Pentanuclear Heterometallic String Complexes with High-Bond-order Units [Ni2 3+?Mo2 4+?Ni2+(bna)4X2]3+ (X = Cl, NCS)

Article abstract of DOI:10.1002/jccs.201700331

Our quest for ligands toward the synthesis of functional metal string complexes started from α-pyridylamido anions. With the modulation of naphthyridylamido anions, the ligands carry less negative charge to be balanced and, thus, to some extent, can tune the oxidation state of the metal centers and the strength of metal–metal interactions. For example, the formation of Ni₂ ³⁺, a mixed-valence moiety, has been demonstrated. In this paper, high-bond-order units of M₂ (Mo₂ or Ru₂) and Ni₂ ³⁺ are introduced and supported by four equatorial ligands of the bisnaphthyridylamido anion (bna^?). The resulting compounds are [Ni₂Mo₂Ni(bna)₄Cl₂](PF₆)₃ (1), [Ni₂Mo₂Ni(bna)₄(NCS)₂](PF₆)₃ (2), and [Ni₂Ru₂Ni(bna)₄Cl₂](ClO₄)₃ (3). X-ray crystallography reveal quadruply bonded characteristics with Mo–Mo distances of 2.133(2) and 2.109(2) ? for 1 and 2, respectively. The Ni₂ moieties have a short Ni–Ni distance of 2.331(5) for 1 and 2.334(4) ? for 2, suggesting the formation of mixed-valence Ni₂ ³⁺ units. It appears that there are no significant metal–metal interactions between Mo₂ and its neighboring Ni centers. Characterization using magnetism, voltammetry, electronic absorption, and single-molecule conductance, however, shows significant influence of Mo₂ on the properties of the metal string complexes.

Full text of DOI:10.1002/jccs.201700331

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Special Issue Paper
Pentanuclear Heterometallic String Complexes with High-Bond-order Units
3
+
4+
2+
3+
[
Ni2 −Mo2 −Ni (bna) X ] (X = Cl, NCS)
4 2
a†
a†
a
a*
Ssu-chieh Hsu, Geng-Min Lin, Gene-Hsiang Lee, Chun-hsien Chen and
a,b
*
Shie-Ming Peng
a
Department of Chemistry and Center for Emerging Material and Advanced Device, National Taiwan University,
No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan
b
Institute of Chemistry, Academia Sinica, 128, Sec. 2, Academia Road, Taipei 11529, Taiwan
(Received: September 25, 2017; Accepted: November 17, 2017; DOI: 10.1002/jccs.201700331)
Our quest for ligands toward the synthesis of functional metal string complexes started from
α-pyridylamido anions. With the modulation of naphthyridylamido anions, the ligands carry less
negative charge to be balanced and, thus, to some extent, can tune the oxidation state of the metal
3+
centers and the strength of metal–metal interactions. For example, the formation of Ni2 , a
mixed-valence moiety, has been demonstrated. In this paper, high-bond-order units of M (Mo or
2
2
3
+
Ru ) and Ni
are introduced and supported by four equatorial ligands of the bisnaphthyridyla-
mido anion (bna ). The resulting compounds are [Ni Mo Ni(bna) Cl ](PF ) (1), [Ni Mo Ni
2
2
−
2
2
4
2
6 3
2
2
(
bna) (NCS) ](PF ) (2), and [Ni Ru Ni(bna) Cl ](ClO ) (3). X-ray crystallography reveal qua-
4 2 6 3 2 2 4 2 4 3
druply bonded characteristics with Mo–Mo distances of 2.133(2) and 2.109(2) Å for 1 and 2,
respectively. The Ni moieties have a short Ni–Ni distance of 2.331(5) for 1 and 2.334(4) Å for 2,
2
3+
suggesting the formation of mixed-valence Ni2 units. It appears that there are no significant
metal–metal interactions between Mo and its neighboring Ni centers. Characterization using
2
magnetism, voltammetry, electronic absorption, and single-molecule conductance, however, shows
significant influence of Mo on the properties of the metal string complexes.
2
Keywords: Heterometallic complexes; Metal–metal interactions; Metal string complexes; Extended
metal-atom chains; Mixed-valent compounds; Molecular wires.
Ssu-chieh Hsu received the B.S. degree from the National Central University, Taiwan, in 2015, and the
M.S. degree from the National Taiwan University in 2017. Her research interests include well-ordered
mesoporous silica and heterometallic string complexes.
Geng-Min Lin received the B.S. degree from Soochow University, China, in 2008, and the Ph.D. degree from
the National Taiwan University (NTU) in 2016. He is currently a Postdoctoral Research Associate with NTU.
His research interests include the synthesis and characterization of extended metal atom chains and single-
molecule electronics by means of scanning tunneling microscopy-based methods.
*Corresponding authors. Email: chhchen@ntu.edu.tw (Chun-hsien Chen), smpeng@ntu.edu.tw (Shie-Ming Peng)
†These authors contributed equally to this work.
J. Chin. Chem. Soc. 2017
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1

Special Issue Paper
Hsu et al.
Gene-Hsiang Lee received the B.Sc. degree from the Department of Chemistry and the M.Sc. degree from the
Graduate Institute of Applied Chemistry, Chinese Culture University, Taiwan, and the Ph.D. degree from the
Institute of Organic Polymeric Materials Engineering, National Taipei University of Technology, Taiwan. He
is currently a Senior Crystallographic Specialist in the X-ray Diffraction Laboratory, Instrumentation Center,
National Taiwan University. He has co-authored more than 360 peer-reviewed articles in scientific journals,
among which seven were with Prof. Tien-Yau Luh.
