Solvent effects on isomerization in a ruthenium sulfoxide complex

Article abstract of DOI:10.1021/ic902047f

We report the structure, electrochemistry, and isomerization kinetics for [Ru(bpy)(biq)(OSO)](PF₆), where bpy is 2,2′-bipyridine, biq is 2,2′-biquinoline, and OSO is 2-methylsulfinylbenzoate. UV-visible and infrared data are suggestive of intramolecular S→O and O→S isomerization of the sulfoxide. Cyclic voltammetry reveals evidence for isomerization triggered by oxidation and reduction. Of particular note is the variation of the S→O isomerization rate constant in different solvents. The rates were found to be 3.2 (±0.4) s^-1 in propylene carbonate, 0.80 (±0.03) s^-1 in acetonitrile, and 0.26 (±0.01) s^-1 in dichloromethane.

Full text of DOI:10.1021/ic902047f

4466 Inorg. Chem. 2010, 49, 4466–4470
DOI: 10.1021/ic902047f
Solvent Effects on Isomerization in a Ruthenium Sulfoxide Complex
Tod A. Grusenmeyer,† Beth Anne McClure,† Christopher J. Ziegler,‡ and Jeffrey J. Rack*,†
†Department of Chemistry and Biochemistry, Nanoscale and Quantum Phenomena Institute, Ohio University,
Athens, Ohio 45701, and ‡Department of Chemistry, Knight Chemical Laboratory, University of Akron,
Akron, Ohio 44325
Received October 15, 2009
We report the structure, electrochemistry, and isomerization kinetics for [Ru(bpy)(biq)(OSO)](PF6), where bpy is
2,20-bipyridine, biq is 2,20-biquinoline, and OSO is 2-methylsulfinylbenzoate. UV-visible and infrared data are
suggestive of intramolecular SfO and OfS isomerization of the sulfoxide. Cyclic voltammetry reveals evidence for
isomerization triggered by oxidation and reduction. Of particular note is the variation of the SfO isomerization rate
constant in different solvents. The rates were found to be 3.2 ((0.4) s-1 in propylene carbonate, 0.80 ((0.03) s-1 in
acetonitrile, and 0.26 ((0.01) s-1 in dichloromethane.
Introduction
polypyridine ruthenium sulfoxide complexes based on SfO
and OfS isomerization.14-18 In our studies, we have found
an unusual solvent dependence on the intramolecular SfO
isomerization rate.
Electron transfer induced conformational changes are
central to the operation of molecular machines and other
types of molecular bistability.1-4 A number of studies have
revealed that reduction or oxidation of interlocked rotaxanes
results in translocation of a molecular unit from one site to
another.5-7 However, certain transition metal complexes
exhibit bistability through isomerization of bound ambiden-
tate ligands.8,9 For example, pentaammine ruthenium com-
plexes of dimethylsulfoxide (dmso) undergo intramolecular
linkage isomerization (S vs O) following oxidation and
reduction of ruthenium.10-13 Recently, we have developed
a class of simultaneously photochromic and electrochromic
Experimental Section
Materials. The ruthenium starting material [(p-cym)-
Ru(bpy)Cl]Cl, where p-cym is para-cymene and bpy is 2,20-bipyr-
idine, was synthesized according to a modified procedure for the
complex [(Bz)Ru(bpy)Cl]Cl by starting from [(p-cym)RuCl2]2 in-
stead of [(Bz)RuCl2]2.19,20 The synthesis of the ligand OSO has been
described previously.14 The reagents 2,20-bipyridine (bpy), 2,20-
biquinoline ligand (biq), 2-methylthiobenzoic acid (OS), lithium
chloride, and 3-chloroperoxybenzoic acid (mCPBA) were pur-
chased from Sigma Aldrich and used as received. Silver hexa-
fluorophosphate (AgPF6) and silver trifluoromethanesulfonate
(AgOTf) were purchased from Strem and used as received. Tetra
n-butyl ammonium hexafluorophosphate (TBAPF6) was pur-
chased from Fluka and recrystallized from hot ethanol three
times before use. The solvents N,N-dimethylformamide (DMF),
ethanol, diethyl ether, acetone, triethylamine, 1,2-dichlor-
oethane, dichloromethane, and methanol were purchased from
VWR and used without further purification. Acetonitrile and
dichloromethane for electrochemical experiments were of spec-
troscopic grade and purchased from Burdick and Jackson.
Anhydrous propylene carbonate used for electrochemical ex-
periments was purchased from Sigma Aldrich and used without
further purification. Deuterated solvents (d6-dmso, d2-D2O, and
*To whom correspondence should be addressed. E-mail: rackj@ohio.edu.
