8]. A Suzuki-type cross-coupling reaction with 4-thio-
methyl-phenyl-boronic acid gave the thiomethyl-functionalized alde-
hyde in good yield. Finally, a double imine-condensation of this latter
with 4,4'-methylenedianiline in ethanol afforded the desired ligand [9].
Both complexes were synthesized by mixing a CH2Cl2/MeOH 5/1
solution of the ligand L1 with a MeOH solution of the corresponding
metal salt. While the perchlorate salt of the L13Zn2 complex
precipitated during the reaction, the hexafluorophosphate salt of
the L13Fe2 complex could be obtained by addition of a methanolic
NH4PF6 solution [10,11].
The two binuclear complexes were characterised by 1 H NMR, IR,
UV–vis spectroscopy, ESI mass spectrometry, elemental analysis, and
single-crystal X-ray diffraction.
In the course for the development of new electronic-active
molecular scaffolds, we designed and synthesized bimetallic helical
complexes based on a 4,4'-methylenedianiline bridging motif
(Scheme 1). More than a combination of three molecular organic
wires with new structural features, this work highlights the potential
possibility to tune the electronic transport properties by varying the
metal ions. In view of the recent studies concerning metal complexes
connected between electrodes, such potential molecular cables are of
highest interest in the field of molecular electronics [7].
⁎
Corresponding author. Tel.: +49 724 782 8173; fax: +49 724 782 6368.
E-mail address: christophe.stroh@kit.edu (C. Stroh).
Scheme 1. Double end-functionalized dinuclear triple helices.
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.09.026

A. Stuparu et al. / Inorganic Chemistry Communications 14 (2011) 42–45
43
Fig. 2. ESI mass spectrum of the dinuclear zinc(II) helix in MeCN. (Inset: isotopic
distribution of the [L13Zn2]4+ complex, L1=C39H32N4S2).
Scheme 2. Synthesis of the functionalized ligand L1 and its two binuclear metal complexes.
helical shape (Fig. 3). The zinc cations are complexed by the pyridine-
imine chelating groups, forcing a distorted octahedral geometry. As
expected, the six sulphur atoms are located on both sides of the helical
structures. The two planes containing respectively the three outer
sulphur atoms are nearly coplanar (10.5°) rendering them almost ideal
for contacting them between metal (gold) electrodes. The intermetallic
distance in this complex is 1.187 nm.
Single crystals of the Fe2-complex suitable for X-ray analysis were
obtained from a MeCN/Et2O solution [13]. In addition to the similar
triple helical arrangement of the ligands around the two metal ions,
this reveals the presence of four PF−6 anions as expected for a bis-iron
(II) complex in perfect agreement with the mass spectrometric
finding of quadruply charged complex cation. The intermetallic
distance in this second complex is 1.144 nm.
A careful look at the crystal structure shows the difference of
distances between the bridging phenyl rings between both complexes
(Fig. 4). The shorter distances of the carbon atoms connected to the
imine nitrogen of the Fe(II) complex are induced by the shorter metal–
nitrogen bond distances. This observation is in agreement with the
hindered rotation of the phenyl rings in this compound as state above.
In the present communication, we report a new synthetic route of
potential nano-cables suited for molecular electronics. Using this
strategy, we succeeded in the preparation, and complete chemical and
structural characterization of two new coordination compounds with
additional groups for metal surface anchoring. Formed by the self-
assembly of three conjugated organic rods around two metal ions, this
work highlights the large variety of the possible structural modifications.
Interestingly, while the zinc(II) helix shows a clearly resolved 1 H
NMR spectrum, the iron(II) helix shows two broad bands at 5.68 and
7.00 ppm (Fig. 1). This can be ascribed to the hindered phenyl ring
rotation between the two metals, as suggested for a similar compound
[12]. Temperature dependent 1 H NMR spectra support this conclusion.
Another different behaviour between the two helical structures in
solution is observed in the ESI mass spectra. While the perchlorate salt
yields predominantly the [L13Zn2]4+ (=[M]4+) and the [M+ClO4]3+
peaks (Fig. 2), the mass spectrum of the hexafluorophosphate salt
displays additional peaks which can be attributed to aggregation of
several helical complexes with its counterion of general form [Mx
(PF6)y](4x − y)+. However, the cation/ligand ratio is always in agree-
ment with a triple helical bimetallic structure, confirming its very
selective formation.
Single crystals of the Zn2-helix suitable for X-ray analysis were
obtained from an acetonitrile solution. The compound crystallises in the
monoclinic space group C2/c with six acetonitrile solvate molecules per
formula unit. As expected, three ligands fold around two metal ions in a
Fig. 3. View of the bis-zinc(II) triple helicate showing the internuclear distance.
Perchlorate anions have been omitted for clarity.
