A novel triruthenium metal string complex with naphthylridylamide ligand: Synthesis, structure, magnetism, and molecular conductance

Article abstract of DOI:10.1016/j.inoche.2013.10.009

A triruthenium metal string, [Ru₃(npa)₄(NCS) ₂][PF₆] (1), supported by naphthylridylamide (npa) ligands was successfully synthesized and is reported in this work. X-ray single crystal analysis shows that compoun

Full text of DOI:10.1016/j.inoche.2013.10.009

Inorganic Chemistry Communications 38 (2013) 152–155
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
A novel triruthenium metal string complex with naphthylridylamide
ligand: Synthesis, structure, magnetism, and molecular conductance
Cherng-Shiaw Tsai a, Isiah Po-Chun Liu a, Fang-Wei Tien a, Gene-Hsiang Lee a, Chen-Yu Yeh b,
Chun-hsien Chen a, Shie-Ming Peng a,c,
⁎
a
Department of Chemistry, National Taiwan University, Taipei, Taiwan
Department of Chemistry, National Chung Hsing University, Taichung, Taiwan
b
c
Institute of Chemistry, Academia Sinica, Taipei, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
A triruthenium metal string, [Ru3(npa)4(NCS)2][PF6] (1), supported by naphthylridylamide (npa) ligands was
successfully synthesized and is reported in this work. X-ray single crystal analysis shows that compound 1 ex-
hibits a nonlinear [Ru3]7+ backbone (∠ = 170.26(3)°) with long Ru–Ru bond lengths (2.3554(8) Å). The long
Ru–Ru distances observed for 1 decrease the Ru–Ru interactions and electric conductance. Magnetic measure-
ments indicate that compound 1 is in a S=1/2 state. DFT calculations suggest that this unpaired electron occupies
Received 16 July 2013
Accepted 8 October 2013
Available online 26 October 2013
Keywords:
Ruthenium
Molecular wire
the π orbital which is stabilized by π-acid NCS− ligands and thus weakening the Ru–Ru π interaction.
⁎
Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.
Metal–metal interaction
Molecular conductance
Metal string complexes possessing a 1D transition metal frame-
work are of great interests because electrons can transport through
these molecules, and metal string complexes are expected to exhibit
the wire-like behavior and function as nano-wires [1]. Because of
this potential application, metal string complexes have attracted
considerable attention from scientists in the field of molecular
electronics [2]. Generally, the approach used to synthesize metal
string complexes is by oligopyridylamine ligands (Scheme 1).
After deprotonation of oligopyridylamine ligands, the resulting
oligopyridylamide can provide enough negative charges to stabilize
the central positive 1D metal core. A series of metal string complexes
supported by these oligopyridylamine ligands have been synthesized
and reported over the past two decades [3]. In 2006, for the first time, our
group attempted to develop oligonaphthylridylamine ligands for synthe-
sis of metal string complexes [4a]. Since oligonaphthylridylamide ligands
are less anionic than oligopyridylamide ligands with same length, the
new metal string complexes tended to form a reduced mix-valent metal
core, which significantly enhances the electric conductance [4]. Because
of this exciting result, our strategy for developing novel metal string
complexes shifted to focus on the oligonaphthylridylamine ligands.
More and more metal string complexes supported by various
oligonaphthylridylamine ligands are being designed and synthesized
[4,5]. Although oligonaphthylridylamide-supported metal strings are
the rising stars in the field, it should be noted that currently reported
examples of 1D metal chains of have mainly consisted of first row tran-
sition metal ions. Considering that heavier transition metal ions (4d and
5d) may show different physical characters, it is important to extend
studies of oligonaphthylridylamide-supported metal strings to the
heavier transition metal ions. In view of this consideration, here we re-
port the preparation, crystal structure, magnetism, electrochemistry
and electric conductance of the naphthylridylamide supported
triruthenium string, [Ru3(npa)4(NCS)2][PF6] (1).
Compound
1
was prepared from the reaction of 2-
naphthyridylphenylamine (Hnpa) and Ru2(OAc)4Cl in the presence of
t-BuOK and NaSCN, followed by stirring with excess KPF6 (Scheme 2)
[6]. The crystal structure of 1 is depicted in Fig. 1, and selected bond
lengths are reported in Fig. 2 [7]. Compound 1 shows a non-linear
[Ru3]7+ core (∠ = 170.26(3)°), which is helically wrapped by four
npa− ligands. The coordination environment of three ruthenium ions
is distorted octahedron. The four npa− ligands in 1 adopt a (3,1) ar-
rangement. Due to different steric hindrances between phenyl groups
and axial NCS− ligands at the two terminals, the two NCS− ligands
⁎
Corresponding author at: Department of Chemistry, National Taiwan University, No. 1,
Sec. 4, Roosevelt Rd., Taipei 10617, Taiwan. Tel.: +886 2 3366 1655; fax: +886 2 8369
3765.
E-mail address: smpeng@ntu.edu.tw (S.-M. Peng).
Scheme 1. Oligopyridylamine ligands.
1387-7003/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2013.10.009

