Experimental and Computational Studies of the Single-Molecule Conductance of Ru(II) and Pt(II) trans-Bis(acetylide) Complexes

Article abstract of DOI:10.1021/acs.organomet.6b00472

The single-molecule conductance of metal complexes of the general forms trans-Ru(C≡CArC≡CY)₂(dppe)₂ and trans-Pt(C≡CArC≡CY)₂(PPh₃)₂ (Ar = 1,4-C₆H₂-2,5-(OC₆H₁₃

Full text of DOI:10.1021/acs.organomet.6b00472

Article
pubs.acs.org/Organometallics
Experimental and Computational Studies of the Single-Molecule
Conductance of Ru(II) and Pt(II) trans-Bis(acetylide) Complexes
Oday A. Al-Owaedi,†,‡,# David C. Milan,§,# Marie-Christine Oerthel,∥,# Soren Bock,⊥ Dmitry S. Yufit,∥
̈
Judith A. K. Howard,∥ Simon J. Higgins,§ Richard J. Nichols,*,§ Colin J. Lambert,*,† Martin R. Bryce,*,∥
and Paul J. Low*,⊥
†Department of Physics, University of Lancaster, Lancaster LA1 4YB, U.K.
‡Department of Laser Physics, Women Faculty of Science, Babylon University, Hilla, Iraq
§Department of Chemistry, University of Liverpool, Crown Street, Liverpool L69 7ZD, U.K.
∥Department of Chemistry, Durham University, South Road, Durham DH1 3LE, U.K.
⊥School of Chemistry and Biochemistry, University of Western Australia, 35 Stirling Highway, Crawley, Perth 6009, Australia
S
* Supporting Information
ABSTRACT: The single-molecule conductance of metal
complexes of the general forms trans-Ru(CCArC
CY)2(dppe)2 and trans-Pt(CCArCCY)2(PPh3)2 (Ar =
1,4-C6H2-2,5-(OC6H13)2; Y = 4-C5H4N, 4-C6H4SMe) have
been determined using the STM I(s) technique. The
complexes display high conductance (Y = 4-C5H4N, M = Ru
(0.4 0.18 nS), Pt (0.8 0.5 nS); Y = 4-C6H5SMe, M = Ru
(1.4 0.4 nS), Pt (1.8 0.6 nS)) for molecular structures of
ca. 3 nm in length, which has been attributed to transport
processes arising from tunneling through the tails of LUMO
states.
magnitude for the 3.0 nm long “five-ring” system NH4C5C
INTRODUCTION
■
C(C6H2R2CC)3C5H4N (10−6.7G0, 0.015 nS), and thereafter
falling only slightly to 10−6.9G0 (0.01 nS) in an analogous 5.8
nm long “nine-ring” system.
Measurements of the electrical characteristics of a wide variety
of saturated, conjugated, and redox-active organic compounds
have served to drive the development of concepts and
techniques in molecular electronics.1−3 However, metal
complexes offer several potential advantages over organic
compounds as components in molecular electronic devices,
including redox activity at moderate potentials, ready tuning of
frontier molecular orbital energy levels to better match the
Fermi levels of metallic electrodes, and magnetic properties.4,5
Consequently, attention has been turned to the construction
and study of metal complexes,6−14 clusters,15−18 extended
metal atom chains,19−21 and organometallic acetylide spe-
cies22−34 within molecular junctions.
In the case of purely organic oligo(aryleneethynylene)-based
compounds with pyridyl contacting groups, the molecular
conductance, as determined by single-molecule STM break
junction (STM-BJ) experiments, decreases with length, initially
in line with the exponential decay expected for a tunneling
mechanism before shifting to a shallower length dependence
more indicative of an incoherent hopping mechanism of charge
transport for compounds of ca. 3 nm in length.35 Conductance
values range from 10−4.5G0 (2.45 nS) for the 1.6 nm long
“three-ring” oligoarylenes NH4C5CCC6H2R2CCC5H4N
(R = OC6H13), decreasing by approximately 3 orders of
In cases where direct comparison is possible, it has generally
been found that the incorporation of a ruthenium metal center
34
29
such as Ru(dppm)2 or Ru(dppe)2 within a π-conjugated
wirelike structure leads to a 2−5-fold increase in conductance
with the conductance value measured likely also being
dependent on the nature of the molecule−electrode contacting
group (e.g., trans-Ru(CCC6H4SAc)2(dppm)2, STM break
junction 19
7 nS;34 trans-Ru(CCC6H4CC-
SiMe3)2(dppe)2, I(s) method [(5.1 0.99) × 10−5]G0/3.9
0.8 nS;29 trans-Ru(CC-4-C5H4N)2(dppe)2, STM-BJ [(2.5
0.4) × 10−4]G0/19 3 nS28).
In contrast, earlier studies have shown that the Pt(II)
complex trans-Pt(CCC6H4SAc)2(PPh3)2 behaves rather
more as an insulating species when it is bound within a
mechanically controlled break junction (MCBJ), with resis-
tances (5−50 GΩ; 0.2−0.02 nS) some 3 orders of magnitude
larger than those of the comparable organic compounds
AcSC6H4CCArCCC6H4SAc (Ar = 9,10-C14H8, 1,4-C6H2-
Received: June 16, 2016
© XXXX American Chemical Society
A
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2-NH2-5-NO2) being reported.22 A later study with a range of
trans-Pt(CCC6H4SAc)2(PR3)2 complexes (R = Cy, Ph, OEt)
revealed little effect of the supporting phosphine or phosphite
ligand on the through-molecule conductance, although
curiously the conductance for these Pt complexes measured
in a cross-wire junction was reported to be some 2−3-fold
greater than that of the simple oligo(phenyleneethynylene)
AcSC6H4CCC6H4CCC6H4SAc.23
a
Scheme 1. Synthesis of 1-Ru, 1-Pt, 2-Ru, and 2-Pt
Here we turn our attention to a family of linearly conjugated,
wirelike organometallic complexes featuring trans-Ru(C
CR)2(dppe)2 and Pt(CCR)2(PPh3)2 moieties embedded
within the oligo(aryleneethynylene) backbone of ca. 3 nm
molecular length and describe the results of single-molecule
conductance studies based on the I(s) method. These metal
complexes are substantially more conductive than their purely
organic analogues of comparable molecular length, with
detailed computational investigation indicating that the
enhanced conductance arises from conductance through the
tails of the LUMO resonances. The conductance values
obtained from the Pt and Ru systems are remarkably similar,
suggesting that the readily synthesized platinum complexes may
have an important role to play in the further development of
metal complexes for applications in single-molecule electronics.
RESULTS AND DISCUSSION
■
Single-molecule measurements using both organic and organo-
metallic compounds have clearly shown that the electronic
properties of the prototypical metal|molecule|metal junctions
not only are strongly influenced by the chemical structure of
the molecular backbone but also are critically dependent on the
combination of the surface and contacting groups.36−43 The
pyridyl-terminated compounds 1-Ru and 1-Pt together with
the analogous methyl thioether terminated compounds 2-Ru
and 2-Pt were chosen to explore both the relative effects of the
Ru(dppe) vs Pt(PPh3)2 fragments on molecular conductance
and the influence of the electrode−molecule contact in a
comparable set of compounds (Scheme 1). The pyridyl and
methyl thioether moieties are already established as surface-
contacting groups in single-molecule studies of oligoynes and
oligo(phenyleneethynylenes).9,35,37,44−47
The complexes 1-Ru, 1-Pt, 2-Ru, and 2-Pt were synthesized
in a convergent fashion as indicated in Scheme 1. The
precursor terminal alkynyl complexes were assembled from the
protected ligand building block 2-((triisopropylsilyl)ethynyl)-5-
ethynyl-1,4-bis(hexyloxy)benzene and [RuCl(dppe)2]OTf, via
a sequence of intermediate vinylidene species which were not
isolated but deprotonated in situ, or PtCl2(PPh3)2, through
simple CuI-catalyzed alkynylation reactions in diethylamine.
