Effect of methylene spacers on the spectral, electrochemical, and structural properties of bis(4,4′-disubstituted-2,2′-bipyridyl) ruthenium(ii) dye analogues

Article abstract of DOI:10.1039/b905551g

Two new ruthenium complexes, [Ru(L₁^OMe) ₂(NCS)₂] and [Ru(L₂^OMe) ₂(NCS)₂] (where L₁^OMe = 2,2′-bipyridine-4,4′-di(methyl ethanoate) and L₂

Full text of DOI:10.1039/b905551g

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Effect of methylene spacers on the spectral, electrochemical, and structural
properties of bis(4,4¢-disubstituted-2,2¢-bipyridyl) ruthenium(II) dye
analogues†
Sheri Lense,a Kenneth I. Hardcastleb and Cora E. MacBeth*a
Received 20th March 2009, Accepted 8th June 2009
First published as an Advance Article on the web 24th July 2009
DOI: 10.1039/b905551g
OMe
Two new ruthenium complexes, [Ru(L1OMe)2(NCS)2] and [Ru(L2OMe)2(NCS)2] (where L1
2,2¢-bipyridine-4,4¢-di(methyl ethanoate) and L2
=
OMe
= 2,2¢-bipyridine-4,4¢-di(methyl propionate)), have
been synthesized and characterized using spectroscopic and electrochemical techniques. The
[Ru(L2OMe)2(NCS)2] complex has also been characterized by X-ray diffraction studies and reveals a
distorted octahedral coordination geometry about the ruthenium center. Cyclic voltammetry studies of
both complexes exhibit quasi-reversible Ru(II/III) couples and a number of ligand reduction events. The
complex with one methylene spacer between the bipyridyl ligand and the methyl ester functional group,
[Ru(L1OMe)2(NCS)2], was particularly unstable under reducing conditions. The properties of these
complexes are compared with the ethyl ester analogue of the N3 photosensitizer [Ru(L0OEt)2(NCS)2].
Re(CO)3Cl(L2OH) complexes deviated from this trend, exhibit-
Introduction
ing nearly identical electron transfer rates. The authors sug-
gest two explanations for this observation: (1) at these very
short bridge lengths, the exponential dependence of electronic
coupling on spacer length may breakdown, as predicted by
theoretical models, or (2) the electron transfer kinetics may
fall into the adiabatic regime, so that solvent relaxation time
dictates electron transfer rates.23 The photophysical properties of
[ReI(CO)3(polypyridyl)(X)] differ significantly from the proper-
ties exhibited by polypyridyl complexes, [MII(polypyridyl)2(X)2]
(where X = anionic ligand), of other d6 metal ions (i.e., RuII
and OsII). Specifically, the strong-field CO ligands give rise to a
larger energy gap between the (dp) Re orbitals and the p* orbitals
of the bipyridyl ligands.25 Given these inherent differences and
the widespread application of ruthenium complexes with 4,4¢-
disubstituted-2,2¢-bipyridyl ligands in photosensitizing applica-
tions, we aimed to explore the effect of methylene linking units
on the fundamental spectroscopic and electrochemical properties
of ruthenium bipyridyl complexes.
Ruthenium complexes containing 2,2¢-bipyridyl and related
polypyridyl ligands have been widely used as photosensitiz-
ing units in the development of molecular machines1–7 and
photovoltaic cells.8–11 Their effectiveness in these applications
stems from their unique photophysical properties, chemical sta-
bility, and well-established synthetic protocols.12 Early work by
Gra¨tzel and co-workers established cis-[bis(4,4¢-dicarboxy-2,2¢-
bipyridine)bis(thiocyanato)] ruthenium(II),13 known as the N3
dye, as a standard photosensitizer for dye sensitized solar cells
(DSSCs).14 In addition to fast electron transfer to wide band-gap
semiconductors, N3 is chemically stable and able to harvest a rela-
tively large portion of the visible-light spectrum. Metal complexes
containing 4,4¢-disubstituted-2,2¢-bipyridyl ligand variants have
since been used extensively to probe the fundamental properties of
sensitizers for wide band-gap semiconductors. A number of these
studies have focused on tuning the spectral absorption properties