(1) Silvi, S.; Venturi, M.; Credi, A. J. Mater. Chem. 2009, 19, 2279–2294.
(2) Rescifina, A.; Zagni, C.; Iannazzo, D.; Merino, P. Curr. Org. Chem.
2009, 13, 448–481.
(3) Bonnet, S.; Collin, J. P.; Koizumi, M.; Mobian, P.; Sauvage, J. P. Adv.
Mater. 2006, 18, 1239–1250.
(4) Stoddart, J. F. Chem. Soc. Rev. 2009, 38, 1802–1820.
(5) McNitt, K. A.; Parimal, K.; Share, A. I.; Fahrenbach, A. C.; Witlicki,
E. H.; Pink, M.; Bediako, D. K.; Plaisier, C. L.; Le, N.; Heeringa, L. P.;
Vander Griend, D. A.; Flood, A. H. J. Am. Chem. Soc. 2009, 131, 1305–1313.
(6) Spruell, J. M.; Paxton, W. F.; Olsen, J. C.; Benitez, D.; Tkatchouk, E.;
Stern, C. L.; Trabolsi, A.; Friedman, D. C.; Goddard, W. A.; Stoddart, J. F.
J. Am. Chem. Soc. 2009, 131, 11571–11580.
(7) Periyasamy, G.; Sour, A.; Collin, J.-P.; Sauvage, J.-P.; Remacle, F.
J. Phys. Chem. B. 2009, 113, 6219–6229.
(8) Hamaguchi, T.; Inoue, Y.; Ujimoto, K.; Ando, I. Polyhedron 2008, 27,
2031–2034.
(9) Johansson, O.; Johannissen, L. O.; Lomoth, R. Chem.;Eur. J. 2009,
15, 1195–1204.
(10) Smith, M. K.; Gibson, J. A.; Young, C. G.; Broomhead, J. A.; Junk,
P. C.; Keene, F. R. Eur. J. Inorg. Chem. 2000, 1365–1370.
(11) Yeh, A.; Scott, N.; Taube, H. Inorg. Chem. 1982, 21, 2542–2545.
(12) Sano, M. Struct. Bonding (Berlin) 2001, 99, 117–139.
(13) Sano, M.; Taube, H. Inorg. Chem. 1994, 33, 705–709.
(14) McClure, B. A.; Mockus, N. V.; Butcher, J., D. P.; Lutterman, D. A.;
Turro, C.; Petersen, J. L.; Rack, J. J. Inorg. Chem. 2009, 48, 8084–8091.
(15) Rack, J. J. Z. Kristallogr. 2008, 223, 356–362.
(16) Rack, J. J. Coord. Chem. Rev. 2009, 253, 78–85.
(17) McClure, B. A.; Rack, J. J. Angew. Chem., Int. Ed. 2009, 48,
8556–8558.
(18) McClure, B. A.; Abrams, E. R.; Rack, J. J. J. Am. Chem. Soc. 2010,
132, 5428–5436.
(19) Bennett, M. A.; Smith, A. K. Dalton Trans. 1974, 233–241.
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r
pubs.acs.org/IC
Published on Web 04/22/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4467
d4-CD3OD) were obtained from Cambridge Isotope Labora-
tories and used as received.
8.89 (d, 1 H), 8.83 (d, 1 H), 8.81 (s, 2 H), 8.75 (d, 1 H), 8.71 (d,
1 H), 8.68 (d, 1 H), 8.51 (d, 1 H), 8.47 (t, 1 H), 8.26 (t, 1 H), 8.12
(d, 1 H), 7.85 (d, 1 H), 7.78 (t, 1 H), 7.73 (t, 1 H), 7.66 (t, 1 H), 7.59
(t, 1 H), 7.49 (t, 1 H), 7.48 (t, 1 H), 7.32 (d, 1 H), 7.20 (t, 1 H), 7.09
(m, 2 H), 6.27 (d, 1 H), 2.04 (s, 3 H). Elemental Analysis:
[Ru(bpy)(biq)Cl2]. This complex was obtained by an altered
synthesis from that described by Heijden et al.21 Orange [(p-cym)-
Ru(bpy)Cl]Cl (502 mg, 1.09 mmol), 2,20-biquinoline (284 mg,
1.10 mmol), and 8 equiv of LiCl (312 mg, 7.35 mmol) were added
to 2 mL of DMF. The reaction was stirred at reflux for 2.5 h
under nitrogen. The solution was allowed to cool to room
temperature and added to 50 mL of acetone. The dark green
product was isolated by vacuum filtration, rinsed with 15 mL
of water, washed with ether (3 ꢀ 15 mL), and air-dried. Yield:
Calculated for [Ru(C10H8N2)(C18H12N2)(C8H7O3S)]PF6 0.5H2O:
3
C, 50.82%; H, 3.32%; O, 6.58%; N, 6.59% S, 3.77%. Found: C,
50.84%; H, 3.25%; O, 6.53%; N, 6.71%; S, 3.96%.
