Photoisomerization in an analogous set of ruthenium sulfoxide complexes

Article abstract of DOI:10.1016/j.jphotochem.2010.11.002

Complexes of the type [Ru(bpy)₂(OSOBnR)](PF₆) where bpy is 2,2′-bipyridine and OSOBnR is a 4-substituted benzylsulfinylbenzoate with R = NO₂, F, Cl, H, CH₃, CF₃ and OCH₃, have been prepared

Full text of DOI:10.1016/j.jphotochem.2010.11.002

Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 341–346
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Photoisomerization in an analogous set of ruthenium sulfoxide complexes
Brianne L. Portera, Beth Anne McClurea, Eric R. Abramsa, James T. Engleb,
Christopher J. Zieglerb, Jeffrey J. Racka,∗
a Department of Chemistry and Biochemistry, Nanoscale and Quantum Phenomena Institute, Ohio University, Athens, OH 45701, USA
b Department of Chemistry, Knight Chemical Laboratory, University of Akron, Akron, OH 44325, USA
a r t i c l e i n f o
a b s t r a c t
Complexes of the type [Ru(bpy)2(OSOBnR)](PF6) where bpy is 2,2ꢀ-bipyridine and OSOBnR is a 4-
substituted benzylsulfinylbenzoate with R = NO2, F, Cl, H, CH3, CF3 and OCH3, have been prepared and
investigated by 1H NMR spectroscopy, cyclic voltammetry and UV–vis spectroscopy. Despite the distance
of the R group from ruthenium, the Ru3+/2+ reduction potential and charge transfer absorption maximum
vary predictably with the electron withdrawing nature of the group.
Article history:
Received 23 June 2010
Received in revised form 2 November 2010
Accepted 7 November 2010
Available online 13 November 2010
© 2010 Elsevier B.V. All rights reserved.
Keywords:
Photochromic
Ruthenium
Polypyridine
Isomerization
1. Introduction
2. Experimental
Electron transfer induced conformational changes are central to
the operation of molecular machines and other types of molecular
bistability [1–3]. A number of studies have revealed that reduc-
tion or oxidation of interlocked rotaxanes results in translocation
of a molecular unit from one site to another [4–6]. However,
certain transition metal complexes exhibit bistability through iso-
merization of bound ambidentate ligands [7–12]. For example,
pentaammine ruthenium complexes of dimethylsulfoxide (DMSO)
undergo intramolecular linkage isomerization (S vs. O) following
oxidation and reduction of ruthenium [13–16]. We have devel-
oped a class of simultaneously photochromic and electrochromic
polypyridine ruthenium sulfoxide complexes based on S → O and
O → S isomerization [17–26]. In our study of these complexes, we
have found that changes to a substituent R group affects the S-
bonded Ru3+/2+ reduction potential, the S-bonded charge-transfer
absorption maximum and the S → O isomerization quantum yield.
Interestingly, the corresponding O-bonded properties show no cor-
relation with the identity of the R group.
2.1. Materials
The reagents RuCl3·xH2O and silver hexafluorophosphate
(AgPF6) were purchased from Strem. The reagents 2,2ꢀ-bipyridine
(bpy), thiosalicylic acid, m-chloroperoxybenzoic acid, triethy-
lamine, 4-trifluoromethylbenzyl bromide, 4-fluorobenzyl bromide,
benzyl bromide, 4-methylbenzyl bromide, 4-chlorobenzyl bro-
mide, 4-methoxybenzyl bromide, and 4-nitrobenzyl bromide were
purchased from Aldrich and used as received. The compounds
cis-[Ru(bpy)2Cl2]·2H2O and [Ru(bpy)2(OSO-Bn)](PF6) were syn-
thesized according to published methods [27,28]. The solvents
acetone, methanol, ethanol, diethyl ether, and dichloromethane
were purchased from VWR and used without further purification.
Tetrabutylammonium hexafluorophosphate (TBAPF6) was pur-
chased from Aldrich and recrystallized three times from ethanol.
Acetonitrile for electrochemical experiments was HPLC grade and
purchased from Burdick and Jackson and used without further
purification. The synthesis and isolation of all ruthenium sulfoxide
complexes were carried out in the dark or under red light condi-
tions.
2.2. Instrumentation
∗
Corresponding author. Te.: +1 740-593-9702.
Electronic absorption spectra were collected on an Agilent 8453
spectrophotometer. Bulk photolysis experiments were conducted
using a 75 W Xenon-arc lamp (Oriel) fitted with a Canon stan-
dard camera UV filter. Proton nuclear magnetic resonance (1H
E-mail addresses: bp293206@ohio.edu (B.L. Porter), bm692806@ohio.edu
(B.A. McClure), ea266105@ohio.edu (E.R. Abrams), jte3@uakron.edu (J.T. Engle),
ziegler@uakron.edu (C.J. Ziegler), rackj@ohio.edu, rack@helios.phy.ohiou.edu
(J.J. Rack).
1010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.11.002