of the dye complexes10,11,14–16 and exploring the influence of the
surface linking unit upon the electron injection kinetics.10,11,17–22
To explore the effect of short alkyl bridges on interfacial
electron coupling, Lian et al. examined the electron transfer
(ET) kinetics for a series of homologous rhenium complexes,
In this report, we describe the synthesis of cis-
OMe
[Ru(LnOMe)2(NCS)2] complexes (where L1
4,4¢-di(methyl ethanoate) and L2
= 2,2¢-bipyridine-
OMe
=
2,2¢-bipyridine-4,4¢-
di(methyl propionate)) in which methylene units have been
inserted between the bipyridine rings and the methyl ester
substituents. These complexes have been characterized by
solution-state spectroscopic and electrochemical methods. In
addition, [Ru(L2OMe)(NCS)2] has been characterized in the
solid-state by X-ray diffraction. The spectral, structural and
electrochemical properties of these complexes are directly
compared to the previously reported ethyl ester analogue of the
OH
Re(CO)3Cl(LnOH) (where Ln = 2,2¢-bipyridyl-4,4¢-dicarboxylate
and n = (1–5) the number of methylene spacers (CH2) between
the carboxylic acid anchoring group and the bipyridyl ligand), to
SnO2 nanocrystalline films.23,24 When the data was analyzed using a
time-scaling approach, the ET rate was found to decrease exponen-
tially with bridge length for Re(CO)3Cl(L3OH), Re(CO)3Cl(L4OH),
and Re(CO)3Cl(L5OH). In contrast, the Re(CO)3Cl(L1OH) and
OEt
N3 dye, [Ru(L0OEt)2(NCS)2] (where L0 = 4,4¢-ethoxycarbonyl-
aDepartment of Chemistry, Emory University, Atlanta, GA, USA. E-mail:
cmacbet@emory.edu; Fax: +1 404-727-6586; Tel: +1-404-727-7033
2,2¢-bipyridine).26 Attempts to hydrolyze these methyl ester
complexes to form their corresponding carboxylic acid analogues
and attempts to synthesize the carboxylic acid derivatives
directly from the carboxylic acid functionalized ligands yielded
complex product mixtures unsuitable for interfacial electron
bX-Ray Crystallography Laboratory, Department of Chemistry, Emory
University, Atlanta, GA, USA
† CCDC reference number 723665. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/b905551g
7396 | Dalton Trans., 2009, 7396–7401
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transfer studies. The possible decomposition pathways for these
complexes under hydrolysis conditions are also described.
downfield for [Ru(L1OMe)2(NCS)2] than for [Ru(L2OMe)2(NCS)2]
due to the closer proximity of the electron withdrawing ester
group. This downfield shift is even more pronounced for the
methylene spacer protons. This trend is expected, as the methy-
lene spacers in [Ru(L2OMe)2(NCS)2] are further from both the
deshielding ester and pyridyl groups than the single methylene
spacer in [Ru(L1OMe)2(NCS)2]. In the 13C NMR spectra, the
thiocyanate carbon peak appears at approximately 134 ppm
for both [Ru(L1OMe)2(NCS)2] and [Ru(L2OMe)2(NCS)2], consistent
with an N-bound isothiocyanate.26 The IR spectra are also
consistent with the proposed structures of [Ru(L1OMe)2(NCS)2] and
[Ru(L2OMe)2(NCS)2], as the complexes exhibit diagnostic n(CN)
stretches at 2103 cm-1 and n(CO) stretches at 1736 and 1731 cm-1,
respectively.
The UV-vis absorption spectra for [Ru(L1OMe)2(NCS)2],
[Ru(L2OMe)2(NCS)2], and N3 recorded in DMSO are shown in
Fig. 1 and the electronic spectral data are summarized in Table 1.
Each complex displays one bipyridine centred p–p* transition
and two metal-to-ligand charge transfer (MLCT) bands charac-
teristic of cis-coordinated ruthenium bipyridyl complexes.8 Both
[Ru(L1OMe)2(NCS)2] and [Ru(L2OMe)2(NCS)2] absorb at similar
wavelengths with similar extinction coefficients. The absorptions
of [Ru(L1OMe)2(NCS)2] and [Ru(L2OMe)2(NCS)2] are significantly
blue shifted with respect to that of the previously reported
[Ru(L0OEt)2(NCS)2] and the N3 dye (Table 1).26,31 This shift arises
due to the disruption in the conjugation between the carbonyl and
pyridyl groups.