Fig. 1. 1H NMR spectra of the two complexes at room temperature.

44
A. Stuparu et al. / Inorganic Chemistry Communications 14 (2011) 42–45
Fig. 4. View of the coordination sphere with the M–N distances (in blue), and the nearest C atoms of the phenyl rings (in purple) for the Zn(II) and the Fe(II) cations respectively.
J.R. Heath, M.A. Ratner, Phys. Today (2003) 43;
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The syntheses of derivatives with different bridging units, anchoring
groups and metal ions appear very promising.
Peng, J. Phys. Chem. B 108 (2004) 959.
[4] L.J. Charbonnière, A.F. Williams, C. Piguet, G. Bernardinelli, E. Rivara-Minten,
Acknowledgements
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We thank the Institute of Nanotechnology (INT) and the KIT
(campus north) for the financial support. We thank Prof. Marcel
Mayor for the support.
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CCDC 667923 and 667924 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via http://www.ccdc.
cam.ac.uk/data_request/cif. Supplementary mzterial associated with this
article can be found, in the online version, at doi:10.1016/j.
inoche.2010.09.026.
References1
[1] M.A. Ratner, Mater. Today (2002) 20;
[9] Synthesis of ligand L1: 4,4'–methylenedianiline (1.23 mmol) and 4-(4’-
methylthio)-phenyl-salicylaldehyde (2.47 mmol) were mixed in EtOH (40 mL)
and stirred at rt for 3 h. The yellow precipitate obtained was filtered, washed with
EtOH (2x2 mL) and recrystallized from EtOH/CH2Cl2 to give the ligand as a
yellowish solid in 91% yield. Calc. for C39H32N4S2 (Mr = 620.83): C: 75.45, H: 5.20,
N: 9.02, S: 10.33, found: C: 75.25, H: 5.17, N: 9.07, S: 10.34; 1H NMR (C2D2Cl2):
δ (ppm) 2.46 (s, 6H), 3.98 (s, 2H), 7.21 (s, 8H), 7.29 (d, J = 8.3 Hz, 4H), 7.51 (d, J = 8.3
Hz, 4H), 7.92 (dd, J1 = 8.3 Hz, J2 = 2.2 Hz, 2H), 8.19 (d, J = 8.2 Hz, 2H), 8.59 (s, 2H),
8.83 (d, J = 2.2 Hz, 2H); MALDI-TOF (no matrix), m/z (%): 621.0 (100) [M+H]+; IR
C. Joachim, J.K. Gimzewski, A. Aviram, Nature 408 (2000) 541;
M. Mayor, H.B. Weber, Chimia 56 (2003) 494;
J.R. Heath, M.A. Ratner, Phys. Today 56 (2003) 43.
[2] F. Rosei, M. Schunack, Y. Naitoh, P. Jiang, A. Gourdon, E. Leagsgaard, I. Stengaard, C.
Joachim, F. Besenbacher, Prog. Surf. Sci. 71 (2003) 95;
N.S. Hush, Ann. N.Y. Acad. Sci. 1006 (2003) 1;
~
1
Perchlorate salts should be handled carefully, in low quantities, used as hydrated salts,
selected (KBr pellet): υ (cm–1) 1625, 1594, 1582, 1500, 1472, 1097, 998, 817, 529;
UV-vis (MeCN) λ, nm (ε, M–1cm–1): 346 (100800), 232 (54400).
and never dehydrated under vacuum before used. Mass spectrometry: Electrospray
ionisation (ESI) mass spectra were taken on a 7T-Fourier-transform ion cyclotron resonance
(FT-ICR) mass spectrometer (APEX II, Bruker Daltonik) from solutions of the Fe2- and the Zn2-
Helix in acetonitrile. Crystal data and structure refinement: Data were collected at 180(2) K on a
STOE IPDS II diffractometer with graphite monochromated MoKα radiation (λ=0.71073 Å).
Zn2-Helix: The structure was solved by direct methods and refined by full-matrix least-
squares analysis. [C117H96N12S6Zn2](ClO4)4·6(C2H3N), M=2637.38, monoclinic, a=51.717
(10), b=10.223(2), c=25.735(5) Å, β=114.10(3)°, V=12420(4) Å3, space group C2/c,
Z=4, μ(MoKα)=0.464 mm-1, 22463 reflections measured, 9446 unique [R(int)=0.1186].
R1=0.0768 (for [IN2σ(I)]), wR2=0.1436 (for all data). CCDC 667923. Fe2-Helix:13 [C117H96-
Fe2N12S6](PF6)4·8(C2H3N), M=2877.39, monoclinic, a=30.542(6), b=20.286(4),
c=11.599(2) Å, β=94.39(3)°, V=7165(2) Å3, space group C2, Z=2, μ(MoKα)=
0.420 mm-1, 18783 reflections measured, 7962 unique [R(int)=0.0650]. R1=0.1003 (for
[IN2σ(I)]), wR2=0.2473 (for all data) FLACK parameter 0.44(4). CCDC 667924.
