Sulfide-capped wire-like metallaynes as connectors for Au nanoparticle assemblies

Article abstract of DOI:10.1039/b412336k

A new series of Ru₂(DMBA)₄(oligo(phenyleneethyne)) ₂ compounds bearing sulfide termini was synthesized; structural characterization revealed both the rigid rod nature and extended π-conjugation in these metallaynes; in the

Full text of DOI:10.1039/b412336k

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Sulfide-capped wire-like metallaynes as connectors for Au nanoparticle
assemblies{
Jie-Wen Ying, David R. Sobransingh, Guo-Lin Xu, Angel E. Kaifer* and Tong Ren*
Received (in Cambridge, UK) 10th August 2004, Accepted 15th October 2004
First published as an Advance Article on the web 30th November 2004
DOI: 10.1039/b412336k
The sulfide capped OPEn ligands were prepared following
literature procedures.19,20 Trimethylsilylethylene (TMSE) was
chosen as the thiol protection group because of its superior
chemical stability over acetyl protection group, and methoxy
phenyl substituents were introduced in both OPE2 and OPE3 to
improve the solubility of metallaynes. Using the weak base
A new series of Ru2(DMBA)4(oligo(phenyleneethyne))2 com-
pounds bearing sulfide termini was synthesized; structural
characterization revealed both the rigid rod nature and
extended p-conjugation in these metallaynes; in the presence
of these metallaynes, Au nanoparticles readily assembled into
dimers and chains with well-defined inter-dimer distances.
protocol,21–23
reactions
between
[Ru2(DMBA)4](NO3)2
(DMBA 5 N,N9-dimethylbenzamidinate) and the appropriate
OPEn ligand resulted in trans-(OPEn)2Ru2(DMBA)4 compounds
1–3 as red, diamagnetic crystalline materials. Cyclic voltammetric
measurements with compounds 1–3 revealed nearly identical
voltammograms (Fig. 1) consisting of an oxidation (A) and a
reduction (B), from which the HOMO–LUMO gap of the solvated
metallaynes was estimated to be ca. 1.57 V. In comparison, the
optical HOMO–LUMO gap of compounds 1–3 estimated from
the lowest lmax is ca. 1.42 eV.
Molecular electronics and functional nanoparticles represent two
of the most active research areas at the interface of chemistry,
physics and material science.1,2 While there is considerable interest
on nanoparticles as novel catalysts,3–6 the integration of nano-
particles with molecular bridges, especially conjugated rigid rod
molecules, has emerged as an exciting new frontier during the
recent years. Organization of nanoparticles using molecular linkers
can be traced to the pioneering work of Alivisatos and others.7,8
Recent results with the emphasis towards electronic materials
applications include hierarchical assemblies of Ag and Au
nanoparticles with sulfide-capped linear and branched oligo(phe-
nyleneethyne)s (OPE),9–11 conductance enhancement between two
Au nanowires using sulfide-capped oligo(phenylenevinylene)
(OPV) bridges,12 transfer of spin coherence between CdSe
nanoparticles mediated by 1,4-benzenedimethanethiol,13 magnetic
flux closure in Co nanoparticles ring templated with resorcinar-
ene14,15 and nano-cells constructed from disordered metallic Au
islands and sulfide-capped OPEs.16 We are interested in achieving
active materials for molecular electronic devices based on
conjugated diruthenium metallaynes and reported high degree of
electronic mobility across both the carbon backbone and
diruthenium units recently.17,18 Here, we wish to report the
synthesis of a new family of sulfide-capped diruthenium–bis(OPE)
compounds (Scheme 1) and their utility for the preparation of
dimers and linear chains of Au nanoparticles.
The molecular structures of compounds 1 and 2,{ shown in
Fig. 2, provide structural insights of these conjugated metallaynes.
The coordination geometry around the Ru2 core in both cases is
similar to that observed for other Ru2(DMBA)4(C2R)2 type
compounds,21 indicating a very minimal structural perturbation by
the OPEn ligand. In addition to the apparent rigid rod nature, the
near co-planarity among phenyleneethyne units is noteworthy and
reflects the extended p-conjugation across the entire metallayne,
which is consistent with both the early structural analysis24–26 and
more recent computational studies of pure organic OPEs.27
The presence of terminal sulfide functional groups at both ends
of these rod shaped molecules opens the possibility of using them
as organometallic tethers between noble metal nanostructures. In
order to test this possibility, we prepared 12-nm gold nanoparticles
using a well-established method relying on the citrate reduction of
2 28
aqueous solutions of AuCl4
.
Colloidal solutions of these Au
nanoparticles were treated with aliquots of solutions containing
compounds 1 to 3 (see ESI{ for experimental details) until the
corresponding surface plasmon resonance band shifted from a
lmax of 518 to 525 nm. At that point, the presence of discrete
nanoparticle aggregates was presumed and substantiated by TEM
analysis. Numerous well-defined nanoparticle dimers were
Scheme 1 Wire-like Ru2–(OPEn) metallaynes 1–3.
{ Electronic supplementary information (ESI) available: Synthesis and
characterization of 1–3, X-ray crystallographic details for 1 and 2, and
experimental details for nanoparticle preparation and assembly experi-
ments. See http://www.rsc.org/suppdata/cc/b4/b412336k/
Fig. 1 Cyclic voltammograms of compounds 1–3 recorded in THF at a
scan rate of 0.10 V s21
.
*akaifer@miami.edu (Angel E. Kaifer)
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Chem. Commun., 2005, 357–359 | 357