Results and discussion
Syntheses
The ligands 2,2¢-bipyridine-4,4¢-di(methyl ethanoate)(L1OMe) and
2,2¢-bipyridine-4,4¢-di(methyl propionate)(L2OMe) were synthe-
sized using modified literature procedures (see Experimental for
details).27 The corresponding Ru complexes were synthesized
OMe
OMe
by refluxing two equivalents of either L1
or L2
with
RuCl3(H2O)x, excess LiCl and excess hydroquinone in a 2 : 1
dimethoxyethane–methanol solution. After refluxing, a dark red
solid was isolated and treated immediately with excess potassium
thiocyanate in refluxing methanol (Scheme 1). It is assumed, based
on similar synthetic protocols,13,26,28 that the dark red products
isolated after the first step are the corresponding dichloride
complexes, [Ru(LnOMe)2Cl2] (where n = 1 or 2). These intermediates
were unstable and were, therefore, converted directly to the
corresponding bis(isothiocyanto) complexes, [Ru(LnOMe)2(NCS)2]
(Scheme 1).Once synthesized, the complexes were stored as solids
in the dark. Solutions (CH3CN) of the complexes were photo-
chemically unstable and exhibited a decrease in their absorbance
maxima after prolonged exposure (ca. 5 h) to sunlight.
Scheme 1 Synthesis of [Ru(L1OMe)2(NCS)2] and [[Ru(L2OMe)2(NCS)2].
It is worthy to note that the analogous carboxylic acid
derivative, [Ru(L1OH)2(NCS)2], could not be prepared directly by
refluxing RuCl3(H2O)x with the ligand, L1OH, in DMF. This classic
route leads only to complex reaction mixtures. Likewise, heating
OMe
the methyl ester ligand derivative L1
with RuCl3(H2O)x in
Fig.
1 ( ),
UV-vis absorption spectra for [RuII(L1OMe)2(NCS)2]
DMF (135 ◦C) leads mainly to decarboxylation of the ligand
scaffold. This decarboxylation pathway has been previously
observed for the parent N3 dye complex under forcing condi-
[RuII(L2OMe)2(NCS)2] ( ), and cis-(NCS)2 bis(2,2¢-bipyridyl-4,4¢-dicarbo-
xylate) ruthenium(II) or N3 dye ( ) recorded in DMSO.
◦
tions (T > 200 C).29,30 The instability of this particular ligand
Both [Ru(L1OMe)2(NCS)2] and [Ru(L2OMe)2(NCS)2] are lumines-
cent in acetonitrile solutions at room temperature. When excited
at 525 nm, the emission maxima exhibited by [Ru(L1OMe)2(NCS)2]
and [Ru(L2OMe)2(NCS)2] at 722 and 715 nm, respectively, are higher
in energy than that of [Ru(L0OEt)2(NCS)2] (875 nm).26 This trend
indicates that the insertion of methylene spacer groups between
the bipyridyl ligand and the ester functional group leads to an
overall increase in the energy of the p* orbital.
derivative is also apparent in the electrochemical studies of the
[Ru(L1OMe)2(NCS)2] complex (vide infra).
Spectroscopy
The metal complexes were characterized by 1H NMR, 13C NMR,
UV-vis, and IR spectroscopies, as well as mass spectrometry, and
elemental analysis. The H NMR spectra of the complexes each
1
show six peaks in the aromatic region, indicative of cis-bipyridyl
coordination, as well as four and six peaks in the alkyl region
for [Ru(L1OMe)2(NCS)2] and [Ru(L2OMe)2(NCS)2], respectively, that
correspond to the protons of the methylene spacers and the
methyl ester protons. The aromatic peaks are shifted more
Electrochemistry
Cyclic voltammetry of the complexes, performed in DMF and
reported vs. Fc/Fc+, show quasi-reversible events centred at
0.18 V and 0.23 V for [Ru(L2OMe)2(NCS)2] and [Ru(L1OMe)2(NCS)2],
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Table 1 Electronic spectral and luminescence data for cis-[RuII(L)2(NCS)2] complexes (L = 4,4¢-substituted bipyridine)a
Complex
Solvent
MLCT
MLCT
p–p*
Emission maximum
[Ru(L0OEt)2(NCS)2]b
[Ru(L1OMe)2(NCS)2]
[Ru(L2OMe)2(NCS)2]
N3bc
DMSO
CH3CN
DMSO
CH3CN
DMSO
CH3CN
DMSO
550 (9.0)
—
520 (9.6)
515 (10.0)
515 (9.3)
515 (8.8)
542 (13.2)
414 (9.5)
—
365 (10.5)
365 (11.0)
365 (10.0)
360 (9.6)
400 (13.0)
320 (27.8)
—
300 (52.3)
295 (54.0)
300 (52.0)
295 (52.0)
318 (45.7)
—
875
—
722d
—
715d
—
a Wavelengths are reported in nm, the values in parentheses are molar extinction coefficients (e ¥ 103 M-1 cm-1). b Ref. 26. c N3 = cis-[bis(4,4¢-dicarboxy-
2,2¢-bipyridine)bis(thiocyanato)] ruthenium(II). d Emission spectra of 3 ¥ 10-5 M solutions were recorded at 25 ◦C.
respectively, which are assigned to the Ru(II/III) couple (Fig. 2).