Chun-hsien Chen obtained the B.S. degree from the National Sun Yat-sen University (NSYU) in 1986, and the
Ph.D. degree from the University of Illinois at Urbana-Champaign, USA, in 1993. After spending 2 years as a
Postdoctoral Research Associate at The University of North Carolina at Chapel Hill, USA, he joined the
Department of Chemistry, NSYU. He moved to the National Tsing Hua University, Taiwan, in 2000 and to
the National Taiwan University in 2006. He is a distinguished professor of NTU and a recipient of the Out-
standing Research Award from NSC (National Science Council, 2008) and MOST (Ministry of Science and
Technology, 2015), and the Outstanding Scholar Award (Foundation for the Advancement of Outstanding
Scholarship, 2016). His current research interests include electrochemistry and scanning probe microscopy on
phenomena at the solid–liquid interface. He and Prof. Luh published their first collaborative work in 2005 and
have thus far co-authored 22 journal articles.
Shie-Ming Peng received the B.S. degree from the National Taiwan University (NTU) in 1970, and the Ph.D.
degree from the University of Chicago, USA, in 1975. After finishing a Postdoctoral Fellowship at Northwest-
ern University, he became an Associate Professor (1976–1980) and a Full Professor (1980) at NTU. He is
currently a Chair Professor of NTU. He was the Chairman of the Department of Chemistry from 1987 to 1990
and the Vice President of NTU from 1999 to 2002 and Academia Sinica from 2011 to 2014. He served as the
President of the Chemical Society, Taipei, during 2001–2002. In 1998, he was elected as the Academician of
Academia Sinica. He was also elected as a member of TWAS (The Third World Academy of Sciences) in
2004. He has been a recipient of the TWAS prize in chemistry in 2002, the Taiwan–French Science Prize in
2008, the Japan Society of Coordination Chemistry International Award in 2011, the Presidential Science Prize
of Taiwan in 2013, the Luigi Tartufari International Chemistry Prize (Italian Academy of Sciences) in 2014,
and the Russian Academy of Engineering Award in 2014. His current research interests include the development
of metal string complexes and investigating their applications in molecular electronics.
INTRODUCTION
Heterometallic complexes have attracted interest
anion) was found to be 40% more conductive than its
analog [Ni (tpda) (NCS) ] (5) (tpda = tripyridyldia-
5 4 2
21
2
−
1
,2
from several disciplines such as catalysis, molecular
mido dianion). The superior conductance of the
former is attributed to a larger bond order of the
3,4
5–9
magnetism, and molecular electronics.
Among the
3+
one-dimensional complexes, many of which are electro-
mixed-valence [Ni
(naph)
2
]
4
moieties (naph = 1,8-
10
statically assembled by ionic building blocks,
naphthyridyl) in which an electron is delocalized
between two nickel centers and renders a stronger
extended metal-atom chains (EMACs) are molecule-
based and the metal cores are linearly organized by four
equatorial ligands. In the string, heterometals can be
site-specifically arranged in a desirable sequence to
some extent via rationally designed synthetic
4
+
nickel–nickel interaction than that of [Ni
natively, metal–metal interactions are heavily associ-
]
in 5. Alter-
2
2
3
ated with the number of d electrons and can be
intriguing because of the incorporation of a multiply
11–14
24,25
procedures.
Additionally, a variety of ligands have
bonded dimetal unit into an EMAC.
Here, we pro-
been developed from prototypical α-oligopyridylamido
pose novel heterometallic EMACs (Scheme 1) by com-
1
5,16
17
anions to heterocyclics,
fused rings,
and
bining the above-mentioned approaches of employing
1
8–22
−
naphthyridyl-modulated ones.
The latter contain
the bna ligand and M
2
units (i.e., Mo
2
or Ru
), where
2
fewer amido units and thus bear fewer negative charges
than those of the prototypical α-oligopyridylamido
anions, leading to a strategy to tune the metal–metal
the former has great potential to stabilize the metal cen-
ters with reduced charges and the latter carries strong
metal–metal interactions. Specifically, the metal strings
2
0–22
interactions of EMACs.
For example, [Ni (b-
studied are [Ni
Mo
2
2
Ni(bna)
Cl
4
](PF
2
)
6
3
(1), [Ni
2
Mo
Cl ]
2
2
Ni
5
−
na) (NCS) ](NCS) (4) (bna = bisnaphthyridylamido
(bna) (NCS) ] (PF
4
2
)
6
3
(2), and [Ni
2
Ru Ni(bna)
2
4
4
2
2
2
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JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Metal Strings with High Bond-Order Units
yield of 1 (<1%) and 3 was unattainable, which are
attributed to the difficulty in substituting the less labile
nickel centers by the more labile molybdenum or ruthe-
26
nium. Hence, the procedures of Scheme 1 have the
advantage of a relatively high yield of ~3% due to the
Scheme 1. High-bond-order units for the design of
new heterometallic strings.
n+
suppression of [Ni (bna) Cl ] formation. In fact, even
5
4
2
with Scheme 1, the yield of EMAC 3 was low (~1%)
and the amount was not enough for the exchange of –
(
ClO ) (3). Their properties are explored through mag-
4 3
n
netic measurements, cyclic voltammetry, and single-
molecule conductance measurements.
NCS-terminated axial ligands [Ni Ru Ni(bna) (NCS) ]
2 2 4 2
+
and also insufficient for magnetic characterization.