Instrumentation. Cyclic voltammetry was performed on a CH
Instruments CH1730A Electrochemical Analyzer. This work-
station contains a digital simulation package as part of the
software package to operate the workstation (CHI version 2.06).
The working electrode was a glassy-carbon electrode (1.5 mm,
BASi), the counter electrode was a Pt wire, and the reference
electrode was a Ag/AgPF6 electrode. Electrochemical measure-
ments were performed in acetonitrile, dichloromethane, and
propylene carbonate solutions containing 0.1 M TBAPF6 elec-
trolyte in a one compartment cell. Electronic absorption spectra
were collected on an Agilent 8453 spectrophotometer. Bulk
photolysis experiments were conducted using a 100 W xenon-arc
lamp (Oriel) fitted with a Canon standard camera UV filter.
Infrared spectra were obtained on a Nicolet 380 FT-IR spectro-
meter by evaporating 1,2-dichloroethane solutions onto 25 mmꢀ
4 mm KBr plates. Proton nuclear magnetic resonance (1H NMR)
spectra were recorded on either a 300 MHz Bruker AG or a
500 MHz Varian INOVA500 spectrometer in deuterated dmso,
water, and methanol (d6- dmso, d2-D2O, d4-CD3OD).
502 mg (74.6%). UV-vis (MeOH) λmax = 590 nm (ε590
=
6140 M-1 cm-1), 1H NMR (d6-dmso, 300 MHz): δ 9.73 (t, 2 H),
8.71 (q, 2 H), 8.59 (d, 1 H), 8.55 (d, 1 H), 8.44 (d, 1 H), 8.25
(d, 1 H), 8.16 (d, 1 H), 8.07 (t, 1 H), 7.88 (d, 1 H), 7.73 (m, 4 H),
7.51 (d, 1 H), 7.42 (t, 1 H), 7.06 (t, 2 H), 6.95 (d, 1 H). Charac-
terization by 1H NMR agrees with previously published results.
[Ru(bpy)(biq)(OH2)2](OTf)2 0.5CH2Cl2 2H2O. Dark green
3
3
[Ru(bpy)(biq)Cl2] (52.2 mg, 0.0893 mmol) was dissolved in
10 mL of water and purged with argon for 10 min. Two
equivalents of silver triflate (AgOTf) (51.5 mg, 0.200 mmol)
were added, and the reaction mixture was stirred under reflux
and argon for 4 h. The solution turned a deep purple as the
reaction progressed during which time solid AgCl precipitated.
The solution was cooled to 0 °C overnight to ensure complete
precipitation of AgCl. The solution was filtered to collect 2 equiv
of AgCl and rinsed with water until the filtrate was colorless.
The filtrate solution was concentrated to approximately 1 mL
via rotary evaporation to which dichloromethane was added
dropwise to induce precipitation. The solution was cooled in an
ice bath for an hour before isolation by vacuum filtration. The
solid was rinsed with cold dichloromethane (2 ꢀ 5 mL) and
diethyl ether (2 ꢀ 5 mL) and air-dried. The solid was further
Electrochemistry. Rates of electrochemically induced isomeri-
zation were obtained by variation of the scan rate in cyclic
voltammetry from approximately 0.1-3 V s-1. The rate con-
stant k was found by determining the slope of the left side of eq 1
plotted as a function of inverse scan rate (ν-1). In eq 1 Ep is the
S-bonded anodic peak potential, E1/2 is the average between the
S-bonded anodic and cathodic peak potentials, F is Faraday’s
constant, T is temperature and was 300 K, n is the number of
electrons transferred (n=1). Equation 1 was derived from eq 2 for
an irreversible chemical process following a reversible electron
transfer event as described by Nicholson and Shain.22 Equation 2
has been modified so that the second term is positive rather than
negative to account for the isomerization reaction following
oxidation rather than reduction as described by Delahay.23,24
1
dried overnight under vacuum at room temperature. The H
NMR spectrum reveals 0.5 equiv of CH2Cl2. Yield: 58.5 mg
(70.3%). UV-vis: (H2O) λmax = 556 nm (8900 M-1 cm-1),
435 nm (shoulder, 2890 M-1 cm-1). 1H NMR (D2O, 300 MHz):
δ 8.89 (d, 1 H), 8.72 (m, 3 H), 8.54 (m, 2 H), 8.27-8.35 (m, 4 H),
7.77-7.90 (m, 5 H), 7.75 (t, 1 H), 7.40 (t, 1 H), 7.11 (t, 1 H), 6.97
(t, 1 H), 6.39 (d, 1 H), 5.42 (s, 2 H, 0.5 CH2Cl2) ppm. Elemental
Analysis: Calculated for [Ru(C10H8N2)(C18H12N2)(OH2)2]-
(OTf)2 0.5CH2Cl2 2 H2O: C, 39.96%; H, 3.11%; N, 6.02%.