Neither the one electron oxidation of [Ru(L1OMe)2(NCS)2] nor
of [Ru(L2OMe)2(NCS)2] is completely reversible at scan rates
ranging from 50 mV s-1 -250 mV s-1. However, the oxidation
of [Ru(L2OMe)2(NCS)2] appears to be more reversible than that of
[Ru(L1OMe)2(NCS)2]; that is, the DEp value (0.077 V) and the ipr/ipf
ratio (0.95) recorded for [Ru(L2OMe)2(NCS)2] are closer to the ideal
values of DEp (0.059 V) and ipr/ipf (1.0) expected for a reversible
process.32 Additionally, the ratio of ipf/n1/2 for [Ru(L2OMe)2(NCS)2]
varies only slightly with increasing scan rate. These results differ
from the Ru(II/III) oxidation reported for [Ru(L0OEt)2(NCS)2]
of which, at -1.480 mV and -1.697 mV (vs. Fc/Fc+), were
chemically and electrochemically reversible.34 It is possible that
additional ligand reduction events occur for [Ru(L1OMe)2(NCS)2]
and [Ru(L2OMe)2(NCS)2], but that they occur at potentials too
negative to be observed in the DMF solvent window.
The degree of reversibility exhibited by complexes of this type
is associated with the stability of the reduced form. The decreased
stability of the reduced forms of [Ru(L1OMe)2(NCS)2] may result
from the presence of the vulnerable methylene group flanked by
both the pyridyl and ester groups, which could stabilize either
a radical or negative charge at the site, facilitating hydrogen or
proton transfer within or between the reduced ligands.
(Ep = 0.56 mV vs. Fc/Fc+ at 100 mV s-1) in DMF, which was
ox
found to be chemically irreversible at all scan rates up to 10 000 mV
s-1.33 The greater reversibility observed for [Ru(L2OMe)2(NCS)2] and
[Ru(L1OMe)2(NCS)2] may be due to the fact that these complexes are
oxidized at a lower potential than [Ru(L0OEt)2(NCS)2], making it
less thermodynamically favourable for the oxidized Ru(III) centres
to oxidize the coordinated isothiocyanate ligands.
Structural characterization
The crystal structure of [Ru(L2OMe)2(NCS)2], was obtained by
diffusing ether into an acetonitrile solution of the complex. The
structure of [Ru(L2OMe)2(NCS)2] is shown in Fig. 3 and is one of
the few examples of a structurally characterized cis-[Ru(L)2(X)2]
(where L = 4,4¢-disubstitued-2,2¢-bipyridyl and X = monodentate,
anionic ligands) type complex.26,35–39 As can be seen in Fig. 3,
there is a large amount of static disorder amongst the bipyridyl
substituents, particularly the methyl ester atoms. Consequently,
the higher angle data collection was limited, and the reflection
count was only 83.7% complete. Attempts to model the disorder
did not improve the structure.
Fig. 2 Cyclic voltammograms (vs. Fc/Fc+) of the one electron ox-
idation of (A) [Ru(L1OMe)2(NCS)2] and (B) [Ru(L2OMe)2(NCS)2] and
(inset) complete voltammograms of (C) [Ru(L1OMe)2(NCS)2] and (D)
[Ru(L2OMe)2(NCS)2]. Scans recorded in DMF (0.2 M n-Bu4NPF6) at
100 mV s-1 and reported vs. Fc/Fc+.
Two quasi-reversible ligand reduction events centred at -1.985
and -2.236 V (vs. Fc/Fc+) were observed for [Ru(L2OMe)2(NCS)2]
and three irreversible ligand reduction events were observed for
[Ru(L1OMe)2(NCS)2] at -2.065, -2.128, and -2.529 V. The cyclic
voltammograms of [Ru(L1OMe)2(NCS)2] and [Ru(L2OMe)2(NCS)2]
are displayed in Fig. 2. Wolfbauer et al. reported observing
four reduction processes for [Ru(L0OEt)2(NCS)2],34 the first two
Fig. 3 Thermal ellipsoid diagram of [Ru(L2OMe)2(NCS)2] with ellipsoids
drawn at 35% probability. Hydrogen atoms have been omitted for clarity.