X-ray characterization
RESULTS AND DISCUSSION
Syntheses of pentanuclear heterometallic string
complexes
Figure 1 shows the Oak Ridge thermal-ellipsoid
plots (ORTEP) of EMACs 1 and 3, in which a linear
string of five metal centers is supported by four equato-
−
Scheme 2 outlines the preparation of EMACs 1–3
via a one-pot approach which has multiple steps yet
without involving intermediate purification procedures.
The synthesis of the equatorial ligand Hbna was reported
rial bna ligands and capped by two axial chloride ions.
The ratio of cationic EMACs to counterions is 1:3 in
the unit cell (Table 1), suggesting that the EMACs
carry three positive charges. The fact that Mo1 (or Ru3
of Figure 1) sits in the central position of the string cre-
ates two orientations in the crystal packing and thus
leads to crystallographic disorder.
21
elsewhere. For EMACs 1 and 3, Hbna was first reacted
1
3,14
with M (OAc) (M = Mo, Ru)
in boiling naphtha-
2
4
lene under argon atmosphere, presumably resulting in
M (bna) . Ni(OAc) ꢀ4H O and LiCl were subsequently
The selected bond distances for 1 and 2 are shown
in Figure 2. The corresponding molybdenum bond
lengths in 1 and 2 are 2.13 and 2.11 Å, which correlate
well with typical quadruple bond lengths of 2.06–2.17 Å
2
4
2
2
introduced, which was followed by an excess amount of
KPF , a counter ion to neutralize the charge for crystalli-
6
zation. EMAC 2 was obtained by replacing the axial
27
ligands with excess NaSCN. Among the five coordina-
reported in the literature. The distances between molyb-
denum and the adjacent nickel are 2.37 and 2.51 Å for
−
tion sites in bna (i.e., four N
and one Namido sites),
naph
28
the most likely one to form M (bna) is Namido, which
1 and 2.41 and 2.53 Å for 2. Although the values of
Ni─Mo spacing are smaller than the sums of the radii of
2
4
bears a negative charge whereas the others are neutral.
In principle, EMACs 1 and 3 could be prepared
by swapping the steps. That is, the first step would yield
29
molybdenum and nickel, we believe that the unusually
−
short spacing is due to the helicity of bna ligands rather
n+
n+
[
Ni (bna) Cl ] (6 ) in which the nickel centers would
than the formation of Mo─Ni bonds because, as the men-
5
4
2
later be displaced by dimolybdenum and diruthenium,
respectively. In practice, this alternative gave a low
tioned above, the dimolybdenum bond lengths are not
longer than those of typical Mo quadruple bonds. In
2
Scheme 2. Synthesis of EMACs 1—3.
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Special Issue Paper
Hsu et al.
2
+
characterization of [Ni (bna) Cl ](PF ) (6 ), and its
5
4
2
6 2
4+
chemically oxidized form [Ni (bna) Cl ](PF ) (6 )
Figure 2). The respective Ni1–Ni2 distances of 1 and
are 2.31 and 2.33 Å, similar to 2.33 Å of [Ni (naph) ]
in 6 but shorter than 2.41 Å of [Ni (naph) ] in 6 .
5
4
2
6 4
21
(
3
2
2
4+
4
+
2+
4+
2
4
Accordingly, the dinickel units in 1 and 2 have a bond
order of 0.5 with a delocalized electron.
Figure 3 shows the selected bond distances for
EMAC 3, which are quite similar to those of EMACs
1, except the diruthenium moiety. Also illustrated for
comparison in Figure 3 is EMAC 7 ([Ni Ru Ni
2
2
(
tpda) (NCS) ]), an α-pyridylamido analog of EMAC
4
2
3
. The diruthenium unit has been determined as a
5+
mixed-valence [Ru2] with a bond order of 2.5 and a
Ru─Ru spacing of 2.26 Å. For typical diruthenium
double bonds, the distance is 2.238–2.408 Å. There-
fore, for EMAC 3, the spacing of 2.31 Å is likely due
to a double-bonded [Ru2] unit.
26
30
4+
Magnetic properties
Measurements of magnetic susceptibility for
EMACs 1 and 2 were carried out on polycrystalline sam-
ples in the temperature range 4–300 K (Figure 4). The
respective values for 1 and 2 at 300 K are 1.44 and
2
.00 emu K/mol, which decrease on cooling. At 4 K, the
magnetic susceptibility reaches 0.44 and 0.58 emu K/mol,
suggesting that their ground states adopt an S of 1/2.
Based on their structural characteristics, some fea-
tures of their electronic configurations can be inferred:
3
+
the mixed-valence Ni
unit has a spin state S12 of 3/2;
2
4+
the Mo2 unit is a diamagnetic unit (S34 = 0); and the
terminal nickel center in square-pyramidal geometry is
a high-spin state (S = 1). Therefore, the behaviors of
5
1
and 2 are described by a model with two magnetic
centers, S = S12 + S . The values of 1.44 and 2.00 emu
5
K/mol of EMACs 1 and 2 are significantly smaller than
the theoretical spin-only value of 2.88 emu K/mol. The
discrepancy indicates relatively strong antiferromag-
netic interactions between the two magnetic centers.
Considering the interactions between the two mag-
netic centers (i.e., S12 and S ) and the Zeeman splitting,
5
the Heisenberg–Dirac–van Vleck Hamiltonian is formu-
lated as follows:
Fig. 1. Molecular structures of EMACs 1 (top) and
3
(bottom). Ellipsoids are all drawn at 50%
probability levels. Hydrogen atoms and inter-
stitial solvents are omitted for clarity.