3
3
ꢀ ꢁ
Found: C, 39.69%; H, 2.79%; N, 6.01%.
[Ru(bpy)(biq)(OSO)](PF6) 0.5H2O. Dark green [Ru(bpy)-
kRT
nF
1
1=2 - EpÞ
e1:560e2nF=RTðE
¼
ð1Þ
ν
3
(biq)Cl2] (158 mg, 0.255 mmol) was allowed to react with
2-methylsulfinylbenzoic acid (OSO) (51.7 mg, 0.281 mmol), an
excess of triethylamine (75 μL), and 2 equiv of AgPF6 (145 mg,
0.572 mmol) in 40 mL of ethanol. The reaction was brought to
reflux for 4.5 h under nitrogen. The solution changed from green
to deep red as the reaction progressed during which time solid
AgCl precipitated. The solution was cooled to -30 °C to ensure
complete precipitation of AgCl and was then filtered to collect
2 equiv of AgCl. The solvent was removed from the filtrate by
rotary evaporation. The resulting residue was dissolved in
50 mL of dichloromethane and 2 mL of acetonitrile. The
solution was extracted with 15 mL (2 ꢀ 7.5 mL) of an aqueous
!
rffiffiffiffiffiffiffiffiffiffi
RT
Ep ¼ E1=2 þ
nF
kRT
0:780 - ln
ð2Þ
nFν
Crystallography. Crystals suitable for structural determina-
tion were obtained by slow addition of diethyl ether to a
saturated acetonitrile solution. Single crystal X-ray diffraction
data were collected at 100 K (Bruker KRYO-FLEX) on a
Bruker SMART APEX CCD-based X-ray diffractometer sys-
˚
tem equipped with a Mo-target X-ray tube (λ=0.71073 A). The
detector was placed at a distance of 5.009 cm from the crystal.
Crystals were placed in paratone oil upon removal from the
mother liquor and mounted on a plastic loop in the oil.
Integration and refinement of crystal data were done using
Bruker SAINT software package and Bruker SHELXTL
(version 6.1) software package, respectively.25 Absorption cor-
rection was completed by using the SADABS program.
solution of LiOH H2O (13 mg). The organic layer was dried
3
with anhydrous magnesium sulfate, and the solvent was re-
moved by rotary evaporation. Approximately 2 mL of ethanol
was added to the residue, and 5 mL of diethyl ether was added to
complete precipitation. The solid was isolated via vacuum
filtration, washed with diethyl ether (2ꢀ15 mL), and air-dried.
Yield: 186 mg (84.8%). UV-vis: (MeOH) λmax = 465 nm
(S-bonded, ε465=3490 M-1 cm-1), 572 nm (O-bonded, ε572
=
6410 M-1 cm-1). ν(SdO)=1101 cm-1 (S-bonded), 1006 cm-1
(O-bonded). 1H NMR (d4-CD3OD, 500 MHz): δ 9.02 (d, 1 H),
(22) Nicholson, R. S.; Shain, I. Anal. Chem. 1964, 36, 706–723.
(23) Delahay, P. J. Am. Chem. Soc. 1953, 75, 1190–1196.
(24) Delahay, P. New Instrumental Methods in Electrochemisty; Inter-
science: New York, 1954.
(21) Heijden, M.; van Vliet, P. M.; Haasnoot, J. G.; Reedijk, J. Dalton
Trans. 1993, 3675–3679.
(25) Sheldrick, G. M. SHELXTL, Crystallographic Software Package,
Version 6.10; Bruker-AXS: Madison, WI, 2000.

4468 Inorganic Chemistry, Vol. 49, No. 10, 2010
Grusenmeyer et al.