C.-S. Tsai et al. / Inorganic Chemistry Communications 38 (2013) 152–155
153
Scheme 2. Synthesis of Hnpa and 1.
bond to Ru ions with linear and bent coordination modes, respectively.
The Ru–Ru distances in 1 are 2.3462(8) and 2.3646(8) Å respectively,
which are longer than that of the [Ru3(dpa)4Cl]+ analog (2, 2.2911(6)
Å) [8]. It should be noted that the bite angle (N\N\N) of supported li-
gands has direct influence on the metal–metal distances [9]. Compared
with the bite angle in 2 (~171.6°), the larger bite angle in 1 (~175.5°)
resulted from the rigidity of npa leading to the elongation of Ru–Ru dis-
tances in 1.
weak intermolecular antiferromagnetic interaction. The EPR spec-
trum recorded at 4 K for 1 is coincident with the feature of the s =
1/2 spin state, which supports the magnetism (Fig. 3, inset). Single
point spin-unrestricted DFT calculation was performed on complex
1 based on its X-ray structure [10]. The DFT calculation suggests
that compound 1 possesses a σ2π4δ2πn4bδn2b
π
3 electronic configura-
⁎
⁎
tion (Fig. S2), which shows stabilized π orbitals in compared with
those of 2 (σ2π4δ2πn4bδn2b
δ
σnb). This stabilization of π orbitals of
1
⁎2
⁎
The temperature dependence of effective magnetic moment (μeff
)
1 may be attributed to the weakening of Ru π-bonds resulting
from the lengthening of Ru–Ru distances (Scheme 3). Moreover,
of 1 is shown in Fig. 3. The μeff is 2.09 B.M. at 300 K, which is close to
the spin-only value of one unpaired electron (1.73 B.M.). With de-
creasing temperature, the μeff decreases slowly resulting from a
⁎
the degenerated π orbitals are asymmetrically occupied by three
⁎
π
electrons, which may also be the cause of non-linear geometry.
Fig. 1. The ORTEP of the cationic part for [Ru3(npa)4(NCS)2][PF6]·(Me2CO)2·(Et2O)2 (1). Thermal ellipsoids are drawn at the 40% probability level. Solvent molecules and hydrogen atoms
are omitted for clarity.

154
C.-S. Tsai et al. / Inorganic Chemistry Communications 38 (2013) 152–155
Fig. 2. Selected interatomic distances observed for 1.
Fig. 5. Cyclic voltammogram of compound 1 in CH2Cl2 containing 0.1 M TBAP with scan
rate = 100 mV s−1
.
The single molecular conductance of 1 was measured by the scan-
ning tunneling microscopy (STM) break-junction method (Fig. 4) [11].
Fig. 4a shows a typical conductance trace. The vertical axis is plotted
in units of G0 (~(12.9 kΩ)−1), defined as the conductance quantum of
a gold nanowire with a cross-section of a single atom. The conductance
value changed in a stepwise fashion as the STM tip was pulled away
from the substrate. Each stepwise drop, as the distance of the substrate
from the tip increases, indicates the loss of a molecule from the tip–
substrate junction. The histogram of counts from more than one
thousand traces shows local maxima at conductance values which are
integer multiples of a fundamental one, suggesting that the number of
molecules in the junctions was one, two and so forth. The conductance
of this [Ru3]7+ complex is 6.9 × 10−4 G0 (18.7 MΩ), which is less con-
ductive than [Ru3(dpa)4(NCS)2]+ (10.2 × 10−3 G0) [12]. The decreased
conductance may be attributed to the elongation of the distance be-
tween Ru ions weakening their interactions.
Fig. 3. Temperature-dependent magnetic effective moment (○) for compound 1. Inset:
Powder X-band EPR spectrum for 1 at 4 K.
It is well-known that Ru ions exhibit various oxidation states from
+2 to +8 [13]. Therefore, the cyclic voltammogram of compound 1
shows multiple redox processes (Fig. 5). Compound 1 displays
three redox couples at −0.39, +0.09, and +0.9 V, corresponding
to [Ru3(npa)4(NCS)2]0/[Ru3(npa)4(NCS)2]−
, /
[Ru3(npa)4(NCS)2]+
[Ru3(npa)4(NCS)2]0, and [Ru3(npa)4(NCS)2]2+/[Ru3(npa)4(NCS)2]+
,
respectively. These three reversible redox couples suggest that the
reduced and oxidized products of 1 may be isolated by performing
suitable methods in the ambient condition.
In conclusion, the triruthenium metal string supported by
naphthylridylamide ligands was successfully synthesized and studied
in this work. Because of the large bite angle resulting from the rigidity
of the npa− ligand, compound 1 exhibits longer Ru–Ru distances than
Scheme 3. Electronic configurations of [Ru3(npa)4(NCS)2]+ (left) and [Ru3(dpa)4Cl2]+
(right). The π-acid ligands NCS− in [Ru3(npa)4(NCS)2]+ stabilize the π orbitals and the
⁎
π-base ligands Cl− in [Ru3(dpa)4Cl2]+ destabilize the π orbitals.
⁎
Fig. 4. Single-molecule conductance of [Ru3(npa)4(NCS)2]+ as measured by STM break-junction at a bias of 50 mV. The conductance of a single [Ru3(npa)4(NCS)2]+ is 6.9 × 10−4 G0 (R=
18.7 MΩ). (a) Typical conductance-distance traces acquired by stretching the molecular junction with arbitrary x axis offsets. (b) The conductance histogram is obtained from more than
1000 measurements, and plotted by using 540 bins for the range presented.