2+
addition, the bonding geometries of Ni as square-
4+
planar and Mo2 as paddlewheel might be more stable
than having a Ni─Mo bond on the axial sites.
For the discussion of the Ni1–Ni2 distance, we
consider the literature results of the crystallographic
^
H = −JS12 ꢀS5 + gβS ꢀH
ð1Þ
4
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Metal Strings with High Bond-Order Units
Table 1. X-ray crystallographic data for 1 and 3
Compound
1ꢀ3CH
2
Cl
2
C
2
H
4
Cl
2
2 4 2
3ꢀ6C H Cl
Formula
Crystal system
Space Group
a (Å)
C H Cl F Mo N Ni P
C H Cl N Ni O Ru
76 63 17 20 3 12 2
Orthorhombic
P2 /c
1
13.7413(6)
21.8453(9)
35.3539(15)
90
93.2750 (9)
90
6
9
50 10 18
2
20
3
3
Orthorhombic
P2
1
2
1
2
1
14.1368(4)
20.8313(6)
27.3607(8)
90
90
90
b (Å)
c (Å)
ꢁ
α ( )
ꢁ
β ( )
ꢁ
γ ( )
V (Å )
3
8057.4(4)
4
10 595.3(8)
4
Z
T (K)
150(2)
0.0451/0.1181
0.0546/0.1258
150(2)
0.0576/0.1414
0.0824/ 0.1605
1
2
R1 /wR2 [I > 2σ(I)]
1
2
R1 /wR2 (all data)
P
P
1
R1 = |F | − |F |/ |F |.
o
c
2
o
2 2
c
P
P
2
2 2 1/2
o
2
2
2
2
2
wR2 = { [w(Fo − F ) ]/ [w(F ) ]} , in which w = 1/σ (F ) + (aP) + bP, P = (F + 2Fc )/3.
o
o
The temperature dependence of the magnetic sus-
ceptibility for 1 and 2 is satisfactorily reproduced
Figure 4) with the fitting parameters listed in Table 2.
The intramolecular antiferromagnetic exchange
coupling between the two magnetic centers has two pos-
(
sible communication pathways: (1) coupling through
2
The fitting results of the exchange parameter J suggest
the central dimolybdenum unit involving the d /σ
z
31
that the antiferromagnetic interactions of EMACs
orbitals and/or (2) interacting along the routes defined
2+
−
2
2
1
6
and 2 are stronger than those of its analogs 6 and
.
by the bna ligands and the d
-like orbitals of the
x −y
4
+ 21
32
metal atoms. Because the Ni─Nnaph bond lengths of
EMACs 1 and 2 show insignificant difference (≤0.03 Å)
from those of the corresponding magnetic centers in
Fig. 2. Selected bond distances of EMACs (top)
Fig. 3. Selected bond distances for 3 (top, the most
1
(red, X = Cl) and 2 (blue, X = NCS);
possible one) and 7 (bottom, [NiRu Ni
(tpda) (NCS) ]).
4 2
2 2
2
+
4+
1
26
(
bottom) 6 (red) and 6 (blue).
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Special Issue Paper
Hsu et al.
Fig. 5. Cyclic voltammograms of EMACs 1 (left)
and 2 (right) in CH Cl2 containing 0.1 M
2
TBAP with a scan rate of 0.1 V/s.
Fig. 4. Plots of χMTM versus T of EMACs 1 ( ) and
properties, the formal oxidation models are proposed to
be Ni2 −Mo2 −Ni for 1 and 2, denoted as M5 in
2
( ) with fitting lines (
). The parameters
3+
4+
2+
9+
are shown in Table 2 with the coefficients of
2
determination, R , being 0.9958 for 1 and
Table 3. We tend to attribute the peaks at ~1.0 and
0
.9983 for 2.
3+/2+
5+/4+
1
.2 V to the redox reactions of Ni2
and Mo2
,
but we are unable to identify further which peak
belongs to which reaction. Likewise, due to the lack of
necessary information, the peak at ~0.0 V might be
Table 2. Fitting parameters of the temperature-dependent
n+
magnetic susceptibility for EMACs 1, 2, and 6
−
1
2+/1+
4+/3+
3+/2+
EMACs
g
J (cm
)
associated with Ni
other event.
Ren and coworkers studied the redox potentials
of a series of quadruply bonded Mo2 coordinated
, Mo2
, Ni2
, or any
[
[
[
[
Ni
Ni
2
2
Mo
Mo
2
2
Ni(bna)
Ni(bna)
4
Cl
2
](PF
6
)
3
(1)
2.07
2.02
2.17
2.01
−45.56
−46.73
−15.86
−34.03
33
4
(NCS)
2
](PF ) (2)
6 3
2
+
21
21
4+
Ni (bna) Cl ](PF ) (6
)
)
5
4
2
6 2
4
+
Ni
5
(bna)
4
Cl
2
](PF
6
)
4
(6
with four diarylformamidinate ligands (ArNC(H)
−
NAr ). The aryls were derivatized with electron-
donating groups (EDGs) and electron-withdrawing
2
+
4+
EMACs 6 and 6 (Figure 2), Pathway 2 is unlikely
to be the dominant one for the exchange coupling.