After removal of the triisopropylsilyl protecting group, the
surface binding groups were readily introduced by the “on-
complex” cross-coupling reactions with 4-iodopyridine or 4-
iodothioanisole (Scheme 1).
The STM I(s) technique was used to measure the single-
molecule conductance of the series of compounds 1-M and 2-
M (M = Ru, Pt) in mesitylene solution, with a flame-annealed
Au(111) gold-on-glass substrate serving as the bottom
electrode and the STM tip creating the top electrode in these
elementary metal|molecule|metal junctions. The current is
recorded at a fixed bias, while the junction is elongated by
retraction of the STM tip to generate conductance traces.48
From analyses of the conductance traces, break-off distances of
3.1 nm (1-Ru) and 3.0 nm (1-Pt) can be determined (Table 1).
The break-off distances quoted correspond to 95th percentile
a
Reagents and conditions: (i) (a) [RuCl(dppe)]OTf/DBU, (b) TlBF4
(76%) or (a) cis-PtCl2(PPh3)2/CuI(cat)/NHEt2 (81%); (ii) NBu4F
([M] = Ru(dppe)2, 60%; [M] = Pt(PPh3)2, 63%); (iii) 4-
iodopyridine/Pd(PPh3)4/CuI (cat.)/NEt3 (1-Ru, 64%; 1-Pt, 30%)
or 4-iodothioanisole/Pd(PPh3)4/CuI (cat.)/NEt3 (2-Ru, 34%; 2-Pt,
17%).
values from the accumulated I(s) scans. These values compare
well with the N···N distance obtained from single-crystal X-ray
diffraction studies of 1-Ru (Figure 1, 2.86 nm) and 1-Pt
(Figure 2, 2.86 nm), noting that in the solid state these
compounds are not perfectly linear but rather exhibit sigmoidal
(1-Ru) or gracefully curved (1-Pt) structures arising from
crystal-packing effects. Nevertheless, the good agreement
between the break-off distance and the calculated molecular
lengths (vide infra) is consistent with the contact of these
molecules almost normal to the electrode surface via the
pyridine lone pair within these molecular junctions.
In contrast, shorter break-off distances are determined for the
methyl thioether complexes 2-Ru (2.4 nm) and 2-Pt (2.5 nm);
cf. the S···S distance of 3.18 nm in the crystallographically
determined molecular structure from a weakly diffracting
B
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DFT
Table 1. Frontier Orbital Energies (eV), Experimental (exp G/G0) and Calculated (th G/G0) Conductances at EF − EF
=
−0.07 eV, Experimental 95th Percentile Break-off Distance Z* (nm), Molecular Length from the DFT-Optimized Junctions L =
r···r (nm), Where r = N or S Atoms, Bond Length between the Top Gold Atoms of Gold Electrodes and the Anchor Atoms in
the Relaxed Junctions, X (nm)
d
molecule
EHOMO (eV)
ELUMO (eV)
exp G/G0
th G/G0
Z* (nm)
L (nm)
X (nm)
contacting group (Y)
1-Ru
1-Pt
2-Ru
2-Pt
−4.42
−4.69
−4.18
−4.40
−1.46
−1.48
−1.07
−1.12
4.5 × 10−6
9.8 × 10−6
1.8 × 10−5
1.8 × 10−5
5.4 × 10−6
8.7 × 10−6
1.8 × 10−5
1.78 × 10−5
3.1
3.0
2.4
2.5
2.9
0.23
4-C5H4N
2.86
2.65
2.68
0.23
4-C5H4N
0.245
0.245
4-C6H4SMe
4-C6H4SMe
Figure 3. Plot of the molecule 2-Pt. Solvent molecules and hydrogen
atoms have been omitted, and only one component of a disordered
hexyloxy side chain is shown for clarity. Torsion angle C7−C6−C11−
C16: 165(1)°. S(1)−S(1′): 31.83(2) Å.
Figure 1. Plot of the molecule 1-Ru with thermal ellipsoids at 50%
probability. Solvent molecules and hydrogen atoms have been omitted
for clarity. Torsion angle C(7)−C(6)−C(11)−C(12): 145.4(2)°.
N(1)−N(1′): 28.624(3) Å.
Figure 2. Plot of the molecule 1-Pt with the thermal ellipsoids at 50%
probability. Solvent molecules and hydrogen atoms have been omitted
for clarity. Torsion angle C7−C6−C11−C15: 164.6(2)°. N(1)−
N(1′): 28.620(7) Å.
Figure 4. I(s) conductance histograms of 1-Ru and 1-Pt constructed
from 500 traces.
sample (Figure 3), which is consistent with a rather more tilted
arrangement of the molecule in the junction as might be
expected from the geometry of the sulfur lone pairs in the
thioether;49 this interpretation has been supported by studies of
the DFT-optimized junctions described in more detail below.
The conductance histograms constructed from 500 molec-
ular junction formation traces with characteristic plateaus are
shown in Figures 4 and 5. The peak conductance values from
these histograms together with key data are summarized in
Table 1. These conductance histograms reveal pronounced
conductance peaks at 0.4 0.18 nS (1-Ru), 0.8 0.5 nS (1-
Pt), 1.4 0.4 nS (2-Ru), and 1.8 0.6 nS (2-Pt), and within
each pair of compounds featuring the same contacting group
these values are indistinguishable. The 2−4-fold increase in
conductance values of 2-Ru and 2-Pt in comparison with 1-Ru
and 1-Pt further indicates the important role of the contacting
group in the electrical response of the junction. However, in
Figure 5. I(s) conductance histograms of 2-Ru and 2-Pt constructed
from 500 traces.
contrast to the thiolate-contacted molecules derived from trans-
Ru(CCC6H4SAc)2(dppm)2 (STM-BJ)34 and trans-
C
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Figure 6. Plots of the HOMO and LUMO of 1-Ru, 1-Pt, 2-Ru, and 2-Pt (isosurfaces 0.02 (e/bohr3)1/2).
Pt(CCC6H4SAc)2(PPh3)2 (MCBJ),22 the differences in
conductance as a function of the metallic moiety are negligible,
and the platinum complexes are as conductive (or resistive) as
the ruthenium analogues. The values for 1-Ru and 1-Pt, while
low, are at least 1 order of magnitude higher than that of the
“five-ring” organic compound NH4C5CC(C6H2R2C
C)3C5H4N (R = OC6H13; 10−6.7G0, 0.015 nS) of comparable
molecular length (3 nm) (MCBJ data).35
In the quest to better understand these trends in
conductance behavior, the electronic properties of the
molecules and the electrical behavior of the junctions have
been investigated by using DFT-based methods. Initial studies
of the electronic structures of 1-Ru, 1-Pt, 2-Ru, and 2-Pt were
carried out at the B3LYP level of theory50 with a split
LANL2DZ (Ru, Pt)/6-31G** (all other atoms) basis set.51,52
Plots of the highest occupied and lowest unoccupied molecular
orbitals (HOMO and LUMO, respectively) are given in Figure
6, and an analysis of the energy and distribution of the frontier
molecular orbitals is summarized in Tables 1 and 2.