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Table 2 Selected bond lengths (A) and angles (◦) for
˚
modified ruthenium dye complexes. The complexes exhibit very
similar electronic absorption spectra that are blue shifted (ca.
30 nm) with respect to the N3 dye. Both complexes displayed
quasi-reversible, one-electron oxidation events (Ru(II/III)) and
several ligand-based reduction events. The complex with one
methylene spacer, [Ru(L1OMe)2(NCS)2], is significantly less stable
under reducing conditions than the complex with two methylene
spacers, [Ru(L2OMe)2(NCS)2].
[Ru(L0OEt)2(NCS)2]·CH3CN (1), [Ru(L2OMe)2(NCS)2] (2), and [Ru(2,2¢-
bpy)2(NCS)2]·CH3CN
1a
2b
3c
˚
Bond lengths/A
Ru–N(NCS)
2.07(2)
2.01(1)
2.04(2)
2.049(6)
2.045(6)
2.027(7)
2.062(6)
2.042(6)
2.026(6)
2.055(5)
2.041(4)
2.051(4)
d
Ru–N(L)cis
d
Experimental
Ru–N(L)trans
General
Bond angles/◦
All solvents and reagents were purchased from Sigma-Aldrich and
used as received unless otherwise stated. Tetrahydrofuran (THF),
dimethoxyethane (DME), methanol (MeOH), and acetonitrile
(CH3CN) were purified by sparging with argon and dried on a
Glass Contour Solvent System before use. THF was further pu-
rified by distillation from CaH2. Reagent grade (99.9%) dimethyl
sulfoxide (DMSO) was purchased from EMD Chemicals and used
without further purification. Anhydrous N,N-dimethylformamide
(DMF) was purchased from DriSolv. MgSO4 was purchased from
EMD Chemicals, Inc. Alumina was purchased from Sorbent Tech-
nologies. LiCl was purchased from Alfa Aesar and ruthenium(III)
chloride hydrate, 40–43% Ru (RuCl3·xH2O) was purchased from
Strem Chemicals. The N3 dye (cis-[bis(isothiocyanato)bis(2,2¢-
bipyridyl-4,4¢-dicarboxylato)]-ruthenium(II)) was purchased from
Solaronix and used as received.
N(NCS)–Ru–N(NCS)¢
86(1)
95(1)
172(1)
174(1)
90.2(2)
88.7(3)
78.7(2)
173.0(3)
168.2(2)
d
N(L)trans–Ru–N(L)trans
91.2(2)
171.8(2)
177.3(6)
172.4(6)
d
N(L)cis–Ru–N(L)¢cis
Ru–N–C(S)
a Ref. 26. b This work. c Ref. 40. d The N(L)cis and N(L)trans notation is used
to refer to the nitrogen ligands of the bipyridyl ligands that are cis and
trans to the coordinated NCS ligands.
The structure of [Ru(L2OMe)2(NCS)2] displays a distorted oc-
tahedral coordination environment about the Ru(II) centre,
which is similar to the previously reported ethyl ester ana-
logue of the N3 dye, [Ru(L0OEt)2(NCS)2],26 and the homolo-
gous complex containing unsubstituted 2,2¢-bipyridyl ligands,
[Ru(2,2¢-bpy)2(NCS)2].26,40 Both [Ru(2,2¢-bpy)2(NCS)2]·CH3CN
and [Ru(L0OEt)2(NCS)2]·CH3CN display crystallographic C2-
symmetry. Each structure in this series displays a distorted
octahedral coordination environment around the ruthenium cen-
tre. The bond lengths and angles observed in the structure of
[Ru(L2OMe)2(NCS)2] are quite similar to those found in both
[Ru(L0OEt)2(NCS)2] and [Ru(2,2¢-bpy)2(NCS)2] (Table 2). The small
structural differences displayed by this series of compounds sug-
gest that this basic ligand set enforces a rather rigid coordination
geometry that is not strongly influenced by the 4,4¢-bipyridyl
substituents.
1H and 13C NMR were recorded on either a Varian Mercury
300 spectrometer operating at 300 or 75 MHz for 1H or 13C,
respectively, or on an Inova 400 spectrometer at 400 or 100 MHz
1
for H or 13C, respectively. Chemical shifts (d) are reported in
ppm downfield from TMS and are referenced to residual solvent.