Thus, the communication between the magnetic centers,
groups (EWGs). It was demonstrated that a stronger
5+/4+
EWG led the redox event of Mo2
to a more posi-
tive potential: for example, at 0.244 V (vs. EAg/AgCl
with the p-methoxyphenyl group and 0.944 V with the
,5-dichlorophenyl group. Given that naphthyridyl is a
strong EWG, it is reasonable that one of the peaks at
)
3+
2+
2
Ni2 and the terminal Ni , is made through the d /σ
orbitals of the Mo unit.
z
2
3
Electrochemistry and electronic absorption spectra
EMACs 1 and 2 exhibit three reversible one-
electron redox couples in the potential range −0.5 to
.5 V against EAg/AgCl (Figure 5). Table 3 summarizes
their E1/2 values. Based on the structural features and
5+/4+
1
.2 and 1.0 V involves Mo2
Table 3 also displays the E1/2 values of EMAC 6
a pentanickel string complex with the equatorial and
.
2
+
,
1
axial ligands identical to those of 1. The formal
Table 3. Redox potentials for 1 and 2
n + 1/n+
5
E
1/2 of M
(V vs. EAg/AgCl)
1
1+/10+
10+/9+
9+/8+
8+/7+
EMAC
Ni(bna) Cl ](PF
M5
M5
M5
M5
Ref.
[
[
[
[
Ni
2
Mo
2
4
2
6
)
3
(1)
1.20
1.26
n.a.
0.65
1.02
1.07
0.46
n.a.
0.01
0.02
0.16
n.a.
n.a.
This work
This work
Ni Mo Ni(bna) (NCS) ](PF ) (2)
n.a.
−0.59
n.a.
2
5
5
2
4
2
6 3
2
+
21
Ni
Ni
(bna)
(tpda)
4
Cl
Cl
2
6 2
](PF ) (6 )
34
4
2
]
6
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Metal Strings with High Bond-Order Units
Fig. 7. Conductance histograms of EMAC 2 obtained
by the method of STM break junction. Inset:
typical G–s (conductance–tip displacement)
traces. The histograms were constructed by
more than 5000 G–s traces without data selec-
tion. Other conditions: Ebias, 0.10 V; speed to
tip displacement, 50 nm/s; solvent, propylene
carbonate.
Fig. 6. UV–vis spectra of EMACs 1 (
), 2 (
),
and Hbna ligand (
) in CH Cl .
2
2
2
+
3+
2+
3+
oxidation model for 6 is Ni2 –Ni –Ni
and is
2
8
+
expressed by M5 in Table 3. The redox event at
inset of Figure 7 displays typical G–s traces, in which
the conductance was monitored as a function of the
distance of the STM tip moving away from the sub-
strate. The conductance value first overshot the scale
1
0+/9+
0.46 V, namely M5
, is attributed to the reaction of
4
+/3+
the second [Ni (naph) ]
moiety whose coordination
environments are similar to those of the corresponding
2
4
2+
2
−1
mixed-valence unit in 1. The fact that for 1 and 6 the
redox potentials of [Ni (naph) ]
and then reached 1 G (1 G = 2e /h ≈ (12.9 kΩ) ),
0 0
4
+/3+
differ by more
indicating the formation of a single conducting chan-
2
4
than 0.55 V shows the significant influence from Mo2,
although crystallographic data show negligible metal–
nel or the minimum cross-section of the tip–substrate
37
contact being a single gold atom. Subsequently, a
sharp drop in the conductance took place and, at
3
+
metal interactions between Ni2
metal center.
and its immediate
−5
~10
G , the trace reached a plateau. The sharp drop
0
The absorption spectra of EMACs 1, 2 and the
shows characteristics of breaking the tip–substrate con-
Hbna ligand (Figure 6) show peaks at ~225 and
nection and the development of the gap rapidly to
2
65 nm, attributed to n–π* and π–π* transitions of the
5–7 Å due to the snapback motion at the apices of
−
38,39
bna ligands. Compared to the corresponding transition
of free Hbna ligands, the peaks are red-shifted. A shoul-
der at 400–450 nm is consistent with the δ–δ* transition
from the quadruple bond of the Mo unit. A broad
and weak band at 500–700 nm is ascribed to the inter-
the gold electrodes.
The conductance plateau
−5
(e.g., at ~10
G
) is a signature of the presence of an
0
EME junction because the dependence of conductance
on the gap size is significantly smaller than that of
electron tunneling through void space.
Figure 7 displays the conductance histograms of
EMAC 2 by pooling more than 5000 G–s traces. The
3
3
2
3
5
valence charge transfer (IVCT) band.
Single-molecule conductance
histograms show a sharp peak at 1.0 G , representing a
0
The conductance of EMAC 2 was measured by
scanning tunneling microscopy (STM)-based break
junction (STM bj), one of the standard approaches for
single gold atom configuration prior to breaking the
tip–electrode contact (see below). The peak centered at
−
5
1.58 × 10
G is ascribed to the most probable molecu-
0
36,37
the measurements of single-molecule conductance.
lar conductance of EMAC 2. With the range of 1σ
To form electrode–molecule–electrode (EME) junc-
tions for the measurements, sulfur-terminated axial
ligands (i.e., EMAC 2), gold STM tips, and a gold
substrate were utilized to take the advantage of the
strong sulfur–gold interactions. The tip was repeatedly
pushed against the substrate and pulled back. The
(standard deviation of the peak distribution), the distri-
−
6
−5
bution spanned from 5 × 10 to 4.3 × 10
value is surprisingly smaller than 74 (ꢂ7) × 10
(or 17.6 (ꢂ1.9) MΩ) of 4 ([Ni (bna) (NCS) ](NCS)
(or 24.4 (ꢂ2.0) MΩ) of the pro-
G . This
0
−
5
G
0
)
2
5
4
2
−5
and 53 (ꢂ4) × 10
totypical [Ni (tpda) (NCS) ].