The HOMOs of the ruthenium complexes display the
familiar pattern of dπ−pπ interactions along the metal−ethynyl
axis53 and extend along the molecular backbone. The nodal
pattern of the HOMOs in the Pt complexes is similar, with a
smaller metal contribution (Figure 6). The LUMOs are also
delocalized over the molecular backbones and can largely be
described as the π* system of the diethynylarylene ligands with
little (Pt) or no (Ru) metal character. These varying metal
contributions are reflected in the relative orbital energies, with
the significant Ru contribution to the HOMO in 1-Ru and 2-
Ru resulting in these orbitals lying some ca. 0.25 eV higher in
energy than in the Pt analogues 1-Pt and 2-Pt. The largely
organic π*-based LUMOs lead to less significant differences in
LUMO energies, which differ by only 0.02−0.05 eV (Table 1).
D
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Table 2. Composition (%) of the HOMO and LUMO of 1-
Ru, 1-Pt, 2-Ru, and 2-Pt
1-Ru
Ru
dppe
CCC6H4(OC6H13)2CCC5H4N
LUMO
HOMO
0
2
3
98
72
25
1-Pt
Pt
PPh3
CCC6H4(OC6H13)2CCC5H4N
LUMO
HOMO
2
6
3
2
95
92
2-Ru
Ru
dppe
CCC6H4(OC6H13)2CCC6H4SMe
LUMO
HOMO
0
2
3
97
76
22
2-Pt
Pt
PPh3
CCC6H4(OC6H13)2CCC6H4SMe
LUMO
HOMO
4
5
10
1
86
94
However, these frontier orbital distributions per se do not
provide evidence relating to the mechanisms of conductance,
which is instead dominated by the alignment of the key
molecular orbitals with the Fermi level of the electrodes. As
noted by Georgiev and McGrady in computational studies of
the conductance properties of extended metal atom chain
complexes, the dominant conductance channel need not
necessarily be associated with a molecular orbital evenly
distributed along the molecular backbone; for example, a
dominant conduction channel in Cr3(dpa)4(NCS)2 (dpa =
dipyridylamide) is derived from a nonbonding combination of
Figure 7. Relaxed geometries of molecular junctions of 1-Ru, 1-Pt, 2-
Ru, and 2-Pt.
2
metal dz orbitals directed along the Cr−Cr−Cr axis and
localized on the terminal chromium atoms.54
To provide further insight into the experimentally observed
trends obtained using the I(s) technique, and to better evaluate
the properties and behavior of these molecular junctions,
calculations using a combination of DFT and a nonequilibrium
Green’s function formalism were also carried out. For the
transport calculations, eight layers of (111)-oriented bulk gold
with each layer consisting of 6 × 6 atoms and a layer spacing of
0.235 nm were used to create the molecular junctions, as shown
in Figure 7 and described in detail elsewhere.55 These layers
were then further repeated to yield infinitely long current-
carrying gold electrodes. Each molecule was attached to two
(111)-directed gold electrodes; one of these electrodes was
pyramidal, representing the STM tip, while the other was a
planar slab representing the electrode formed by the idealized
Au(111) substrate in the I(s)-based molecular junction. The
molecules and first layers of gold atoms within each electrode
were then allowed to relax again, to yield the optimal junction
geometries shown in Figure 7. From these model junctions the
transmission coefficient, T(E), was calculated using the
GOLLUM code.55
It is well-known that the Fermi energy predicted by DFT is
often not reliable, and as such the room-temperature electrical
conductance G was computed for a range of Fermi energies EF;
the calculated G is plotted as a function of EF − EFDFT in Figure
8. This multipoint fitting of the Fermi energy is a commonly
accepted procedure in DFT-based calculations in molecular
electronics.56 To determine EF, the predicted conductance
values of all molecules were compared with the experimental
values and a single common value of EF was chosen, which gave
Figure 8. Plots showing selected comparisons of calculated
conductance as a function of the Fermi energy for molecular junctions
1-Ru, 1-Pt, 2-Ru, and 2-Pt. Black dashed lines show the chosen Fermi
energy (EF = −0.07 eV).
the closest overall agreement. This yielded a surprisingly small
value of EF − EFDFT = −0.07 eV, which has been used in all of
the theoretical results described below. Thygesen and
colleagues have discussed similar situations for C60-contacted
molecular wires and shown that critical molecular orbitals can
become pinned close to the Fermi level due to partial charge
transfer and leading to good quantitative agreement between
E
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calculated and experimentally determined conductance.57 As
shown below, the LUMO states of 1-M and 2-M (M = Ru, Pt)
tail near the Fermi level in a manner similar to that in the
Thygesen system, and partial charge transfer may also be
responsible for the good agreement observed here.
influenced by the electrode−molecule contact. The energies
and distributions of the molecular LUMOs are qualitatively
similar in all of the compounds studied here and can be well
described as the ethynylarylene ligand π* orbitals. Given the
rather straightforward synthetic chemistry associated with the
preparation of long-chain ethynylarylene ligands, this work
opens new avenues for the design of metal complex based
molecular wires, including those based on readily available
trans-bis(alkynyl) Pt(II) complexes.
The optimized junction geometries conform well to a
description of the pyridine contacted compounds 1-Ru and 1-
Pt forming point contacts between the pyridine nitrogen atom
and the undercoordinated gold atoms of the gold electrodes. As
expected, Figure 7 shows that the methyl thioether contacted
compounds 2-Ru and 2-Pt are not oriented normal to the
idealized, flat electrode surface within the molecular junction.
Rather, they are tilted within molecular junctions to
accommodate the directionality of the lone pairs of electrons
on the sulfur atoms that bind to the gold electrodes.49,58 The
calculated molecular lengths and experimental break-off
distances are consistent with these interpretations (Table 1).
The results of the room-temperature conductance calcu-
lations are summarized in Table 1, and comparisons between
pairs of molecules according to contacting group and metal
complex fragment are illustrated in Figure 8. It is immediately
apparent that the conductance of the methyl thioether
contacted molecules 2-M is approximately 3−4 times higher
than that of the analogous pyridine contacted species 1-M, in
good agreement with the experimental trends (Figure 8, top
row, and Table 1). The greater conductance of the methyl
thioether contacted compounds 2-M likely arises from the
greater Au−S bond strength and the broadening of the LUMO
resonances arising from these interactions versus the pyridine-
contacted analogues 1-M.
More surprising is the limited influence of the metal−
phosphine fragment on the molecular conductance (Figure 8,
bottom row), which can be explained by the relative energy of
the Fermi level and the molecular LUMOs together with a
conductance mechanism based on a tunneling process through
the tails of the respective LUMO states. Although tunneling
through pyridine terminated compounds is usually attributed to
LUMO-based transport,43,59,60 the methyl thioether contact has
been shown to permit both HOMO- and LUMO-based
conductance mechanisms, depending on the nature of the
molecular backbone.61 Here it appears that the similar
conductance values obtained from both series of compounds
reflect the similar natures, energies, and compositions of the
LUMOs, which provide a conductance channel between the
electrodes. This contrasts with the recently reported single-
molecule conductance studies of trans-Ru(CCC5H4N)(LL)2
(LL = dppe, dmpe, {P(OMe)3}2) with the shorter alkynylpyr-
idine ligands, in which the ligand π* levels are likely to be much
higher in energy than the extended alkynyl-based ligands in
compounds 1-M and 2-M, and a HOMO-mediated con-
ductance channel is proposed.28
EXPERIMENTAL SECTION
■
General Conditions. All reactions were carried out in oven-dried
glassware under an oxygen-free argon atmosphere using standard
Schlenk techniques. Diisopropylamine and triethylamine were purified
by distillation from KOH; other reaction solvents were purified and
dried using an Innovative Technology SPS-400 system and degassed
before use. The compounds [RuCl(dppe)2]OTf,62 cis-PtCl2(PPh3)2,63
and 1,4-bis(hexyloxy)-2,5-diiodobenzene64 were prepared by literature
methods. Other reagents and intermediates were prepared by
variations on literature methods as described below or purchased
commercially and used as received.