Table 1. Crystal Data for [Ru(bpy)(biq)(OSO)](PF6) 2CH3CN
3
formula weight
temperature, K
˚
wavelength, A
923.82
100(2)
0.71073
crystal
space group
orthorhombic
P2(1)2(1)2(1)
a = 7.9092(13), R = 90
b = 17.075(3), β = 90
c = 28.341(5), γ = 90
3827.5(11)
˚
unit cell dimensions, A, deg
3
˚
volume, A
Z
4
1.603
0.584
1872
0.35 ꢀ 0.08 ꢀ 0.07
1.39 to 27.50°
-10 e h e 10
-22 e k e 2
-36 e l e 36
31898
density (calculated), Mg/m3
absorption coefficient, mm-1
F(000)
crystal size, mm3
θ range for data collection
index ranges
reflections collected
independent reflections
completeness to θ =27.50°
absorption correction
max. and min transmission
refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2sigma(I)]
R indices (all data)
8678 [R(int) = 0.0547]
99.5%
SADABS
0.9603 and 0.8217
full-matrix least-squares on F2
8678/0/526
Figure 1. Molecular structure of [Ru(bpy)(biq)(OSO)](PF6) 2CH3CN.
3
Thermal ellipsoids have been rendered at 50% probability. Anion,
solvent, and certain hydrogen atoms have been omitted for clarity.
˚
˚
Selected metrical parameters: Ru-S 2.2304(8) A; S-O 1.479(2) A; O-
S-Ru 122.04°.
0.954
R1 = 0.0320, wR2 = 0.0638
R1 = 0.0402, wR2 = 0.0741
0.00(2)
absolute structure parameter
largest diff. peak and hole
Results and Discussion
-3
˚
0.459 and -0.570 e A
The tris-heteroleptic ruthenium complex, [Ru(bpy)(biq)-
(OSO)]þ, where bpy is 2,20-bipyridine, biq is 2,20-biquinoline,
and OSO is 2-methylsulfinylbenzoate, is prepared following
modification to the general procedure originally described
by Freedman and Mann.19,20 This complex is simply pre-
pared by reaction of bipyridine with the ruthenium cymene
dichloride dimer to yield [Ru(bpy)(p-cym)Cl]Cl, where p-cym
is para-cymene. Subsequent reaction of this monomer with
biquinoline yields [Ru(bpy)(biq)Cl2], which is then allowed to
react with 2-methylsulfinylbenzoic acid in the presence of
NEt3 and AgPF6 to yield [Ru(bpy)(biq)(OSO)](PF6). Alter-
nately, the biquinoline ligand may be added to the ruthenium
cymene dimer prior to the addition of bipyridine.26 In our
experience, the OSO ligand must be added last. We surmise
that the monoanionic nature of the OSO ligand makes iso-
lation and handling of the intermediary complexes difficult.
Shown in Figure 1 is the molecular structure of
[Ru(bpy)(biq)(OSO)]þ with crystallographic data presented
in Table 1. The complex crystallizes in space group P212121,
and the complex is clearly chiral. The biquinoline displays a
pronounced saddling, and its bulk leads to a distorted
octahedral geometry about ruthenium. The angles formed
by the chelates are 79.2° (bipyridine, N1-Ru1-N2), 77.6°
(biquinoline, N3-Ru1-N4), and 84.5° (OSO, O1-Ru1-S2).
Curiously, despite the bulk of the biquinoline ligand, the
benzoate ring of OSO is oriented toward the outer ring of
the biquinoline ligand. Indeed, the centroid-to-centroid dis-
and bipyiridine complexes, respectively. There are only a
few crystallographic reports of ruthenium biquinoline struc-
tures.26,28-30 These structures feature substantial buckling or
a “banana-shaped curvature” of the biquinoline chelate.30
For example, the torsion angle defined by N3-C19-C20-
N4 in [Ru(bpy)(biq)(OSO)]þ is -þ4.5° which is similar to
-8.6° observed in [Ru(tpy)(biq)Cl] , where tpy is 2,20:60,200-
terpyridine. The reported structure of [Ru(p-cym)(biq)Cl2] is
of poor quality so that it does not allow possible extraction of
1
this metric for comparison.26 Elemental analyses and H
NMR spectra of the sample are consistent with the crystal
structure.