4 2
G
0
21
5
J. Chin. Chem. Soc. 2017
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7

Special Issue Paper
Hsu et al.
The conductance histogram at the right panel in
oxidation because the redox event is ~.5 V more posi-
3+
Figure 7 is plotted against tip displacement which, in
tive than that of 6 . The conductance of 2 was
40,41
−5
some examples,
can resolve the conductance peaks
1.58 × 10 G (≈810 MΩ). This finding is counterintui-
0
arising from different origins. In this tip displacement–
tive in that the conductance of the higher bond-ordered
conductance map, there is, however, only one group of
2 is an order of magnitude less than that of [Ni (tp-
5
−5
−5
conductance distribution at 1.3 × 10 G where the
da) (NCS) ] (53 (ꢂ4) × 10 G or 24.4 (ꢂ2.0) MΩ)
0
4
2
0
gap extended by an additional 7 Å after the EME junc-
tions were formed. It is intriguing that EMAC 2 bears
high-bond-order units yet appears an order of magni-
tude less conductive than [Ni (tpda) (NCS) ] in which
whose metal–metal interaction is considered among the
weakest in EMACs. Although it was determined that
no Mo─Ni bond is present in 1 and 2, the strong effect
of Mo on the molecular properties is evident. To corre-
5
4
2
2
the metal–metal interactions are among the weakest in
EMACs. More comprehensive information, such as
simulations of transmission spectra or measurements of
conductance as a function of bias potentials, might be
helpful to reveal the reason why the incorporation of
late comprehensively the properties with the structures
of heterometallic EMACs, more characterizations and
simulations on tailored compounds are necessary.
EXPERIMENTAL
Materials
3
+
Mo and mixed-valence Ni
provides 2 with an infe-
2
2
rior conductance.
All chemicals were of reagent grade and used
without further purification unless otherwise noted. The
precursors of 2-chloro-1,8-naphthyridine, 2-amino-1,8-
naphthyridine, Mo (OAc) , and Ru (OAc) Cl were pre-
CONCLUSIONS
To a pentanickel framework, high-bond-order
units were incorporated to develop new heterometallic
strings [Ni Mo Ni(bna) X ](PF ) (X = Cl, 1; X =
2
4
2
4
42,43
pared according to standard literature procedures.
21
Bisnaphthyridylamide (Hbna). A mixture of 2-amino-
1,8-naphthyridine (14.5 g, 100 mmol), 2-chloro-1,8-
naphthyridine (16.4 g, 100 mmol), t-BuOK (11.2 g,
2
2
4
2
6 3
NCS, 2) and [Ni Ru Ni(bna) Cl ](PF ) (3), by having
2
2
4
2
6 3
−
bna coordinated with the dimetal precursor (Mo or
2
Ru ) first, followed by the reaction with Ni(OAc) . The
100 mmol), Pd (dba)3 (0.91 g, 1 mmol), and dppp
2
2
2
−
equatorial bna ligand is a naphthyridyl-based penta-
dentate with one negative charge less than the prototyp-
(0.82 g, 2 mmol) was placed in a flask under argon and
refluxed in THF (300 mL) for 72 h. The solvent was
removed under reduced pressure. The solid sediment
was washed by methanol and ethyl ether. Subsequent
extraction by dichloromethane yielded a light yellow
2
−
ical tpda , tripyridyldiamido dianion, and gives the
EMAC a total of six negative charges (i.e., four from
the equatorial ligands and two from the axial ones) to
be balanced by five metal centers. Consequently, the
1
product. Yield: 22.96 g, 84.1%. H NMR (400 MHz;
3+
product contains a mixed-valent moiety [Ni (naph) ] ,
[D]DMSO): δ = 11.02 (s, 1H), 8.92 (dd, J = 5.97,
2.28 Hz, 2 H), 8.33 (m, 6 H), 7.44 ppm (dd, J = 12.23,
2
4
which is also a magnetic center as well as an electro-
chemical redox center. The yield of 3 was low (<1%),
and the chromatographically purified amount was suffi-
cient only for characterization by X-ray crystallogra-
phy, mass spectrometry, and IR spectroscopy. For
+
3.56 Hz, 2H). MS (ESI-MS): m/z: 274.11 [M + H] . IR
(KBr): v~ = 3431vs, 1603s, 1558s, 1535s, 1491s, 1420s,
1382s, 1337s, 1307s, 1234vs, 1135s, 852s, 836s, 807s,
−
1
778vs, 739vs, 593vs cm .