NMR spectra were recorded in deuterated solvent solutions on
Bruker Avance 400 MHz and Varian VNMRS 700 MHz spectrometers
1
and referenced against residual protio solvent resonances (CHCl3, H
1
7.26 ppm and 13C 77.00 ppm; CH2Cl2, H 5.32 ppm and 13C 53.84
ppm). In the NMR assignment, the phenyl ring associated with the
dppe and PPh3 is denoted Ph. Ar indicates any arylene group
belonging to the alkynyl ligands.
Matrix-assisted laser desorption ionization (MALDI) mass spectra
were recorded an using Autoflex II TOF/TOF mass spectrometer with
a 337 nm laser. Infrared spectra were recorded on a Thermo 6700
spectrometer from CH2Cl2 solution in a cell fitted with CaF2 windows.
2-Iodo-5-((trimethylsilyl)ethynyl)-1,4-bis(hexyloxy)-
benzene.64 In a 250 mL Schlenk flask, a solution of 1,4-
bis(hexyloxy)-2,5-diiodobenzene (6.0 g, 11 mmol), (trimethylsilyl)-
acetylene (490 mg, 0.7 mL, 5 mmol), PdCl2(PPh3)2 (140 mg, 0.2
mmol), and CuI (38 mg, 0.2 mmol) in degassed dry Et3N (120 mL)
was stirred overnight at room temperature. The solvent was removed
and the residue purified on a silica column. Elution with hexane
allowed recovery of unreacted 1,4-bis(hexyloxy)-2,5-diiodobenzene,
followed by elution with CH2Cl2/hexane (1/9), which after
evaporation of the solvent produced a yellowish oil of the desired
1
monoalkyne. Yield: 1.88 g (76%). H NMR (400 MHz, CDCl3): δ
7.25 (s, 1H, Ar); 6.83 (s, 1H, Ar); 3.95−3.92 (td, (J = 6.4, 1.4 Hz, 4H,
−OCH2); 1.81−1.76 (m, 4H, CH2); 1.52−1.48 (m, 4H, CH2); 1.36−
1.33 (m, 8H, CH2); 0.93−0.88 (m, 6H, CH2CH3); 0.25 (s, 9H, SiMe3)
ppm.
2-((Triisopropylsilyl)ethynyl)-5-((trimethylsilyl)ethynyl)-1,4-
bis(hexyloxy)benzene.22 To
a solution of 2-iodo-5-
((trimethylsilyl)ethynyl-1,4-bis(hexyloxy)benzene (1.88 g, 3.8 mmol)
in degassed Et3N (30 mL) were added (triisopropylsilyl)acetylene
(TIPSA; 638 mg, 0.78 mL, 3.5 mmol), Pd(PPh3)4 (219 mg, 0.19
mmol), and CuI (36 mg, 0.19 mmol). The reaction mixture was stirred
overnight at room temperature. The solvent was removed, and the
residue was purified by passage through a silica pad and elution by
ethyl acetate EtOAc/hexane (1/9) to give a yellow oil, which solidified
In summary, the single-molecule conductance of two pairs of
trans-bis(alkynyl) organometallic complexes based on Ru-
(dppe)2 and Pt(PPh3)2 fragments and methyl thioether and
pyridyl surface contacting groups have been studied in
molecular junctions formed by the I(s) method. Perhaps
surprisingly, the nature of the metal moiety is a less significant
point of chemical control over the electrical properties of the
junction, with Pt(PPh3)2-based complexes being essentially as
conductive (or as resistive) as the analogous Ru(dppe)2
derivatives. The conductance of these compounds is more
dependent on the position of the LUMO resonance with
respect to the Fermi level of the junction and is largely
1
to give an off-white solid on standing. Yield: 1.30 g (60%). H NMR
(400 MHz, CDCl3): δ 6.88 (s, 1H, Ar); 6.87 (s, 1H, Ar); 3.97−3.91
(dt, J = 12.7, 6.4 Hz, 4H, −OCH2); 1.82−1.72 (m, 4H, CH2); 1.53−
1.43 (m, 4H, CH2); 1.35−1.30 (m, 8H, CH2); 1.13 (s, 21H, SiPri3);
0.92−0.88 (m, 6H, CH2CH3); 0.25 (s, 9H, SiMe3) ppm.
2-((Triisopropylsilyl)ethynyl)-5-ethynyl-1,4-bis(hexyloxy)-
benzene (1).22 Potassium caronate (298 mg, 2.16 mmol) was added
to a solution of 2-((triisopropylsilyl)ethynyl)-5-((trimethylsilyl)-
ethynyl)-1,4-bis(hexyloxy)benzene (1.20 g, 2.16 mmol) in THF/
MeOH (1/1) (160 mL). The solution was stirred for 2 h before
CH2Cl2 was added. The solution was washed with water and the
organic layer was collected and dried over MgSO4, before the solvent
was removed to yield an orange solid, which was used without further
F
DOI: 10.1021/acs.organomet.6b00472
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
1
purification. Yield: 950 mg (91%). H NMR (700 MHz, CDCl3): δ
6.91, 6.89 (2s, 2 × 1H, Ar), 3.98 (t, J = 6.5 Hz, 2H, −OCH2); 3.92 (t, J
= 6.5 Hz, 2H, −OCH2); 3.31 (s, 1H, CCH); 1.83−1.72 (m, 4H,
CH2); 1.49−1.44 (m, 4H, CH2); 1.35−1.30 (m, 8H, CH2); 1.13 (s,
21H, SiPri3); 0.92−0.87 (m, 6H, CH2CH3) ppm. 13C{1H} NMR (101
MHz, CDCl3): δ 154.1 (O-CAr); 153.9 (O-CAr); 117.6, 117.2 (HCAr);
114.6, 112.6 (CAr); 102.7, 96.6 (C); 82.1 (H-C); 80.1 (C);
69.7, 69.3 (O-CH2); 31.7, 31.5, 29.4, 29.1, 25.8, 25.6, 22.62, 22.57
(CH2); 18.7 (H3CSiPr3); 14.1, 14.0 (CH3); 11.4 (HCSiPr3) ppm.
trans-Ru[CC{1,4-C6H2(OC6H13)2}CCSiPri3]2(dppe)2 (2).
The complex salt [RuCl(dppe)2]OTf (100 mg, 0.09 mmol) was
added to a degassed solution of CH2Cl2 (4 mL) containing 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU; 4 drops). The solution
changed from red to orange with the addition of 1 (96 mg, 0.20
mmol). The reaction mixture was stirred for 1 h at room temperature
before TlBF4 (27 mg, 0.09 mmol) was added. After 20 min, the
resulting solution had turned yellow and formed a precipitate (TlCl).
The precipitate was removed by filtration through a Millex syringe
filter (Millipore) to give an orange solution, which was reduced to the
minimum volume, whereupon methanol (5 mL) was added. A yellow
precipitate was obtained upon further concentration of the mixture.