Infrared (IR) spectra were recorded on a Varian 800 FT-IR. UV-
vis absorption spectra were recorded on a Cary 50 Scan UV-vis
spectrophotometer using 1 cm quartz cells. Mass spectrometry
analyses (positive and negative electrospray ionization) were
recorded on a Thermo Finnigan LTQ-FTMS. The emission
spectra were recorded on a Horiba Jobin Yvon FluoroMax-3
spectrofluorometer by excitation at 525 nm. Room temperature
solution measurements (3.0 ¥ 10-5 M) were recorded using a 1 cm
quartz cuvette. Samples were prepared under an N2 atmosphere
utilizing degassed and anhydrous CH3CN.
Elemental analysis was performed by either Columbia Analyt-
ical Services, Tucson, AZ or Atlantic Microlab, Inc., Norcross,
GA. Electrochemical experiments were conducted using a CH
Instruments Potentiostat (CHI600C) in a three-component cell
consisting of a Pt auxiliary electrode, a non-aqueous reference elec-
trode (Ag/AgNO3 in (0.1 M n-Bu4NPF6) CH3CN), and a glassy
carbon working electrode. All electrochemical measurements were
conducted under a nitrogen atmosphere and are referenced and
reported vs. the ferrocene/ferrocenium couple. 2,2¢-Bipyridine-
4,4¢-di(2-ethanoic acid) (L1OH) was synthesized using a literature
procedure.27
Attempts
to
hydrolyse
[Ru(L1OMe)2(NCS)2]
and
[Ru(L2OMe)2(NCS)2] to form the carboxylic acid analogues
were unsuccessful. Subjecting both [Ru(L1OMe)2(NCS)2] and
[Ru(L2OMe)2(NCS)2] to typical hydrolysis procedures resulted
in the formation of the desired product plus a single impurity
1
(~20% by H NMR spectroscopy) that was not separable using
chromatography or recrystallization procedures. Spectroscopic
data suggest that the impurity is most likely the NCS linkage
isomer, which has been observed in the synthesis of the N3
dye and other analogues.10,41 These findings suggest that milder
synthetic protocols using the carboxylic acid versions of the
ligands and alternative Ru(II) precursors may be necessary to
isolate these complexes in analytically pure form.
Conclusions
In conclusion, we have synthesized and characterized
two new ruthenium complexes, [Ru(L1OMe)2(NCS)2] and
[Ru(L2OMe)2(NCS)2], to study the effects of methylene spacers on
the spectroscopic, electrochemical, and structural properties of
Preparations
OH
2,2¢-Bipyridine-4,4¢-di(methyl
(662 mg, 2.47 mmol) and 10.7 equivalents of SOCl2 were refluxed
ethanoate),
(L1OMe). L1
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in 26 mL MeOH under N2 for 16 h. The reaction was then
cooled to room temperature and the solvent removed under
reduced pressure. The resulting residue was extracted into
dichloromethane (200 mL) and washed three times with aqueous
saturated NaHCO3 (100 mL each). The organic layer was then
dried over Na2SO4, filtered and the solvent removed under reduced
pressure to yield a cream coloured solid The product was purified
by column chromatography using Silica-60, eluting with 66.25%
Et2O, 33.25% pentane and 0.5% Et3N, by volume. The product
eluted at Rf = 0.14. Yield = 79%. IR (KBr) nmax/cm-1 3005.1 w
(sp2 CH), 2953.9 (sp3 CH), 1742.2 s (CO), 1602.0 m, 1461.0 m,
1430.0 (pyridyl CN, CC)). lmax/nm (DMSO) (e/cm-1 M-1): 230.0
(2570); 284.9 (1570). dH(CDCl3, 300 MHz)/ppm: 8.64 (d, J =
5.1 Hz, 2 H), 8.33 (s, 2 H), 7.27 (dd, J = 5.1 Hz, 1.5 Hz, 2 H),
3.74 (s, 4 H), 3.71 (s, 6 H). dC(CDCl3, 400 MHz)/ppm: 170.92,
156.50, 149.71, 144.19, 125.01, 122.49, 52.68, 40.91. HRMS-EI
m/z: M+calculated for C16H16N2O4 300.1188; found 300.1185.
3.69 (s, 4 H). dC(CDCl3, 400 MHz)/ppm: 170.24, 169.89, 158.81,
157.10, 153.63, 151.63, 143.31, 142.55, 133.81, 127.80, 126.85,
123.68, 123.58, 52.94, 52.82, 40.24. MS (postive ESI): m/z (%) =
818.075 (70, M+), 787.112 (51, M+ - OCH3). Elemental analysis:
calculated for C34H32N6O8RuS2: C 49.93, H 3.94, N 10.28. Found
C 49.40, H 3.87, N 10.15. Electrochemistry CV (DMF, 298 K vs.