[10] Synthesis of Zn2-Helix: Zn(ClO4)2·6H2O (59.8 μmol) in MeOH (5 mL) was added to
a solution of the ligand L1 (89.8 μmol) in CH2Cl2/MeOH 5/1 (30 mL). After stirring
overnight, the precipitate was filtered and washed with MeOH to yield the desired
Zn2-helical complex in 70% yield. Calc. for [C117H96N12S6Zn2](ClO4)4·2H2O C:
57.90, H:4.15, N:6.93, found: C:57.96, H:4.30, N:6.95; 1H NMR (CD3CN): δ (ppm)
2.52 (s, 18H), 4.07 (s, 6H), 6.32 (d, J = 8.4 Hz, 12H), 7.02 (d, J = 8.3 Hz, 12H), 7.36
(d, J = 8.6 Hz, 12H), 7.54 (d, J = 8.6 Hz, 12H), 8.11 (d, J = 2.1 Hz, 6H), 8.30 (d, J =
8.1 Hz, 6H), 8.61 (s, 6H), 8.68 (dd, 3J = 8.2, 4J = 2.2 Hz, 6H); ESI (MeCN): positive
mode m/z (%) 498.1 (100) [Zn2L3]4+; negative mode m/z (%): 98.8 (100) [ClO4]1–
;
~
IR selected (KBr pellet,): υ (cm–1) 1629, 1593, 1567, 1501, 1479, 1197, 1098,
1003, 818, 623; UV-vis (MeCN) λ, nm (ε, M–1cm–1): 379 (138500), 272 (104300).
[11] Synthesis of Fe2-Helix: To a solution of L1 (16.1 μmol) in CH2Cl2/MeOH, 5/1 (60 mL)
was added FeCl2·4H2O (12.0 μmol) in MeOH (10 mL). After stirring at r.t. for 3 h,
the resulting solution was filtered. Addition of NH4PF6 (1.07 mmol) in MeOH (10

A. Stuparu et al. / Inorganic Chemistry Communications 14 (2011) 42–45
45
mL) gave a dark violet precipitate. After overnight stirring, the solid was collected
and washed with MeOH to yield the Fe2-helical complex in 46% yield. Calc. for
[C117H96Fe2N12S6](PF6)4·2H2O, C:54.26, H:3.89, N:6.49, found: C:54.23, H:3.97, N:
6.47; Calc. for [C117H96N12S6Fe2](PF6)4 (Mr = 2554.03): C, 55.02, H, 3.79, N, 6.58;
S, 7.53, found (dried at 110°C, 10–3 mbar): C, 54.85, H, 3.96, N, 6.60, S, 7.68; 1H
NMR (CD3CN, 25°C): δ (ppm) 2.52 (s, 18H), 4.08 (s, 6H), 5.68 (br s, 12H), 7.00 (br
s, 12H), 7.33 (d, J = 8.7 Hz, 12H), 7.44 (d, J = 8.7 Hz, 12H), 7.61 (s, 6H), 8.60-8.67
[Fe2L3(PF6)3]1+; negative mode m/z (%) = 144.7 (100) [PF6]1– ; IR selected
~
(KBr pellet): υ (cm–1) 3439, 1589, 1558, 1541, 1499, 1473, 1199, 1097, 842,
556; UV-VIS (MeCN) λ, nm (ε, M–1cm–1): 589 (16800), 372 (119800), 272
(119900).
[12] M. Pascu, G.J. Clarkson, B.M. Kariuki, M.J. Hannon, Dalton Trans. (2006) 2635.
[13] The quality of the crystals of the Fe complex was very poor with only diffuse
diffraction for angles 2θ N 48°. The FLACK parameter indicates that the measured
crystal was a racemic twin. Some phenyl rings and the couterions (PF6–) were
refined disorded. Also the solvent molecules were difficult to localise because of
disorder.
(m, 12H), 9.07 (s, 6H); ESI (MeCN): positive mode m/z (%) 493.4 (100) [Fe2L3]4+
,
664.2 (31) [Fe2L3F]3+ 1068.8 (5) [Fe2L3(PF6)F]2+
(PF6)]4+ 1387.3 (19) [3(Fe2L3)7(PF6)]5+
1770.2 (14) [3(Fe2L3)8(PF6)]4+
,
,
1132.0 (24) [2(Fe2L3)24
,
,
1557.4 (11) [2(Fe2L3)5(PF6)]3+
,
,
1898.3 (9) [4(Fe2L3)11(PF6)]5+
, 2409 (15)