The product was collected by filtration and dried in air to give 2 as a
bright yellow solid. Yield: 131 mg (76%). 1H NMR (400 MHz,
CDCl3): δ 7.44 (m, 16H, Pho); 7.08−7.04 (m, 8H, Php); 6.86−6.82
(m, 18H, (16H, Phm + 2H, Ar); 5.86 (s, 2H, Ar); 3.84 (t, J = 6.9 Hz,
4H, O−CH2); 3.64 (t, J = 6.4 Hz, 4H, O−CH2); 2.89 (m, 8H,
PCH2CH2P); 1.73−1.61 (m, 8H, CH2); 1.48−1.46 (m, 4H, CH2);
1.34−1.30 (m, 12H, CH2); 1.18 (bs, 50H, (42H, SiPri3 + 8H, CH2);
0.92 (t, J = 7.0 Hz, 6H, CH2CH3); 0.81 (t, J = 7.0 Hz, 6H, CH2CH3)
ppm. 31P{1H} NMR (162 MHz, CDCl3): δ 52.07 (s) ppm. 13C{1H}
NMR (101 MHz, CDCl3): δ 154.3, 152.6 (−OCAr); 137.3 (t, J = 11.4
Hz, Phi); 134.1 (Ph); 128.3 (Ph); 126.8 (Ph); 121.8 (C or CAr);
117.2, 115.2 (HCAr); 114.7, 106.5, 104.9, 93.2 (C or CAr); 68.9
(−OCH2); 68.7 (−OCH2); 31.74 (P-CH2) overlapping with CH2;
31.69, 29.6, 27.5, 25.9, 25.8, 22.7, 22.6 (CH2); 18.8 (H3CSiPr3); 14.1
(CH3); 14.0 (CH3); 11.5 (HCSiPr3) ppm. IR (CH2Cl2): ν(CCSiPri3)
2138 (m); ν(RuCC) 2050 (s) cm−1. MS+ (MALDI-TOF; m/z):
898.1 [Ru(dppe)2]+; 1861.9 [M]+. HR-ESI+-MS: m/z calcd for
C114H146O4P496RuSi2 1856.8895; found 1856.8856.
the solution turned orange with a precipitate developing. The
precipitate was removed by filtration, and the solid was washed with
methanol to remove ammonium salts, giving a yellow powder. Yield:
85 mg (64%). Crystals suitable for X-ray diffraction were grown by
slow diffusion of MeOH into a CH2Cl2 solution of 1-Ru containing
1
5% Et3N. H NMR (400 MHz, CD2Cl2): δ 8.57 (d, J = 5.2 Hz, 4H,
C6H4N); 7.52−7.40 (m, 16H, Pho); 7.37 (d, J = 5.2 Hz, 4H, C6H4N);
7.13−7.11 (m, 8H, Php); 6.95 (s, 2H, Ar); 6.90−6.87 (m, 16H, Phm);
5.84 (s, 2H, Ar); 3.93 (t, J = 6.5 Hz, 4H, −OCH2); 3.68 (t, J = 7.2 Hz,
4H, O-CH2); 2.96−2.93 (m, 8H, PCH2CH2P); 1.79−1.74 (m, 8H,
CH2); 1.52−1.50 (m, 4H, CH2); 1.38−1.36 (m, 12H, CH2); 1.26−
1.23 (m, 8H, CH2); 0.94−0.92 (pseudo-t, 6H, CH2CH3); 0.84−0.82
(pseudo-t, 6H, CH2CH3) ppm. 31P{1H} NMR (162 MHz, CD2Cl2): δ
51.7 (s) ppm. 13C{1H} NMR (101 MHz, CD2Cl2): δ 154.3, 153.3
(-OCAr); 150.1 (HCC5H4N); 137.7 (t, J = 10.9 Hz, Phi); 134.5 (Ph);
132.6 (CC5H4N); 128.9 (Ph); 127.3 (Ph); 125.4 (HCC5H4N); 123.3
(C or CAr); 117.9, 114.9 (HCAr); 105.3 (C or CAr); 93.2, 90.7
(C); 69.4, 69.3 (O-CH2); 32.1 (P-CH2); 32.0, 29.5, 29.4, 25.8, 22.7,
22.6 (CH2); 13.9 (CH3); 13.8 (CH3) the other quaternary 13C were
not detected. IR (CH2Cl2): ν(CCC5H4N) 2208 (m); ν(RuCC)
2044 (s) cm−1. MS+ (MALDI-TOF; m/z): 898.0, [Ru(dppe)2]+;
1702.6, [M]+. HR-ESI+-MS: m/z calcd for C106H112N2O4P496Ru
1697.6682; found 1697.6688. Anal. Calcd for C106H112N2O4P4Ru: C,
74.76; H, 6.63; N, 1.64. Found: C, 74.66; H, 6.72; N, 1.70. Crystal data
for 1-Ru: C106H112N O P Ru, M = 1524.70, triclinic, space group P1,
̅
2
4
4
r
a = 12.3676(7) Å, b = 12.9676(7) Å, c = 13.9333(8) Å, α =
83.888(2)°, β = 83.489(2)°, γ = 80.585(2)°, U = 2181.4(2) Å3, F(000)
= 898.0, Z = 1, Dc = 1.296 mg m−3, μ = 0.309 mm−1; 47816 reflections
collected yielding 12134 unique data (Rint = 0.0244). Final wR2(F2) =
0.0952 for all data (531 refined parameters), conventional R1(F) =
0.0356 for 11015 reflections with I ≥ 2σ, GOF = 1.065.
trans-Pt[CC{1,4-C6H2(OC6H13)2}CCSiPri3]2(PPh3)2 (5). A
mixture of 1 (250 mg, 0.52 mmol) and CuI (4 mg) was added to a
solution of cis-PtCl2(PPh3)2 (200 mg, 0.26 mmol) in dry and degassed
diethylamine (NHEt2; 20 mL). The orange reaction mixture was
heated to 100 °C for 2 h. The solvent was removed, and the remaining
residue was purified on a silica column with CH2Cl2 as eluent. The
resulting product was obtained as an amorphous orange solid. Yield:
1
320 mg (81%). H NMR (400 MHz, CDCl3): δ 7.82−7.77 (m, 12H,
trans-Ru[CC-{1,4-C6H2(OC6H13)2}CCH]2(dppe)2 (3). Tetra-
n-butylammonium fluoride (TBAF; 1.0 M in tetrahydrofuran; 0.24
mL, 0.24 mmol) was added to a solution of 2 (180 mg, 0.1 mmol) in
THF (15 mL). The solution was stirred overnight at room
temperature. The resulting mixture was dried and purified on neutral
alumina with CH2Cl2/hexane (50/45) as eluent with 5% Et3N to give
a yellow solid (100 mg, 0.06 mmol, 60%). Crystals suitable for X-ray
diffraction were grown by slow diffusion of MeOH into a CH2Cl2
Ph); 7.31−7.24 (m, 18H, Ph); 6.63 (s, 2H, Ar); 5.71 (s, 2H, Ar); 3.60
(t, J = 6.5 Hz, 4H, O-CH2); 3.49 (t, J = 6.8 Hz, 4H, O-CH2); 1.71−
1.63 (m, 4H, CH2); 1.46−1.39 (m, 4H, CH2); 1.32−1.27 (m, 24H,
CH2); 1.10 (s, 42H, SiPri3); 0.91 (t, J = 7.0 Hz, 6H, CH2CH3); 0.86 (t,
J = 7.0 Hz, 6H, CH2CH3) ppm. 31P{1H} NMR (162 MHz, CDCl3): δ
17.43 (s, JP−Pt = 2654.12 Hz) ppm. 13C{1H} NMR (101 MHz,
CDCl3): δ 154.1 (−OCAr); 152.2 (−OCAr); 135.3 (t, J = 6.0 Hz, Pho);
131.3 (t, J = 29.3 Hz, Phi); 130.1 (Php); 127.6 (t, J = 5.4 Hz, Phm);
120.9 (C or CAr); 118.9, 116.6 (HCAr); 109.1, 104.0, 93.8 (C or
CAr); 70.0, 68.9 (O-CH2); 31.7, 31.6, 29.5, 29.2, 25.9, 25.5, 22.7, 22.6
(CH2); 18.7 (H3CSiPr3); 14.1 (CH3) (one visible); 11.4 (HCSiPr3) ppm,
the other quaternary 13C were not detected. IR (CH2Cl2): ν(C
CSiPri3) 2145 (m); ν(PtCC) 2103 (m) cm−1. MS+ (MALDI-TOF;
m/z): 1682.5, [M]+. HR-ESI+-MS: m/z calcd for C98H128O4P2194PtSi2
1682.8558; found 1682.8484.