Fc/Fc+, 0.2 M TBAPF6, glassy carbon w.e.): E1/2(Ru(II/III)) =
0.23 V. [Ru(L1OMe)2(NCS)2] is photochemically unstable. Exposure
of a solution sample of the complex to direct sunlight for 5 h
resulted in ~3% decomposition of the complex (monitored by UV-
vis absorption spectroscopy).
[Ru(L2OMe)2(NCS)2]. LiCl (650.0 mg, 15.3 mmol) was trans-
ferred to a 3 neck flask, flame dried under N2, and cooled
OMe
under vacuum. Under a constant flow of N2, L2
(337.9 mg,
1.03 mmol), RuCl3·xH2O (134.8 mg, 0.51 mmol), and hydro-
quinone (1.14 g, 10.3 mmol), and a 2 : 1 DME–MeOH solution
(38 mL) were added. The reaction was sparged gently with N2 and
then brought to reflux for 5 h. After cooling to room temperature
the solvent was removed under reduced pressure. The reaction
was then extracted into 265 mL dichloromethane and washed
with (5 ¥ 53 mL) deionized H2O. The organic layer was dried over
MgSO4, and concentrated under reduced pressure. The purple
liquid was then dropped into Et2O (~110 mL), yielding a purple
precipitate that was collected on a fritted glass filter and washed
with ether and water. The purple solid was further washed with
Et2O and then dried overnight. In a three-necked flask, KNCS
(821.5 mg, 8.44 mmol) was dissolved in dry MeOH (~240 mL)
and the solution was sparged with argon for 15 min. The purple
solid was added to the solution and the reaction brought to
reflux under N2 for 15 h. The reaction was then cooled to room
temperature and the MeOH removed under reduced pressure. The
red solid that remained was collected on a fritted glass funnel and
washed with deionized H2O and Et2O. The dark-red solid was
then extracted into dichloromethane and filtered through a fritted
glass funnel The dichloromethane was removed using a rotary
evaporator, leaving a dark fuschia solid (306.7 mg, 69% yield).
X-Ray quality crystals were obtained by the slow diffusion of
ether into an acetonitrile solution of product. IR (KBr) nmax/cm-1
2103.4 s (isothiocyanate CN), 1732.6 s (CO), 1616.4 m, 1478.0
m, 1434.8 m (pyridyl CN and CC), 804.2 w (isothiocyanate CS).
UV-vis (DMSO): lmax/nm (e/cm-1 M-1): 515 (9300); 365 (10 000);
300 (52 000). UV-vis (CH3CN): lmax/nm (e/cm-1 M-1): 515 (8800);
360 (9600); 295 (52 000). dH(CDCl3, 400 MHz)/ppm: 9.41 (d, J =
5.6 Hz, 2 H), 8.03 (s, 2 H), 7.89 (s, 2 H), 7.46 (d, J = 5.6 Hz, 2
H), 7.32 (d, J = 5.6 Hz, 2 H), 6.85 (d, J = 5.6 Hz, 2 H), 3.78 (s, 6
H), 3.65 (s, 6 H), 3.20 (t, 7.2 Hz, 4 H), 2.95 (t, 7.2 Hz, 4 H), 2.87
(t, 7.2 Hz, 4 H), 2.67 (t, 7.2 Hz, 4 H). dC(CDCl3, 400 MHz)/ppm:
172.7, 172.6, 158.8, 157.2, 153.5, 151.4, 150.1, 149.5, 133.6, 126.8,
126.0, 123.0, 122.8, 52.3, 52.2, 34.2, 33.9, 30.3, 30.0. MS (postive
ESI): m/z (%) = 874.1397 (100, M+). Elemental analysis calculated
for C38H40N6O8RuS2: C 52.22, H 4.61, N 9.62%. Found C 52.46,
H 4.58, N 10.00%. CV (DMF, 298 K vs. Fc/Fc+, 0.2 M TBAPF6,
glassy carbon w.e.): E1/2 (Ru(II/III)) = 0.18 V.
2,2¢-Bipyridine-4,4¢-di(methyl propionate), (L2OMe). Beginning
with 1.9 g of the olefin precursor, the literature procedure was
followed until the purification step.27 The crude product was
chromatographed on Si-60 with 0.5% triethylamine in Et2O.
The product eluted at Rf = 0.38. Yield 1.2 g, 51%. dH(CDCl3,
300 MHz)/ppm: 8.59 (d, J = 4.8 Hz, 2 H), 8.30 (d, 2 H), 7.20 (dd,
4.8 Hz, 1.2 Hz, 2 H), 3.69 (s, 6 H), 3.06 (t, J = 7.8 Hz, 4 H), 2.74
(t, J = 7.8 Hz, 4 H).