The visible spectrum of S-[Ru(bpy)(biq)(OSO)]þ in pro-
pylene carbonate features a broad, low energy absorption at
474 nm (ε=3240 ( 10 M-1 cm-1; orange trace; Figure 2),
which is appropriately shifted to the red in comparison to
[Ru(bpy)2(OSO)]þ (λmax = 396 in methanol).14,27 This ab-
sorption is ascribed to a metal-to-ligand charge-transfer
(MLCT) transition based on its intensity and energy. Con-
sistent with other photochromic ruthenium polypyridine
sulfoxide complexes, charge-transfer excitation results in
the formation of a new absorption at 572 nm (ε=5260 (
140 M-1 cm-1; purple trace; Figure 2). For comparison,
[Ru(bpy)(biq)(OH2)2]2þ exhibits λmax at 435 and 556 nm in
aqueous solution. This is significant as the inner coordination
sphere contains two O-bonded ligands on the [Ru(bpy)-
(biq)]2þ fragment. The absorption results are indicative of
intramolecular SfO isomerization.
˚
tance between these two rings is 3.69 A, suggestive of an
intramolecular π-π stacking interaction. Of particular note
˚
Structural evidence in support of isomerization comes
from infrared spectroscopy (Figure 3). The black trace shows
the ground state S-bonded complex which featuresν(SdO) at
is the Ru-S bond distance of 2.2304(8) A and the S-O bond
˚
distance of 1.479(2) A. For comparison, closely related
[Ru(bpy)2(OSO)]þ features Ru-S and S-O bond distances
of 2.213(1) and 1.479(2) A, respectively.27 The Ru-Obenzoate
˚
(28) Gut, D.; Rudi, A.; Kopilov, J.; Goldberg, I.; Kol, M. J. Am. Chem.
Soc. 2002, 124, 5449–5456.
˚
bonds are identical, 2.085(2) and 2.086(2) A, for the biquinoline
(29) Youssef, A. O.; Khalil, M. M. H.; Ramadan, R. M.; Soliman, A. A.
Trans. Met. Chem. 2003, 28, 331–335.
(30) Spek, A. L.; Gerli, A.; Reedijk, J. Acta Crystallogr., Sect. C 1994, 50,
394–397.
(26) Lalrempuia, R.; Kollipara, M. R. Polyhedron 2003, 22, 3155–3160.
(27) Butcher, J., D. P.; Rachford, A. A.; Petersen, J. L.; Rack, J. J. Inorg.
Chem. 2006, 45, 9178–9180.

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4469
Figure 2. UV-visible spectra of S-bonded (orange) and O-bonded
(purple) isomers of [Ru(bpy)(biq)(OSO)]þ in propylene carbonate at
40 °C. Intermediate traces show thermal reversion from O- to S-[Ru-
(bpy)(biq)(OSO)]þ. Inset: kinetic trace (data black squares) at 585 nm
with biexponential fit (red line).
Figure 4. Cyclic voltammograms of [Ru(bpy)(biq)(OSO)]þ in propylene
carbonate (top), acetonitrile (middle), and dichloromethane (bottom).
Working electrode: glassy carbon; Counter electrode: platinum; Refer-
ence electrode: Ag/AgPF6 in CH3CN; scan rate: 0.2 V/s.
1004 cm-1, respectively for the three reports.34-36 These
values are suspiciously close to that reported for [Ru-
(bpy)(biq)(OSO]þ and suggest that the metastable state is
more accurately characterized as an η2- or an asymmetric η2-
sulfoxide ligand. Also present in the IR spectra is a demon-
strable shift in the 1378 cm-1 feature to 1340 cm-1. We
tentatively assign this peak to the carboxylate (RCOO-)
moiety based on its position and intensity. Such a shift may
be expected given the isomerization proposed here. In ag-
gregate, the electronic and infrared spectra permit assign-
ment of the metastable isomer to an O-bonded or η2-bonded
sulfoxide.
Complete reversion of metastable O-[Ru(bpy)(biq)(OSO)]þ
to ground state S-[Ru(bpy)(biq)(OSO)]þ occurs over a period
of hours at room temperature in organic solvents (Figure 2,
inset). For propylene carbonate, a biexponential decay best
fits the absorption versus time data yielding rate constants
Figure 3. Infrared spectra of S- (black) and O-[Ru(bpy)(biq)(OSO)]-
(PF6) (red). Spectra are obtained from 1,2-dichloroethane solutions (non-
irradiated and irradiated) on KBr disks.