1
solution of 3 containing 5% Et3N. H NMR (400 MHz, CDCl3): δ
7.45−7.43 (m, 16H, Pho); 7.09−7.05 (m, 8H, Php); 6.89 (s, 2H, Ar);
6.87−6.83 (m, 16H, Phm); 5.83 (s, 2H, Ar); 3.86 (t, J = 7.0 Hz, 4H,
O−CH2); 3.67 (t, J = 7.0 Hz, 4H, O-CH2); 3.31 (s, 2H, CCH);
2.93−2.89 (m, 8H, PCH2CH2P); 1.75−1.64 (m, 8H, CH2); 1.43−1.41
(m, 4H, CH2); 1.36−1.30 (m, 12H, CH2); 1.23−1.20 (m, 8H, CH2);
0.92 (t, J = 7.0 Hz, 6H, CH2CH3); 0.82 (t, J = 7.0 Hz, 6H, CH2CH3)
ppm. 31P NMR {1H} NMR (162 MHz, CDCl3): δ 51.85 (s) ppm.
13C{1H} NMR (101 MHz, CDCl3): δ 154.0, 152.6 (−OCAr); 137.2 (t,
J = 15.5 Hz, Phi); 134.1 (Ph); 128.4 (Ph); 126.9 (Ph); 122.3 (C or
CAr); 117.7, 115.3 (HCAr); 114.5, 104.9 (C or CAr); 81.7 (H-C);
80.0 (C); 69.0 (−OCH2); 68.9 (−OCH2); 31.6 (P-CH2) over-
lapping with CH2; 31.5, 30.1, 29.5, 29.3, 25.8, 25.6, 22.64, 22.58
(CH2); 14.05 (CH3); 14.02 (CH3) ppm (one quaternary 13C was
not detected). MS+(MALDI-TOF; m/z): 898.0 [Ru(dppe)2], 1548.4
[M]+. IR (CH2Cl2): ν(CH) 3301 (m); ν(RuCC) 2049 (s) cm−1.
HR-ESI+-MS: m/z calcd for C96H106O4P4Ru 1548.6113; found
1548.6082.
trans-Pt[CC{1,4-C6H2(OC6H13)2}CCH]2(PPh3)2 (6). A solu-
tion of TBAF (1.0 M in THF; 0.38 mL, 0.38 mmol) was added to a
solution of 5 (150 mg, 0.096 mmol) in THF (25 mL). The reaction
mixture was stirred for 24 h at room temperature. The solvent was
removed, the residue was redissolved in CH2Cl2, and this solution was
washed with water, ammonium chloride (NH4Cl, aqueous), and brine.
The organic phase was dried (MgSO4) and the solvent removed to
give an amorphous yellow solid. The solid was purified on a short silica
pad, with 5% NEt3 in CH2Cl2 as eluent, and compound 6 was obtained
1
by precipitation in CH2Cl2/MeOH. Yield: 130 mg (63%). H NMR
(400 MHz, CDCl3): δ 7.83−7.78 (m, 12H, Ph); 7.32−7.25 (m, 18H,
Ph); 6.65 (s, 2H, Ar); 5.74 (s, 2H, Ar); 3.64 (t, J = 7.0 Hz, 4H, O-
CH2); 3.48 (t, J = 7.0 Hz, 4H, O-CH2); 3.19 (s, 2H, CCH); 1.73−
1.66 (m, 4H, CH2); 1.44−1.40 (m, 4H, CH2); 1.34−1.13 (m, 24H,
CH2); 0.91 (t, J = 6.3 Hz, 6H, CH2CH3); 0.86 (t, J = 6.3 Hz, 6H,
CH2CH3). 31P{1H} NMR (162 MHz, CDCl3): δ 17.61 (s, JP−Pt = 2648
trans-Ru[CC-{1,4-C6H2(OC6H13)2}CCC5H4N]2(dppe)2 (1-
Ru). Compound 3 (120 mg, 0.077 mmol), 4-iodopyridine (39 mg,
0.192 mmol), Pd(PPh3)4 (4.6 mg, 0.004 mmol), and CuI (0.8 mg,
0.004 mmol) were added to a degassed solution of NHiPr2 (10 mL).
The yellow solution was heated at 80 °C for 20 h, during which time
G
DOI: 10.1021/acs.organomet.6b00472
Organometallics XXXX, XXX, XXX−XXX

Organometallics
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Hz) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ 153.9, 152.3
(−OCAr); 135.2 (t, J = 6.2 Hz, Pho); 131.2 (t, J = 29.3 Hz, Phi); 130.1
(Php); 127.6 (t, J = 5.4 Hz, Phm); 121.3 (CAr or C); 119.4 (t, J =
15.1 Hz, Cα); 118.7, 116.9 (HCAr); 110.2, 107.6 (CAr or C); 80.9
(H-C); 80.4 (C); 69.9, 69.2 (O-CH2); 31.6, 29.2, 29.1, 25.6, 25.4,
22.61, 22.56 (CH2); 14.1, 14.0 (CH3) ppm. IR (CH2Cl2): ν(C−H)
3300 (w); ν(PtCC) 2098 (m) cm−1. MS+ (MALDI-TOF; m/z):
719.4 [Pt(PPh3)2]+, 1371.1, [M]+. HR-ESI+-MS: m/z calcd for
C80H88O4P2194Pt 1369.5863; found 1369.5836.
mg, 0.15 mmol), Pd(PPh3)4 (4 mg, 0.003 mmol), and CuI (1 mg)
were placed in a Schlenk flask charged with degassed HNiPr2 (8 mL),
and the reaction mixture was heated for 2 h at 100 °C. The yellow
solution was evaporated to dryness, and the residue was purified on a
silica column with CH2Cl2/hexane (1/1 v/v) followed by pure CH2Cl2
as eluent to give yellow crystals. Yield: 17 mg, 17%. X-ray-quality
crystals were grown by slow diffusion of methanol into a solution of
the complex in 95/5 CH2Cl2/NEt3 (v/v). 1H NMR (600 MHz,
CDCl3): δ 7.84−7.81 (m, 12H, Ph), 7.38 (d, J = 9.1 Hz, 4H,
C6H4SMe), 7.33−7.27 (m, 18H, Ph), 7.17 (d, J = 9.1 Hz, 4H,
C6H4SMe), 6.68 (s, 2H, Ar), 5.77 (s, 2H, Ar), 3.78 (t, J = 6.5 Hz, 4H,
O-CH2), 3.54 (t, J = 6.9 Hz, 4H, OCH2), 2.48 (s, 6H, SCH3), 1.76−
1.71 (m, 4H, CH2), 1.51−1.47 (m, 4H, CH2), 1.36−1.27 (m, 16H,
CH2), 1.21−1.14 (m, 8H, CH2), 0.91−0.86 (m, 12H, CH2CH3) ppm.
31P{1H} NMR (162 MHz, CDCl3): δ 17.6 (s, JP−Pt = 2653.1 Hz) ppm.