[Ru(L1OMe)2(NCS)2]. LiCl (1.47 g, 34.96 mmol) was transferred
to a 3 neck flask, flame dried under N2, and cooled under
vacuum. Under a constant flow of N2, L1OMe(700.0 mg, 2.33 mmol),
RuCl3·xH2O (304.7 mg, 1.166 mmol), hydroquinone (2.57 g,
23.3 mmol), and a 2 : 1 DME–MeOH solution (69 mL) were
added to the flask. The reaction was sparged gently with N2 and
then refluxed for 5 h. The reaction was then cooled to room
temperature and the solvent was removed under reduced pressure.
The reaction was then extracted into 580 mL dichloromethane and
washed (5 ¥ 116 mL) with deionized H2O. The organic layer was
dried over MgSO4, and concentrated using a rotary evaporator.
The resulting purple liquid was then added dropwise to Et2O
(~250 mL), giving a purple precipitate that was collected on a
fritted glass filter. The solid was further washed with Et2O and
then dried overnight under vacuum to yield a purple solid. In
a 3-necked flask, KNCS (24.84 mmol, 2.417 g) was added to
dry MeOH (~250 mL) and the solution sparged with argon for
15 min. The purple solid was then added and the reaction brought
to reflux under N2 for 15 h. The reaction was then cooled to room
temperature and the MeOH removed under reduced pressure. The
resulting red solid was collected on a fritted glass funnel and
washed with deionized H2O and Et2O. The dark red solid was
then extracted into dichloromethane and filtered through a fritted
glass funnel. The dichloromethane was removed using a rotary
evaporator, leaving a dark fuschia solid (759 mg, 79.6% yield).
IR (KBr) nmax/cm-1 2103.2 s (isothiocyanate CN), 1735.7 s (CO),
1617.7 m, 1477.4 m, 1433.7 m (pyridyl CN and CC), 804.5 w
(isothiocyanate CS). UV-vis (DMSO) lmax/nm (e/cm-1 M-1): 520.1
(9600); 364.9 (10 500); 299.9 (52 300). UV-vis (CH3CN) lmax/nm
(e/cm-1 M-1): 515 (10 000); 365 (11 000); 295 (54 000). dH(CDCl3,
400 MHz)/ppm: 9.48 (d, J = 5.6 Hz, 2 H), 8.16 (s, 2 H), 8.00 (s,
2 H), 7.56 (d, J = 6.0 Hz, 2 H), 7.44 (d, J = 5.6 Hz, 2 H), 6.97
(d, J = 6.0 Hz, 2 H), 3.93 (s, 4 H), 3.85 (s, 6 H), 3.72 (s, 6 H),
Crystal structure determination of [Ru(L2OMe)2(NCS)2]. A suit-
able crystal was coated with Paratone N oil, suspended on a small
fiber loop and placed in a cooled nitrogen gas stream at 173 K on a
Bruker D8 APEX II CCD sealed tube diffractometer with graphite
˚
monochromated Cu Ka (l = 1.54178 A) radiation. Data were
7400 | Dalton Trans., 2009, 7396–7401
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measured using a series of combinations of phi and omega scans
with 10 s frame exposures and 0.5◦ frame widths. Data collection,
indexing and initial cell refinements were all carried out using
APEX II software.42 Frame integration and final cell refinements
were done using SAINT software.43 The final cell parameters were
determined from least-squares refinement on 2159 reflections. The
structure was solved using Direct methods and difference Fourier
techniques (SHELXTL, v6.12).44 Hydrogen atoms were placed
their expected chemical positions using the HFIX command and
were included in the final cycles of least squares with isotropic Uij
values related to the atom ridden upon. All non-hydrogen atoms
were refined anisotropically. Scattering factors and anomalous
dispersion corrections are taken from the International Tables for
X-Ray Crystallography.45 Structure solution, refinement, graphics
and generation of publication materials were performed by using
SHELXTL, v6.12 software.44
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Crystal data. C38H40N6O8RuS2, M = 873.95 g mol-1, mon-
˚
oclinic, a = 29.507(3), b = 15.487(2), c = 17.678(2) A, b =
◦
3
˚
106.297 , V = 7753(6) A , T = 173(2) K, space group C2/c,
Z = 8, m(Cu Ka) = 4.783 mm-1, 15 363 reflections measured, 5353
unique (Rint = 0.0459) which were used in all calculations. The
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©