Inorganic Chemistry
Article
[{(L1)Ru}(μ-tpp){Ru(L1)}][PF6]4 [3][PF6]4. The compounds L1 (128
mg, 0.36 mmol) and {Cl3Ru}(μ-tpp){RuCl3} (100 mg, 0.12 mmol)
were added to ethylene glycol (4 mL). This suspension was degassed
by bubbling nitrogen through it before being heated by microwave to
160 °C for 30 min. The purple solution was poured into an aqueous
saturated KPF6 solution, forming a purple precipitate. The precipitate
was collected by filtration, washed thoroughly with water, and air-
dried. The purple solid was washed from the frit with acetone, the
filtrate was collected, and solvent was removed, leaving a purple
residue. This was eluted down a neutral alumina column with
acetonitrile/CH2Cl2 (1:1) collecting the purple band; the solvent was
removed leaving the title product. Crystals were grown by vapor
diffusion of diethyl ether into an acetonitrile solution. Yield: 97 mg
(43%). ESMS: 325 m/z [M]4+. 1H NMR(CDCl3): δ 9.15 (s, 4H), 8.99
(d (J1HH = 8 Hz), 4H), 8.79 (d (8), 4H), 8.26 (d (8), 4H), 8.10 (t (7),
4H), 7.95 (t (8), 4H), 7.85 (d (6), 4H), 7.76 (d (6), 4H), 7.70 (d (7),
4H), 7.45 (t (7), 4H), 7.32 (t (7), 4H), 2.70 (s, 6H) ppm. 13C{1H}
NMR (CDCl3): δ. 157.8, 155.0, 154.7, 154.0, 153.1, 149.8, 149.3,
143.3, 139.0, 137.8, 132.3, 129.4, 129.2, 128.2, 127.6, 126.4, 125.0,
121.7, 14.2 ppm. Anal. Calc. for C68H50F24N12P4Ru2S2: C, 43.41; H,
2.68; N, 8.93%. Found: C, 43.50; H, 2.84; N, 8.91%.
complexes are shown to be capable of serving as wirelike
assemblies of up to 3 nm long.
EXPERIMENTAL SECTION
■
General Details. Microwave reactions were performed in a Biotage
Microwave Synthesizer (model Initiator 2.5). NMR spectra were
recorded in deuterated solvent solutions on Bruker DRX-400 and
Varian Inova 300, 400, and 500 spectrometers and referenced against
solvent resonances (1H, 13C). Electrospray mass spectra (ESMS) were
recorded on a TQD mass spectrometer (Waters Ltd., U.K.) in
acetonitrile. Atmospheric solids analysis probe mass spectra (ASAP-
MS) were collected on a LCT Premier XE mass spectrometer (Waters
Ltd., U.K.) in dichloromethane. Microanalyses were performed by
Elemental Analysis Service, London Metropolitan University, U.K.
Electrochemical analyses of the complexes were performed using a
PalmSens EmStat2 potentiometer, with platinum working, platinum
counter, and platinum pseudo reference electrodes, from solutions in
acetonitrile containing 0.1 M supporting electrolyte (tetrabutyl-
ammonium hexafluorophosphate, NBu4PF6), scan rate = 100 mV
s−1. The ferrocene/ferrocinium couple was used as the internal
reference.
[{(L2)Ru}(μ-tpp){Ru(L2)}][PF6]4 [4][PF6]4. The compound was
prepared using the same procedure as that described for [3][PF6]4,
Analytical grades of solvents were used. The compounds 4′-
(((trifluoromethyl)sulfonyl)oxy)-2,2′:6′,2″-terpyridine (tpyOTf),32 4-
ethynylthioanisole (MeSC6H4CCH),33 4′-(4-(methylthio)phenyl)-
2,2′:6′,2″-terpyridine (L1),34 [Ru(L1)2][PF6]2 ([1][PF6]2),34 and
{Cl3Ru}(μ-tpp){RuCl3}35 were synthesized according to literature
methods. All other chemicals were sourced from standard suppliers.
Hydrated ruthenium chloride, RuCl3·nH2O, was assumed to be of
approximate composition RuCl3·3H2O.
except L2 was used in place of L1. Yield: 106 mg (46%). ESMS: 337
1
m/z [M]4+. H NMR(CDCl3): 9.03 (s, 4H), 9.00 (d (J1 = 8 Hz),
HH
4H), 8.67 (d (8), 4H), 8.11 (t (8), 4H), 7.97 (t (8), 4H), 7.83 (d (6),
4H), 7.78−7.76 (m, 8H), 7.48−7.45 (m, 8H), 7.36 (t (6), 4H), 2.62
(s, 6H) ppm. 13C{1H} NMR (CDCl3): δ. 157.1, 154.8, 154.7, 154.0,
153.3, 142.9, 139.1, 137.9, 132.6, 132.5, 129.5, 129.3, 127.9, 127.4,
125.7, 125.0, 116.8, 98.1, 86.2, 14.0 ppm. Anal. Calc. for
C72H50F24N12P4Ru2S2·4H2O: C, 43.21; H, 2.92; N, 8.40%. Found:
C, 43.21; H, 2.57; N, 8.72%.