13C{1H} NMR (101 MHz, CDCl3): δ 153.3, 152.5 (OCAr), 138.4 (S-
CAr), 135.3 (t, J = 6.2 Hz, Ph), 131.6 (HCC6H4SMe), 131.3 (t, J =
29.2 Hz, Ph), 130.1 (Ph), 127.6 (t, J = 5.5 Hz, Ph), 125.9
(HCC6H4SMe), 120.7, 120.5 (CAr), 117.7, 117.2 (HCAr), 109.1,
92.9, 86.9 (C), 69.8, 69.2 (OCH2), 31.64, 31.58, 29.4, 29.1, 25.8,
25.5, 22.7, 22.6 (CH2), 15.5 (SCH3), 14.12, 14.08 (CH3) ppm. MS+
(MALDI-TOF; m/z): 719.4 [Pt(PPh3)2]+, 1614.3 [M + H]+. IR
(CH2Cl2): ν(Pt−CC) 2104 (s) cm−1. HR-(ESI+)-MS: calcd for
C94H100O4P2PtS2Na 1637.6107; found 1637.6124. Crystal data for 2-
Pt: C94H100O4P2PtS2, Mr = 1614.89, monoclinic, space group P2/n, a
= 22.659(10) Å, b = 9.469(4) Å, c = 22.765(10) Å, β = 118.005(5)°, U
= 4313(3) Å3, F(000) = 1672.0, Z = 2, Dc = 1.622 mg m−3, μ = 1.244
mm−1, crystal size 0.01 × 0.01 × 0.001 mm3; 42922 reflections
collected yielding 8352 unique data (Rint = 0.2997). Final wR2(F2) =
0.2517 for all data (371 refined parameters), conventional R1(F) =
0.0949 for 4614 reflections with I ≥ 2σ, GOF = 1.024.
trans-Pt[CC{1,4-C6H2(OC6H13)2}CCC5H4N]2(PPh3)2 (1-Pt).
Compound 6 (90 mg, 0.064 mmol), 4-iodopyridine (30 mg, 0.15
mmol), Pd(PPh3)4 (4 mg, 0.003 mmol), and CuI (0.8 mg, 0.004
mmol) were placed in a Schlenk flask charged with degassed Et3N (10
mL), and the reaction mixture was heated for 2 h at 100 °C. The
solvent was removed from the yellow solution and the residue purified
by column chromatography on silica with CH2Cl2/hexane/Et3N (8.5/
1.5/0.5) as eluent to give a yellow solid. The solid was dissolved in the
minimum amount of CH2Cl2, and MeOH (5 mL) was added.
Concentration of the solution caused the desired 1-Pt to precipitate.
Yield: 30 mg (30%). Crystals suitable for X-ray diffraction were grown
by slow diffusion of MeOH into a CH2Cl2 solution of 1-Pt containing
5% NEt3. 1H NMR (400 MHz, CDCl3): δ 8.54 (pseudo-d, 4H,
C5H4N), 7.83−7.81 (m, 12H, Ph), 7.33−7.26 (m, 22H, (18H, Ph +
4H, C5H4N), 6.69 (s, 2H, Ar), 5.78 (s, 2H, Ar), 3.68 (pseudo-t, 4H, O-
CH2), 3.53 (pseudo-t, 4H, O-CH2), 1.76−1.72 (m, 4H, CH2), 1.50−
1.47 (m, 4H, CH2), 1.38−1.16 (m, 24H, CH2), 0.92−0.85 (m, 12H,
CH2CH3) ppm. 31P{1H} NMR (162 MHz, CDCl3): δ 17.67 (s, JP−Pt
=
2643.5 Hz) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ 153.8, 152.5
(−OCAr), 149.5 (HCC5H4N), 135.2 (t, J = 6.2 Hz)) (Pho), 131.2 (t, J =
29.1 Hz)) (Phi), 130.1 (Php), 127.6 (t, J = 5.4 Hz)) (Ph), 125.3
(HCC5H4N), 117.9, 116.9 (HCAr), 107.6 (C or CAr), 92.0, 90.5, 69.9,
69.2 (C) other quaternary 13C were not seen, 31.60, 31.57, 29.3,
29.1, 25.7, 25.4, 22.65, 22.56 (CH2), 14.1, 14.0 (CH3) ppm. IR
(CH2Cl2): 2112 (m) ν(CCC5H4N); 2102 (s) ν(PtCC) cm−1.
MS+ (MALDI-TOF; m/z): 1524.5 [M]+. HR-ESI+-MS: m/z calcd for
C90H95N2O4P2194Pt 1523.6394; found 1523.6362. Anal. Calcd for
C90H94N2O4P2Pt: C, 70.89; H, 6.21; N, 1.84. Found: C, 70.72; H,
6.13; N, 1.93. Crystal data for 1-Pt: C90H94N2O4P2Pt, Mr = 1702.93,
Single-Molecule Conductance Measurements. Gold-on-glass
substrates (Arrandee, Schroer, Germany) were cleaned with acetone
̈
and flame-annealed with a butane torch until a slight orange hue was
obtained. The slide was kept in this state for 20 s, during which time
the torch was kept in motion around the sample to avoid overheating.
This procedure was performed three times to generate flat Au (111)
terraces.65 The freshly annealed substrates were immersed in a 10−4
M
mesitylene solution of the complex under investigation for 1 min, after
which time the gold sample was removed and washed with ethanol and
then dried under an argon flow. The short immersion time and low
concentration of solution were chosen to promote low molecular
coverage of the gold surface, which increases the formation of single-
molecule events over aggregate phenomena.
triclinic, space group P1, a = 9.5706(4) Å, b = 13.1673(6) Å, c =
̅
16.6608(9) Å, α = 71.273(5)°, β = 86.786(4)°, γ = 71.249(4)°, U =
1880.3(2) Å3, F(000) = 788.0, Z = 1, Dc = 1.347 mg m−3, μ = 1.962
mm−1; 17913 reflections collected yielding 8632 unique data (Rint
=
0.0719). Final wR2(F2) = 0.1048 for all data (450 refined parameters),
conventional R1(F) = 0.0535 for 7746 reflections with I ≥ 2σ, GOF =
1.007.
Conductance values of the compounds and the break-off distances
were obtained with an STM (Agilent 5500 SPM microscope), using
the I(s) technique, in which an electrochemically etched gold tip is
approached close to the substrate surface and then retracted with the
tunneling current (I) recorded against distance (s).48 The Agilent 5500
SPM was fitted with a low-current preamplifier, and set point
conditions of I = 30 nA and bias voltage Utip = 0.6 V were employed.
The I(s) method involves repeatedly moving the STM tip toward the
gold surface to given set-point values and then rapidly away from the
surface. During these cycles molecular junctions are occasionally
formed, which can be recognized by deviations from the usual
exponential decay of current in the form of current plateaus. In this
case as the junction is stretched beyond its maximum length, the
molecular bridge breaks, leading to a sharp decrease in current and
current steps. Hence, these junction formation and cleavage processes
are recognized by plateaus and steps in the current-distance currents.
Since the I(s) technique is a “non-contact” method (no metallic
contact between the gold STM tip and gold surface), the molecular
junction formation probability, as recognized by the plateau-step
traces, is significantly smaller than for break junction techniques. The
I(s) tip retraction cycles were repeated many times (normally 4000−
5000 traces) in order to record sufficient traces where molecular
junctions form, called molecular junction formation scans, as opposed
to most traces for which no junction forms. Molecular junction
formation scans are recognized by recording only traces which exhibit
a plateau longer than 1 Å, present in about 15% of all traces for both
trans-Ru[CC{1,4-C6H2(OC6H13)2}CC(4-C5H4SMe)]2(dppe)2
(2-Ru). Compound 3 (40 mg, 0.026 mmol), 4-iodothioanisole (13 mg,
0.052 mmol), Pd(PPh3)4 (1.5 mg, 0.001 mmol), and CuI (0.2 mg,
0.001 mmol) were added to a degassed solution of NHiPr2 (5 mL).