1
and 2, the structural features suggest no significant
[Ni Mo Ni(bna) Cl ](PF )
6 3
0.6 mmol), Mo (OAc) (0.1284 g, 0.3 mmol), and naph-
2 4
(1). Hbna (0.1638 g,
2
2
4
2
4
+
bonding interactions between the Mo2 unit and the
2
neighboring nickel centers, specifically, the terminal Ni
thalene (0.04 g) were stirred and refluxed under argon
atmosphere until the color of the solution turned deep
reddish-brown in about 3 h. The solution was cooled to
+
3+
and the mixed-valent Ni2 . Interesting properties are
−
found when 1 and 2 are compared with their bna ana-
log, [Ni2 − Ni − Ni2 (bna) Cl ]
3+
2+
3+
4+
4+
ꢁ
(6 ). For
~120 C and quenched with n-hexane (200 mL) to wash
4
2
EMAC 1, the antiferromagnetic exchange coupling
out naphthalene. After filtration, the product was
extracted with CH Cl (75 mL) in an Erlenmeyer flask
and evaporated to dryness under reduced pressure. The
4+
across Mo2 is found to be −45.56/cm, which is larger
2
2
2
+
2+
4+
than those across Ni
(6 , −15.86/cm; 6 ,
3
+
−
34.03/cm). The [Ni (naph) ] unit in 1 is less prone to
initial product, Ni(OAc) ꢀ4H O (0.0149 g, 0.06 mmol),
2
4
2
2
8
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JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Metal Strings with High Bond-Order Units
LiCl (0.0509 g, 1.2 mmol), and naphthalene (0.04 g)
were reacted for 3 h again. After quenching with
n-hexane and extracting with dichloromethane in
round-bottomed flask, the product was electrostatically
dichloromethane/acetone (2:3) as the eluent to afford
[Ni Ru Ni(bna) Cl ](ClO )
4 3
which appeared dark
2
2
4
2
green. The resulting solution was stirred for a day and
dried under vacuum. Then, the solid was extracted with
C H Cl and layered with n-hexane. After 1 week, deep
neutralized by KPF in dichloromethane (50 mL). To
6
2
4
2
purify this material, column chromatography was per-
formed with aluminum oxide using dichloromethane/
acetone/methanol (8:2:0.2) as the eluent to afford
green crystals were obtained. Yield: trace (<1%). MS
(MALDI): m/z: 1538.25 [M] . IR (KBr): v~ = 3424s,
+
1609s, 1556vs, 1496s, 1458vs, 1429s, 1374s, 1292vs,
1213vs, 1145s, 1110s, 1088s, 846vs, 780vs, 693vs,
[
Ni Mo Ni(bna) Cl ](PF ) (1) as a dark green solution.
2 2 4 2 6 3
−1
The resulting solution was stirred for a day, and then
dried under reduced pressure. The solid was extracted
with dichloromethane and 1,2-dichloroethane and lay-
ered with n-hexane. After 1 week, deep green crystals
were obtained. Yield: 0.0082 g, 3%. MS (MALDI): m/
625vs cm .
Physical measurements
Magnetic susceptibility data were collected on a
Quantum Design SQUID-VSM system in the range
4–300 K with an instrument with external magnetic
field 3000 G. The MALDI MS was performed on a
Bruker MALDI-TOF mass spectrometer ( New Ultra-
+
z: 1528.00 [M] . IR (KBr): v~ = 3424vs, 1608s, 1553s,
1
8
497s, 1429s, 1365m, 1294s, 1219vs, 1153s, 1028vs,
38m, 803s, 779s, 699vs, 649vs, 558s cm .
−
1
1
[
Ni Mo Ni(bna) (NCS) ](PF ) (2). Excess NaSCN was
fleXtreme™, Bremen, Germany). H NMR spectra for
2
2
4
2
6 3
added to a solution of 1 in dichloromethane under
argon atmosphere. The resulting solution was stirred
for a day and dried under reduced pressure. The solid
was extracted with dichloromethane until dissolved and
layered with n-hexane. After 1 week, deep green crystals
organic and inorganic complexes were recorded in
DMSO and CD Cl2 with a Bruker AMX 400 MHz
2
spectrometer. Electronic spectra of 1 and 2 in CH Cl2
2
were measured in the range 200–800 nm on a Hitachi
(U-3310) UV–vis spectrophotometer. Electrochemistry
was carried out on a CH instrument (Model 750 A),
using CH Cl solvent with 0.1 M TBAP and 1 mM
were obtained. Yield: 99%. MS (MALDI): m/z: 1573.26
+
[
M] . IR (KBr): v~ = 3431s, 2064s, 1609s, 1559s, 1497s,
2
2
1
8
428s, 1364s, 1292vs, 1247vs, 1220vs, 1152s, 1029vs,
EMACs. Cyclic voltammetry was performed with a
homemade three-electrode cell equipped with a BAS
glassy carbon disk (0.07 cm ) as the working electrode,
a platinum wire as the auxiliary, and a homemade
Ag/AgCl (saturated) reference electrode. The reference
electrode was separated from the bulk solution by a
double-junction frit filled with electrolyte solution.
Potentials were reported versus EAg/AgCl(saturated) and
−1
39s, 803vs, 781vs, 699vs, 558vs cm .
(3). Hbna (0.1638 g,
.6 mmol), Ru (OAc) Cl (0.1421 g, 0.3 mmol), and
2
[
Ni Ru Ni(bna) Cl ](ClO )
4 3
2
2
4
2
0
2
4
naphthalene (0.04 g) were stirred and refluxed under
argon atmosphere until the color of solution turned to
deep brown. After 3 h of reaction, the solution was
ꢁ
cooled to ~120 C and quenched with n-hexane
(
200 mL) to wash out naphthalene. After filtration, the
referenced to the ferrocene-ferrocenium ([Cp Fe]/
2
+
product was extracted with dichloromethane (75 mL) in
an Erlenmeyer flask and dried under reduced pressure.
The initial product, Ni(OAc) ꢀ4H O (0.0896 g,
[Cp Fe] ) couple, which took place at an E
of
2
1/2
+0.54 V against EAg/AgCl(saturated). The working elec-
trode was polished with 0.03 μm alumina on Buehler
felt pads and was subjected to ultrasound for 1 min
prior to each experiment. The reproducibility of indi-
vidual potential values was within ꢂ5 mV.