1101 cm-1, in accord with an S-bonded sulfoxide.31-33 This
spectrum isobtainedfollowing evaporation of a 1,2-dichloro-
ethane solution containing the ruthenium complex onto a
KBr plate. The red trace is obtained following evaporation of
an irradiated 1,2-dichloroethane solution containing the
ruthenium complex onto a KBr plate. The concentration of
the two solutions is similar. The absorption spectrum of the
irradiated solution is consistent with full conversion to the
O-bonded isomer. The red trace shows a disappearance of
the band at 1101 cm-1 concomitant with the appearance of a
broad feature at 1006 cm-1, consistent with an O-bonded
sulfoxide.31-33 For comparison, [RuCl2(dmso)4] features
ν(SdO) at 1120 cm-1 and 1090 cm-1 for three S-bonded
dmso ligands and ν(SdO) at 915 cm-1 for a single O-bonded
dmso ligand. The typical range for O-bonded sulfoxides is
(kOfS) of 1.13 ((0.05)ꢀ10-2 s-1 and 2.76 ((0.18)ꢀ10-3 s-1
.
In methanol, these rate constants are 1.53 ((0.01)ꢀ10-3 s-1
and 1.79 ((0.07)ꢀ10-4 s-1. Despite the increased viscosity of
propylene carbonate (η=2.50 mPa s) relative to methanol
3
(η=0.59 mPa s), the reversion rates are faster by a factor of
3
10 in propylene carbonate than in methanol. These rates are
significantly faster than those observed for [Ru(bpy)2-
(OSO)]þ, where kOfS are 9.4(1) ꢀ 10-4 s-1 and 6.6(1) ꢀ
10-5 s-1 in propylene carbonate.14,27 In accord with our
previous interpretation of this biexponential behavior, we
ascribe the fast rate constant to a relatively small molecular
rearrangement prior to the isomerization and the slow rate
constant to the isomerization.14
Cyclic voltammograms of S-[Ru(bpy)(biq)(OSO)]þ are
broadly consistent with an electron-transfer triggered isomeri-
zation of the bound sulfoxide. First described by Taube11 and
∼850 to∼1000 cm
.
-1 32 Interestingly, there are threecrystallo-
graphically characterized complexes that display a bridging
S,O bidentate sulfoxide between two ruthenium(II) cations.
For this unusual bonding mode, ν(SdO) is 1017, 1010, and
(34) Geremia, S.; Mestroni, S.; Calligaris, M.; Alessio, E. Dalton Trans.
1998, 2447–2448.
(35) Lessing, S. F.; Lotz, S.; Roos, H. M.; van Rooyen, P. H. Dalton
Trans. 1999, 1499–1502.
(36) Tanase, T.; Aiko, T.; Yamamoto, Y. Chem. Commun. 1996,
2341–2342.
(31) Alessio, E. Chem. Rev. 2004, 104, 4203–4242.
(32) Calligaris, M. Coord. Chem. Rev. 2004, 248, 351–375.
(33) Calligaris, M.; Carugo, O. Coord. Chem. Rev. 1996, 153, 83–154.

4470 Inorganic Chemistry, Vol. 49, No. 10, 2010
Grusenmeyer et al.
Table 2. Rate Constants, Solvent, and Reduction Potentials for [Ru(bpy)2(OSO)]þ and [Ru(bpy)(biq)(OSO]þ
[Ru(bpy)(biq)(OSO]þ
[Ru(bpy)2(OSO)]þ
PC η = 2.50
CH3CN η = 0.344
CH2Cl2 η = 0.393
PC ηs = 66.14
CH3CN εs = 36.64
CH2Cl2 εs = 8.93
kSfO (s-1
kOfS (s-1
)
)
3.2 ((0.4)
1.13 ꢀ 10-2
2.76 ꢀ 10-3
0.91
0.80 ((0.03)
na
0.26 ((0.01)
na
2.01 ((0.4)
9.4 ꢀ 10-4
6.6 ꢀ 10-5
0.84
1.90 ((0.15)
na
1.51 ((0.04)
na
ES°0 (V)
EO°0 (V)
0.94
0.58
1.12
0.74
0.87
0.50
1.06
0.63
0.52
0.40
later elaborated by Sano,12,13 SfO and OfS isomerization
can be triggered by oxidation of RuII to RuIII and reduction
of RuIII to RuII, respectively. The S-bonded RuIII sulfoxide
is thermodynamically unstable and intramolecularly iso-
merizes to generate O-bonded RuIII. Similarly, the thermo-
dynamically unstable O-bonded RuII sulfoxide isomerizes to
yield S-bonded RuII. The driving force forthis reaction is the
greater covalency of the RuII-S bond relative to RuII-O
bond.37 Isomerization rates from these voltammetric scans
are typically obtained either through simulation or plots of
current ratios versus time. The appearance of the voltammo-
gram is related to the scan rate, the isomerization rate(s), and
the switching potential(s).