The yellow solution was heated at 80 °C for 24 h, and the precipitate
was removed by filtration. The crude solid was purified on a neutral
alumina column with CH2Cl2/5% NEt3 as eluent to give a yellow
1
powder after removing the solvent. Yield: 15 mg (34%). H NMR
(700 MHz, CD2Cl2): δ 7.45−7.43 (m, 20H, Ph (16H) + C6H4SMe
(4H)), 7.23 (d, J = 7.8 Hz, 4H, C6H4SMe), 7.10 (t, J = 7.4 Hz, 8H,
Ph), 6.92 (s, 2H, Ar), 6.88 (t, J = 7.6 Hz, 16H, Ph), 5.85 (s, 2H, Ar),
3.92 (t, J = 6.9 Hz, 4H, OCH2), 3.68 (t, J = 6.4 Hz, 4H, OCH2), 2.93
(s, 6H, C6H4SMe), 1.78−1.69 (m, 8H, CH2), 1.38−1.35 (m, 12H,
CH2), 1.26−1.20 (m, 12H, CH2), 0.96−0.89 (t, J = 6.6 Hz, 6H,
CH2CH3), 0.82 (t, J = 6.9 Hz, 6H, CH3). 31P{1H} NMR (162 MHz,
CDCl3): δ 51.8 (s) ppm. 13C{1H} NMR (700 MHz, CDCl3): δ 153.9,
153.3 (O-CAr), 139.1, 137.8 (S-CAr), 134.5 (Ph), 131.8
(HCC6H4SMe), 128.9 (Ph), 127.3, 126.3 (Ph), 122.2, 120.9 (CAr),
118.1, 114.8 (HCAr), 106.7, 92.8, 88.2 (C), 69.41, 69.36 (OCH2),
32.09, 32.07, 30.0, 29.9, 26.2, 23.1, 23.0 (CH2), 15.7 (SCH3), 14.3,
14.2 (CH3). MS+ (MALDI-TOF; m/z): 898.1 [Ru(dppe)2]+, 1793.3
[M + H]+. IR (CH2Cl2): 2055s ν(Ru−CC) cm−1. HR-ESI+-MS:
calcd for C110H118O4P4RuS2 1792.6495; found 1792.6510.
trans-Pt[CC{1,4-C6H2(OC6H13)2}CC(4-C5H4SMe)]2(PPh3)2
(2-Pt). Compound 6 (90 mg, 0.064 mmol), 4-iodothioanisole (37.5
H
DOI: 10.1021/acs.organomet.6b00472
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
F
anchor groups. The resulting I(s) curves are binned in current steps
(16 pS) and plotted to give a conductance histogram comprised of at
least 500 I(s) scans showing plateaus. The error associated with each
current value reported has been statistically obtained from the
standard deviation of the points comprising the conductance peak.
Single-Crystal X-ray Crystallography. The X-ray single-crystal
data for 1-Ru have been collected using λ(Mo Kα) radiation (λ =
0.71073 Å) on a Bruker D8Venture diffractometer (Photon100
CMOS detector, IμS-microsource, focusing mirrors) and for a crystal
of 1-Pt on an Agilent XCalibur diffractometer (Sapphire-3 CCD
detector, fine-focus sealed tube, graphite monochromator) equipped
with a Cryostream (Oxford Cryosystems) open-flow nitrogen cryostat
at 120.0(2) K. The data for the extremely small and weakly diffracting
crystal of 2-Pt were collected at 100.0(2) K on a Rigaku Saturn 724+
diffractometer at station I19 of the Diamond Light Source (UK)
synchrotron (undulator, λ = 0.6889 Å, ω scan, 1.0°/frame) and
processed using Bruker APEXII software. All structures were solved by
direct methods and refined by full-matrix least squares on F2 for all
data using Olex266 and SHELXTL67 software. All nondisordered non-
hydrogen atoms were refined anisotropically; the hydrogen atoms
were placed in calculated positions and refined in riding mode.
Disordered atoms in the structure of 2-Pt were refined isotropically
with fixed SOF = 0.6 and 0.4. The structure of 2-Pt also contains
severely disordered solvent molecules (probably DCM) which could
not be reliably identified and modeled properly. Their contribution to
the structural factors was taken into account by applying MASK
procedure of Olex2 program package. Crystallographic data for the
structures have been deposited with the Cambridge Crystallographic
Data Centre as supplementary publications CCDC 1483157−
1483159.
where f(E) = [eβ(E−E ) + 1]−1 is the Fermi function, β = 1/kBT, EF is
the Fermi energy and G0 = (2e2/h) is the quantum of conductance.
Since the quantity −(df(E)/dE) is a probability distribution peaking at
E = EF, with a width of the order kBT, the above expression shows that
G/G0 is obtained by averaging T(E) over an energy range of order kBT
DFT
in the vicinity of E = EF. It is well-known that the Fermi energy EF
predicted by DFT is not usually reliable, and therefore plots are shown
of G/G0 as a function of EF − EFDFT. To determine EF, we compared
the predicted values of all molecules with the experimental values and
chose a single common value of EF which gave the closest overall
agreement. This yielded a value of EF − EF
used in all theoretical results.
DFT
= −0.07 eV, which is
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00472.
■
S
Crystallographic data (CIF)
All computed molecule Cartesian coordinates (XYZ)
AUTHOR INFORMATION
Corresponding Authors
*E-mail for R.J.N.: nichols@liv.ac.uk.
*E-mail for C.J.L.: c.lambert@lancaster.ac.uk.
*E-mail for M.R.B.: m.r.bryce@durham.ac.uk.
*E-mail for P.J.L.: paul.low@uwa.edu.au.
■
Theoretical Methods. Gas-phase optimizations were performed
with the Gaussian 09 program package,68 using the B3LYP
functional50 and LANL2DZ basis set on Ru and Pt51 and 6-31G**
on all other atoms.52 Results were further analyzed using the
GaussSum package.69
Author Contributions
#These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
The DFT-Landauer approach used in the modeling of molecular
junctions assumes that, on the time scale taken by an electron to
traverse the molecule, inelastic scattering is negligible. This is known
to be an accurate assumption for molecules up to several nanometers
in length.57 All molecules in this work have been relaxed in isolation.
Geometry optimizations were carried out using the DFT code
SIESTA, with a generalized gradient approximation (PBE func-
tional),70 double-ζ polarized basis set, 0.01 eV/A force tolerance, and a
real-space grid with a plane wave cutoff energy of 250 Ry, zero bias
voltage, and 1 k points.
ACKNOWLEDGMENTS
■
C.J.L. and O.A.A. acknowledge financial support from the
Ministry of Higher Education and Scientific Research of Iraq.
C.J.L. and M.R.B. acknowledge funding from the EU through
the FP7 ITN MOLESCO (project number 212942). P.J.L.
holds an ARC Future Fellowship (FT120100073) and
gratefully acknowledges funding for this work from the ARC
(DP140100855). S.B. holds an International Postgraduate
Research Scholarship and gratefully acknowledges support
from the University of Western Australia. R.J.N. and S.J.H.
thank the EPSRC for funding (grant EP/H035184/1 and EP/
K007785/1). We thank the Diamond Light Source for an
award of instrument time on the Station I19 (MT 6749) and
the instrument scientists for support.
To compute the electrical conductance, the molecules were then
placed in the vicinity of the metal|molecule|metal junctions. Each
molecule has been attached to two (111)-directed gold electrodes; one
of these electrodes is pyramidal, while the other is a planar electrode.
Then the molecules and the first layer of electrodes were allowed to
relax again, yielding the optimal junction geometries as shown in
Figure 7. These layers were then used to extend the gold electrodes to
infinity. For each structure, the transmission coefficient T(E)
describing the propagation of electrons of energy E from the left to
the right electrode was calculated by first obtaining the corresponding
Hamiltonian and overlap matrices using SIESTA and then using the
GOLLUM code to compute T(E) via the relation T(E) = Tr{ΓR(E)
GR(E) ΓL(E) GR†(E)}; in this expression, ΓL,R(E) = i(∑L,R(E) −
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