2
2
0
.36 mmol), LiCl (0.0509 g, 1.2 mmol), and naphtha-
lene (0.04 g) were reacted for another 0.5 h. A solution
of t-BuOK (0.0404 g, 0.36 mmol) in n-butanol (10 mL)
was added dropwise. After 3 h, the solution was cooled
and quenched with n-hexane (200 mL). After filtration,
the product was extracted with 1,2-dichloroethane
X-ray structure determinations
Crystallographic data were collected at 150 (2) K
on a NONIUS Kappa CCD diffractometer with mono-
chromatized Mo Kα radiation (λ = 0.71073 Å) for
EMACs 1, 2, and 3. The cell parameters were retrieved
and refined with DENZO-SMN software on all
(200 mL) in round-bottomed flask, and NaClO was
4
added to provide anions to balance the charge of the
compound. To purify this material, column chromatog-
raphy was performed with aluminum oxide using
J. Chin. Chem. Soc. 2017
© 2017 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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9

Special Issue Paper
Hsu et al.
4
4
observed reflections. Data reduction was performed
with the DEMZO-SMN software. Empirical absorption
was based on the symmetry-equivalent reflection, and
absorption corrections were applied with the SORTAV
7. T.-C. Ting, L.-Y. Hsu, M.-J. Huang, E.-C. Horng,
H.-C. Lu, C.-H. Hsu, C.-H. Jiang, B.-Y. Jin, S.-M. Peng,
C.-h. Chen, Angew. Chem. Int. Ed. 2015, 54, 15734.
8
. N. Xin, J. Wang, C. Jia, Z. Liu, X. Zhang, C. Yu, M. Li,
S. Wang, Y. Gong, H. Sun, G. Zhang, Z. Liu, G. Zhang,
J. Liao, D. Zhang, X. Guo, Nano Lett. 2017, 17, 856.
. G. Turkoglu, M. E. Cinar, T. Ozturk, Molecules 2017,
45
program. All structures were solved and refined with
the SHELX-97 programs.
4
6,47
The hydrogen atoms
9
were included in calculated positions and refined with a
riding mode.
Crystallographic data (excluding structure factors)
for the structures in this paper have been deposited in
the Cambridge Crystallographic Data Centre (CCDC,
2
2, 1522.
10. S.-A. Hua, M.-C. Cheng, C.-h. Chen, S.-M. Peng, Eur.
J. Inorg. Chem. 2015, 2015, 2510.
1. P.-J. Chen, M. Sigrist, E.-C. Horng, G.-M. Lin,
1
G.-H. Lee, C.-h. Chen, S.-M. Peng, Chem. Commun.
2
017, 53, 4673.
12 Union Road, Cambridge CB21EZ, UK). Copies of
1
2. W.-Z. Wang, D. Zhao, T.-B. Tsao, R. Ismayilov, G.-H. Lee,
S.-M. Peng, Eur. J. Inorg. Chem. 2015, 2015, 4329.
3. M.-C. Cheng, C.-L. Mai, C.-Y. Yeh, G.-H. Lee,
S.-M. Peng, Chem. Commun. 2013, 49, 7938.
14. M. Nippe, G. H. Timmer, J. F. Berry, Chem. Commun.
2009, 4357.
the data can be obtained free of charge on quoting the
depository numbers CCDC-1576075 for EMAC 1,
CCDC-1576076 for EMAC 2, and CCDC-1576077 for
EMAC 3 (Email: deposit@ccdc.cam.ac.uk, http://www.
ccdc.cam.ac.uk).
1
1
5. C.-C. Yang, I. P.-C. Liu, Y.-J. Hsu, G.-H. Lee, C.-h. Chen,
S.-M. Peng, Eur. J. Inorg. Chem. 2013, 2013, 263.
6. W.-Z. Wang, R. H. Ismayilov, R.-R. Wang,
Y.-L. Huang, C.-Y. Yeh, G.-H. Lee, S.-M. Peng, Dalton
Trans. 2008, 6808.
ACKNOWLEDGMENTS
1
Mass spectra were recorded by Dr. Mei-Chun
Tseng at the Mass Spectrometry Facility and
SQUID measurements were made by the group of
Dr. Ming-Hsi Chiang at the Institute of Chemistry,
Academia Sinica. This work was supported by the
Ministry of Science and Technology and the Minis-
try of Education, Taiwan.
1
7. W.-Z. Wang, C.-L. Hsieh, R. H. Ismayilov, C.-H. Hsu,
I. P.-C. Liu, Y.-H. Liu, G.-H. Lee, S.-M. Peng, New
J. Chem. 2012, 36, 2340.
8. C.-S. Tsai, I. P.-C. Liu, F.-W. Tien, G.-H. Lee,
C.-Y. Yeh, C.-h. Chen, S.-M. Peng, Inorg. Chem. Com-
mun. 2013, 38, 152.
1
1
9. I. P.-C. Liu, C.-H. Chen, C.-F. Chen, G.-H. Lee,
S.-M. Peng, Chem. Commun. 2009, 577.
Supporting information
2
0. R. H. Ismayilov, W.-Z. Wang, G.-H. Lee, C.-Y. Yeh,
S.-A. Hua, Y. Song, M.-M. Rohmer, M. Benard,
S.-M. Peng, Angew. Chem. Int. Ed. 2011, 50, 2045.
1. I. P.-C. Liu, M. Benard, H. Hasanov, I.-W. Chen,
W.-H. Tseng, M.-D. Fu, M.-M. Rohmer, C.-h. Chen,
G.-H. Lee, S.-M. Peng, Chem.-Eur. J. 2007, 13, 8667.
Additional supporting information is available in
the online version of this article.
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11