C.-S. Tsai et al. / Inorganic Chemistry Communications 38 (2013) 152–155
155
those of [Ru3(dpa)4Cl]+. This longer Ru–Ru distance decreases the elec-
tron transport through the central triruthenium framework. Although
the conductivity of 1 is lower than those of dpa-supported triruthenium
analogs, the rich-redox property of 1 suggests that complex 1 with var-
ious oxidation states may be generated by applying suitable oxidization
or reduction potential. Since variation in oxidation state of metal string
complexes may significantly change their electric conductance, com-
plex 1 may have potential application as an electrically-controlled mo-
lecular switch. Further studies extending the scope of this work are
currently being undertaken by our group.
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Lee, S.-M. Peng, Chem. Commun. 46 (2010) 5018;
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Lee, S.-M. Peng, Chem. Commun. 46 (2010) 5018;
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[6] Ligand Hnpa (111 mg, 0.5 mmol), excess Ru2(OAc)4Cl (178 mg, 0.375 mmol) and
naphthalene (40 g) were placed in an Erlenmeyer flask under argon atmosphere.
The mixture was heated to 180 °C for 30 min, and then raised to 220 °C. A solution
of t-BuOK (56 mg, 0.5 mmol) in n-BuOH (5 mL) was added dropwise. After 180 min,
excess NaSCN was introduced into the flask and the reaction was continued for an-
other 60 min. After the mixture was cooled to 80 °C, n-hexane (100 mL) was added
and the precipitate was filtered. The solid was extracted with CH2Cl2 (200 mL). The
mixture was filtered again and the solvent were removed by a rotary evaporator.
Then purified through silica gel and eluted with acetone and CH2Cl2. The crude ma-
terial was dissolved in CH2Cl2, and then the potassium hexafluorophosphate was
added and stirred for 30 min. The mixture was filtered and the solvent were re-
moved by a rotary evaporator. Crystallization from acetone and diethyl ether
afforded 15 mg of dark green crystal (yield 10 %). MS(FAB): m/z: 1302
[Ru3(npa)4(NCS)2]+, 1244 [Ru3(npa)4(NCS)]+, 1184 [Ru3(npa)4]+; elemen-
tal analysis calcd (%) for [Ru3(npa)4(NCS)2][PF6]: C 48.20, H 2.79, N 13.57; found:
C 48.22, H 2.91, N 13.74; IR (KBr): ṽ = 2044, 1617, 1589, 1552, 1484, 1425,
Acknowledgments
The authors thank the National Science Council and the Ministry of
Education of Taiwan for financial support.
Appendix A. Supplementary data
CCDC 946659 contains the supplementary crystallographic data for
this article. This data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via http://www.ccdc.cam.ac.
uk/data_request/cif. Supplementary data to this article can be
found online at http://dx.doi.org/10.1016/j.inoche.2013.10.009.
1382, 1328, 1211, 1151, 844, 698, 555 cm−1
. UV/Vis (CH2Cl2) λmax/nm
(ε/104 –1 cm−1): 239 (7.8), 352 (2.9), 450broad (1.3), 597broad (0.8).
M
[7] The chosen crystals were mounted on a glass fiber, and the X-Ray crystallographic
data were collected at 150 K on a NONIUS Kappa CCD diffractometer installed
with monochromated Mo-Kα radiation, λ = 0.71073 Å. Cell parameters were re-
trieved and refined by using DENZO-SMN software on all observed reflections. Ab-
sorption correlations were applied with SORTAV program. The structure was solved
by direct method with SHELX-97 program and refined by full-matrix least squares
on F2 values. Hydrogen atoms were fixed at calculated positions and refined using
a riding mode. All DFIX restrains are used to model disordered PF−6 anion and the
cocrystallized acetone molecules. The disordered PF−6 anion was modeled as a pair
of rigid octahedron, with 70 % and 30 % occupancy respectively. And one of the ac-
etone molecules was also disordered. Crystal data for 1 (deep green crystal): C72H72
F6N14O4PRu3S2, Mr = 1709.74, monoclinic, space group P21/n, a = 17.2886(4)
Å, b = 24.3132(6) Å, c = 17.6177(4) Å, β = 91.518(2) °, V = 7402.8(3) Å3, Z = 4,
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