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(Fig. 3) with all linkers and the corresponding inter-particle
distances included in the statistical calculations of the values
reported above.
In summary, we have prepared a series of three new conjugated
Ru2 metallaynes with two terminal trimethylsilylethylene sulfide
groups. These compounds offer interesting and rare examples of
extended conjugation over rather long molecular distances, with
integral Ru2 metallaynes, which are known to facilitate the
electronic communication between the two molecular ends.17,18
We have also used these rigid organometallic disulfides as
connectors of variable length for the preparation of Au
nanoparticle dimers and chains. To the best of our knowledge,
these are the first examples of variable-length nanoparticle
tethers containing transition metal compounds in their
backbones. Further investigation of these systems continues in
our laboratories.
Fig. 2 ORTEP plots of compound 1 (top) and 2 (bottom) at 30%
probability level.
Financial support from the Office of Naval Research (N00014-
03-1-0531) is gratefully acknowledged.
observed in the TEM images of these preparations, confirming
that compounds 1–3 serve as effective linkers between the Au
nanoparticles (see Fig. 3). From the TEM images we measured
1.7 ¡ 0.2, 2.6 ¡ 0.4 and 3.7 ¡ 0.9 nm as the averaged distances
Jie-Wen Ying, David R. Sobransingh, Guo-Lin Xu, Angel E. Kaifer* and
Tong Ren*
Department of Chemistry, University of Miami, Coral Gables, Florida,
33124-0431, USA. E-mail: akaifer@miami.edu
between nanoparticles prepared with tethers 1,
2 and 3,
respectively. The corresponding S–S distances estimated based
on the crystal structures of 1 and 2 and a molecular model of 3 are
2.08, 3.44 and 4.80 nm. Therefore, the measured distances
separating adjacent nanoparticles in the observed dimer and chain
aggregates correlate well with the S–S distances in the organome-
tallic linkers (see plot in ESI{), providing additional support to the
proposed structures. Notice that the average interparticle distances
measured by TEM are shorter than the linker S–S distances, which
may be due to dimer orientation effects and positioning away from
the plane perpendicular to the direction of observation. Energy
dispersive spectroscopy verified the presence of Ru in Au
nanoparticle aggregates that were observed when excess linker
was utilized in the nanoparticle assembly experiments. This finding
further confirms the tethering activity of the Ru2 metallyne
disulfides 1–3 as nanoparticle connectors. We also verified in
control experiments that any excess citrate that might be present in
the Au nanoparticle sols does not react with compounds 1–3.
From our TEM data we estimate that the yield of nanoparticle
dimers under the experimental conditions utilized in this work is
32 ¡ 3% in all cases. Other well-defined aggregates were readily
observed in the TEM images recorded after deposition on copper
grids of drops of Au nanoparticle sols treated with linkers 1–3.
Specifically, linear chains of Au nanoparticles were easily detected
Notes and references
{ Crystal data. 1?C6H14: C68H92N8Ru2S2Si2, monoclinic, space group C2,
˚
a 5 11.2193(17), b 5 18.897(3), c 5 16.872(3) A, b 5 106.173(3)u,
3
23
˚
˚
V 5 3435.6(9) A ; Z 5 2, Dc 5 1.299 g cm , l(Mo-Ka) 5 0.71074 A,
m 5 0.580 mm21. A total of 5540 independent reflections were collected on
a SMART1000 CCD diffractometer at 300 K with the 2h range of 4.8–50u.
The structure was solved with direct method and refined with full-matrix
least squares on F2 to R1 5 0.041 and wR2 5 0.095. 2?C6H14:
¯
C88H108N8O4Ru2S2Si2, triclinic, space group P1, a 5 12.9758(12),
b 5 13.8400(12), c 5 14.2445(13) A, a 5 106.950(2), b 5 104.661(2),
˚
c 5 107.477(2)u, V 5 2165.2(3) A ; Z 5 1, Dc 5 1.276 g cm23, l(Mo-
3
˚
Ka) 5 0.71074 A, m 5 0.477 mm21. A total of 7536 independent reflections
˚
were collected on a SMART1000 CCD diffractometer at 300 K with the 2h
range of 4.8–50u. The structure was solved and refined as above to
R1 5 0.060 and wR2 5 0.105. CCDC 246907 and 246908. See http://
www.rsc.org/suppdata/cc/b4/b412336k/ for crystallographic data in .cif or
other electronic format.
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