Shown in Figure 4 are voltammograms of [Ru(bpy)(biq)-
(OSO)]þ collected in propylene carbonate, acetonitrile, and
dichloromethane solutions containing 0.1 M TBAPF6 at a
scan rate of 0.2 V/s. As is evident, the appearance of the
voltammogram at an identical scan rate and scan range in
three different solvents is quite different. In propylene car-
bonate, upon scanning to positive potentials a one-electron
wave is observed, representing oxidation of S-bonded RuII.
Reversing the polarity at the switching potential reveals only
a small current wave corresponding to reduction of S-bonded
RuIII. A more prominent cathodic wave is observed at less
positive potentials, representing reduction of RuIII-O. Re-
versing the polarity again reveals current waves attributed to
both S- and O-bonded Ru3þ/2þ couples. The O-bonded
couple is only formed following oxidation of the S-bonded
couple at more positive potentials. The relatively fast iso-
merization rate results in the small current wave correspond-
ing to reduction of S-bonded RuIII. The voltammogram of
the same compound in acetonitrile features more reversible
behavior, indicating a slower SfO isomerization rate. In
dichloromethane, the couple is nearly reversible pointing to a
much slower isomerization rate constant.
follow the same trend for the two complexes. Solvent
effects on reaction rates are typically ascribed to static
dielectric constant (ε) and viscosity (η) considerations, and
these are greatest for propylene carbonate. While both are
likely operative here, we favor a more prominent role for
viscosity relative to dielectric.38 This is due in part to
recognizing that the dipole moments of the oxidized pro-
ducts, [RuIII(bpy)(biq)(OSO)]2þ and [RuIII(bpy)2(OSO)]2þ
,
are not expected to be greatly different, yet the solvent effect
is more pronounced for the biquinoline complex. Thus, it
appears that the effect reported here is due to a specific
solute-solvent interaction involving [Ru(bpy)(biq)(OSO)]þ.
It is interesting to note that Feringa and co-workers have
reported fast isomerization rates in overcrowded alkenes in
more viscous solvents relative to less viscous solvents.39 They
advance a hypothesis that the molecule displaces a larger
volume in more viscous solvents. Isomerization occurs within
a void, and there is less solvent friction acting upon molecular
motion. While speculative, a similar effect may be operative
in this complex.
In summary, we find that the data presented here are
consistent with sulfoxide isomerization in [Ru(bpy)(biq)-
(OSO)]þ. In particular, the infrared spectroscopic data
support an O-bonded or η2-bonded sulfoxide structure for
the metastable state. Also, we observe that the SfO and
OfS rate constants in [Ru(bpy)(biq)(OSO)]þ vary by
more than an order of magnitude depending upon solvent.
Future studies will focus on obtaining a more detailed
understanding of the role of solvent in these intramolecular
isomerizations.
Acknowledgment. We thank Carl P. Myers from Penn-
sylvania State University and Preston Roeper (OU) for
experimental assistance. We acknowledge NSF (CHE
0809699), Ohio University, the Condensed Matter and
Surface Science institute and NanoBioTechnology Initiative
for funding. T.A.G. recognizes the Provost’s Under-
graduate Research Fund (PURF) for support of this
work. B.A.M. recognizes NDSEG for a graduate fellowship.
Isomerization rate constants (kSfO) were determined from
plots of Ep versus 1/ν, where ν is the scan rate (Table 2). The
rate constants were found to be 3.2 ((0.4) s-1 in propylene
carbonate, 0.80 ((0.03) s-1 in acetonitrile, and 0.26 ((0.01) s-1
in dichloromethane. In comparison to [Ru(bpy)2(OSO)]þ,
the rate constants were found to be 2.01 ((0.4) s-1 in pro-
pylene carbonate, 1.90 ((0.15) s-1 in acetonitrile, and 1.51
((0.04) s-1 in dichloromethane. While the same trend is ob-
served, the magnitude of the effect is greatly diminished. More-
over, the S- and O-bonded formal potentials (ES°0 and EO°0)
Supporting Information Available: Crystallographic Informa-
tion File (.cif). This material is available free of charge via the
Internet at http://pubs.acs.org.
(38) Attempts to expand the study in more solvents with different
dielectric constants, donor properties, and viscosity are not possible because
of solubility problems.
(37) Lutterman, D. A.; Rachford, A. A.; Rack, J. J.; Turro, C. J. Phys.
Chem. A. 2009, 113, 11002–11006.
(39) Klok, M.; Janssen, L. P. B. M.; Browne, W. R.; Feringa, B. L.
Faraday Discuss. 2009, 143, 319–334.