Synthesis. 4′-((4-(Methylthio)phenyl)ethynyl)-2,2′:6′,2″-terpyri-
dine (L2). Triethylamine (NEt3, 7 mL) was added to a solution
containing tpyOTf (250 mg, 0.65 mmol) and MeSC6H4CCH (96
mg, 0.65 mmol) in tetrahydrofuran (THF, 20 mL). The solution was
freeze−pump−thawed three times before Pd(PPh3)4 (75 mg, 0.065
mmol) was added. The solution was refluxed overnight in the dark,
after which time the solvent was removed. The solid residue was
extracted in dichloromethane and filtered. The filtrate was passed
down a silica column initially with neat CH2Cl2 then CH2Cl2/
acetonitrile (1:1) to elute the product. The main fraction was
collected, dried, and washed with methanol to remove the remaining
impurities, giving a white solid. Crystals were grown via the slow
evaporation of a hexane/dichloromethane solution. Yield: 193 mg
Single-Molecule Conductance Measurements. Gold-on-glass
substrates (Arrandee, Schroer, Germany) were rinsed with acetone
̈
and then flame-annealed with a butane torch until the slide glowed
with a very slight orange hue. The slide was retained in this state for
∼20 s during which time the torch was kept in motion across the
sample to avoid deleterious overheating. This procedure was
performed three times to generate extended Au (111) terraces, as
seen by STM imaging. The freshly annealed substrates were immersed
in a 1 × 10−4 M acetonitrile (99.9% Chromasolv Plus for HPLC)
solution of the complex under investigation for 1 min, after which time
the gold sample was removed, washed with ethanol, and then dried in
a flow of argon. The short immersion time and low concentration of
solution were chosen to promote low molecular coverage of the gold
surface, which favor the formation of single molecule over multi-
molecular junctions.
1
(78%). ASAP-MS: 380 m/z [MH]+. H NMR (CDCl3): δ 8.70 (d
(J1 = 5 Hz), 2H), 8.60 (d (7), 2H), 8.53 (s, 2H), 7.85 (t (7), 2H),
HH
7.44 (d (7), 2H), 7.33 (t (7), 2H), 7.20 (d (7), 2H), 2.48 (s, 3H) ppm.
13C{1H} NMR (CDCl3): δ. 155.5, 155.3, 149.3, 140.5, 137.0, 133.5,
132.1, 125.7, 124.0, 122.7, 121.2, 118.5, 93.8, 87.5, 15.2 ppm. Anal.
Calc. for C24H17N3S: C, 75.96; H, 4.52; N, 11.07%. Found: C, 75.86;
H, 4.45; N, 11.02%.
Conductance values of those compounds and the break-off distance
were obtained with an STM (Agilent 5500 SPM microscope), using
the I(s) technique.36,37 In this method an electrochemically etched
gold tip is approached close to the substrate surface and then retracted
with the tunnelling current (I) recorded against distance (s). In the
case where molecular junctions are formed, significant deviations from
the usual exponential decay of current are observed, with marked
current plateaus and steps appearing as the tip was retracted. The step
is seen as the tip is retracted beyond the maximal stretched length of
the junction, with the molecular bridge breaking, which leads to the
sharp decrease in current and the observed steplike feature. The
resulting I(s) curves are binned in current divisions (0.025 nS) and
plotted to give a conductance histogram comprised of hundreds of
scans that show plateaus synonymous with molecular junction
formation.
X-ray Crystallography. The single-crystal X-ray data for all
compounds were collected at 120.0(2) K on a Bruker D8Venture 3-
circle diffractometer (Photon100 CMOS detector, IμS microsource,
focusing mirrors, λMo Kα, λ = 0.710 73 Å) equipped with Cryostream
(Oxford Cryosystems) open-flow nitrogen cryostat. Following multi-
scan absorption corrections and solution by direct methods, the
structures were refined against F2 with full-matrix least-squares using
the SHELXTL38 and OLEX239 software. Anisotropic displacement
[Ru(L2)2][PF6]2 [2][PF6]2. The ligand L2 (100 mg, 0.26 mmol) and
RuCl3·3H2O (34 mg, 0.13 mmol) were added to ethylene glycol (4
mL). The resulting suspension was degassed by bubbling nitrogen
through it before being heated by microwave to 160 °C for 30 min.
The red solution was poured into an aqueous, saturated KPF6 solution,
forming a red precipitate. The precipitate was collected by filtration,
washed thoroughly with water, and air-dried. The red solid was
extracted in acetone and filtered, and the filtrate was taken to dryness,
leaving a red residue. This residue was dissolved in the minimum
volume of CH2Cl2 and purified by elution through a neutral alumina
column with an acetonitrile/CH2Cl2 (1:1). The orange band was
collected, and the solvent was removed, leaving the title product.
Crystals were grown by vapor diffusion of diethyl ether into an
acetonitrile solution. Yield: 119 mg (80%). ESMS: 430 m/z [M]2+. 1H
NMR(CDCl3): δ 8.87 (s, 4H), 8.52 (d (J1HH = 8 Hz), 4H), 7.96 (t (8),
4H), 7.71 (d (8), 4H), 7.44−7.42 (m, 8H), 7.20 (t (7), 4H), 2.59 (s,
6H) ppm. 13C{1H} NMR (CDCl3): δ. 157.5, 155.1, 152.6, 142.6,
138.2, 132.4, 130.5, 127.6, 125.7, 125.1, 124.5, 117.0, 97.11, 86.3, 14.0
ppm. Anal. Calc. for C48H34F12N6P2RuS2: C, 50.13; H, 2.98; N, 7.31%.
Found: C, 50.00; H, 3.00; N, 7.31%.
B
Inorg. Chem. XXXX, XXX, XXX−XXX