342
B.L. Porter et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 341–346
NMR) spectra were collected in deuterated acetone (d6-acetone)
on either a 300 MHz Bruker spectrometer or a 500 MHz Varian
INOVA500 spectrometer. Cyclic voltammetry was performed on a
CH Instruments CHI 730A Electrochemical Analyzer. This worksta-
tion contains a digital simulation package as part of the software
package to operate the workstation (CHI version 2.06). The work-
ing electrode was a Pt disk (Cypress Systems) electrode (1 mm).
The counter and reference electrodes were Pt wire and Ag/Ag+,
respectively. Electrochemical measurements were performed in
acetonitrile solutions containing 0.1 M TBAPF6 electrolyte in a one
compartment cell.
chloroperoxybenzoic acid (453 mg 60% peroxo reagent by 1H NMR,
1.58 mmol) was dissolved. The solution of m-chloroperoxybenzoic
acid was added dropwise to the solution of OS-BnCH3 over a
period of 5 min. The solution was stirred at 25 ◦C for approximately
15 min. The acetone was removed by rotary evaporation. A mini-
mum amount of diethyl ether was added to the residue and a white
precipitate was isolated via vacuum filtration and air dried. Yield:
333 mg (79%). 1H NMR (d6-acetone, 300 MHz) ı: 8.19 (d, 1 H), 7.91
(d, 1 H), 7.74 (t, 1 H), 7.63 (t, 1 H), 7.10 (m, 4 H), 4.43 (d, 1 H), 3.75
(d, 1 H), 2.30 (s, 3 H).
[Ru(bpy)2(OSOBnCH3)](PF6)·1.8H2O (3). This procedure and
purification are done in the dark or under red light. The dark
purple complex [Ru(bpy)2Cl2]·2H2O (212 mg, 0.408 mmol) was dis-
solved in 50 mL of ethanol with OSO-BnCH3 (124 mg, 0.455 mmol),
2 equivalents of silver hexafluorophosphate (AgPF6) (219 mg,
0.852 mmol) and excess triethylamine (150 ␮L). The solution was
brought to reflux under nitrogen gas for approximately 4 h. The
solution was cooled to −30 ◦C overnight to allow maximum pre-
cipitation of the AgCl. The AgCl was isolated by vacuum filtration
and rinsed with ethanol and dichloromethane until filtrate was col-
orless. The solvent was removed by rotary evaporation. In order to
remove the byproduct NEt3HPF6, the oily residue was dissolved
in dichloromethane and extracted with an aqueous solution of
LiOH·H2O (∼25 mg in 10 mL). The dichloromethane layer was dried
with anhydrous magnesium sulfate and the solvent was removed
by rotary evaporation. A minimal amount of methanol was added
to the resulting residue. Approximately 5 mL of diethyl ether was
added to the concentrated solution to precipitate the product. The
solution was cooled to −30 ◦C for 15 min for complete precipita-
tion. The product was isolated by vacuum filtration and air dried.
Yield: 277 mg (82%). UV–vis (MeOH) ꢀmax(ε) = 399 nm (7400) S-
bonded, 349 nm (9400) and 496 nm (9400) O-bonded. E◦ꢀ Ru3+/2+ vs.
Ag/Ag+ = 0.89 V S-bonded, 0.52 V O-bonded. 1H NMR (d6-acetone,
500 MHz) ı: 9.50 (d, 1 H), 9.05 (d, 1 H), 8.90 (d, 1 H), 8.69 (m, 3 H),
8.52 (t, 1 H), 8.21 (t, 1 H), 8.10 (m, 3 H), 8.05 (t, 1 H), 7.90 (d, 1 H),
7.60 (m, 2 H), 7.45 (m, 5 H), 6.90 (d, 2 H), 6.60 (d, 2 H) 4.25 (d, 1
H), 3.94 (d, 1 H), 2.22 (s, 3 H). Elemental Analysis: Calculated for
[Ru(C10H8N2)2(C15H13O3S)]PF6·1.8H2O: Calculated: C: 48.64%, H:
3.81%, N: 6.48%. Found: C: 48.27%, H: 3.41%, N: 6.76%.
2.3. Quantum yield measurements
Quantum yield of isomerization measurements were deter-
mined by irradiating solutions of the complexes [Ru(bpy)2(OSO-
BnR)]+ in methanol at room temperature. Photolysis was achieved
using a PTI C-60 fluorimeter at the lower energy isosbestic point
between S-bonded and O-bonded isomers. Incident radiation
intensity, Io, was determined using potassium ferrioxalate actinom-
etry. Quantum yields of isomerization were calculated according to
Eq. (1) [29] in which d[O]/dt is the slope of the O-bonded concen-
tration, [O] as a function of time for less than 10% converted to
O-bonded, Aꢀ is the absorbance at the isosbestic point (ꢀ) and V
is the volume of irradiated solution (3 mL). The concentration of
O-bonded isomer was determined using multi-linear regression:
(d[O]/dt)
(Io/V)(1 − 10−Aꢀ
˚
s→o
=
(1)
)
2.4. Crystallography
Crystals suitable for structural determination were obtained
by slow evaporation of a concentrated methanol 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
˚
system equipped with a Mo-target X-ray tube (ꢀ = 0.71073 A). The
detector was placed at a distance of 5.009 cm from the crystal. Crys-
tals 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 soft-
ware package and Bruker SHELXTL (version 6.1) software package,
respectively [30]. Absorption correction was completed by using
the SADABS program.
2.6. Synthesis of [Ru(bpy2)(OSO-BnCF3)](PF6)·1.3H2O
2-(4-Trifluoromethyl-benzylsulfanyl)-benzoic acid, OSBnCF3 (4).
4 was prepared following the procedure as described above
for complex 1 using 300 mg thiosalicylic acid and 520 mg of
4-trifluoromethylbenzyl bromide. Yield: 220 mg (54%). 1H NMR
(d6-acetone, 300 MHz) ı: 8.02 (d, 1 H), 7.70 (dd, 4 H), 7.51 (m, 2
H), 7.25 (t, 1 H), 4.35 (s, 2 H).
2-(4-Trifluoromethyl-benzylsulfinyl)-benzoic acid, OSOBnCF3 (5).
5 was prepared following the procedure as described above for
complex 2 starting with 184 mg of 4. Yield: 147 mg (76%). 1H NMR
(d6-acetone, 300 MHz) ı: 8.18 (d, 1 H), 7.80 (d, 1 H), 7.59–7.74 (m,
4 H), 7.37 (d, 2 H), 4.55 (d, 1 H), 4.02 (d, 1 H).
[Ru(bpy2)(OSOBnCF3)](PF6)·1.3H2O (6). 6 was prepared follow-
ing the procedure as described above for complex 3 starting with
76 mg [Ru(bpy)2Cl2]·2H2O and 64 mg of 5. Yield: 102 mg (77%).
UV–vis (MeOH) ꢀmax(ε) = 392 nm (7100) S-bonded, 350 nm (9200)
and 496 nm (9300) O-bonded. E◦ꢀ Ru3+/2+ vs. Ag/Ag+ = 0.93 V S-
bonded, 0.53 V O-bonded. 1H NMR (d6-acetone, 500 MHz) ı: 9.54
(d, 1 H), 8.99 (d, 1 H), 8.87 (d, 1 H), 8.68 (m, 3 H), 8.53 (t, 1 H),
8.26 (t, 1 H), 8.16 (m, 3 H), 8.10 (t, 1 H), 7.93 (d, 1 H), 7.58 (m,
2 H), 7.52 (t, 2 H), 7.46 (t, 1 H), 7.38 (m, 3 H), 7.29 (d, 1 H), 6.88
(d, 2 H), 4.46 (d, 1 H), 4.18 (d, 1 H). Elemental Analysis: Calcu-
lated for [Ru(C10H8N2)2(C15H10O3F3S)]PF6·1.3H2O: Calculated: C:
46.23%, H: 3.18%, O: 7.57%, N: 6.16%, S: 3.53%. Found: C: 45.92%, H:
3.13%, O: 7.59%, N: 6.44%, S: 3.30%.
2.5. Synthesis of ruthenium complexes
2-(4-Methyl-benzylsulfanyl)-benzoic acid, OSBnCH3 (1). Thecom-
pound thiosalicylic acid (598 mg, 3.88 mmol) was dissolved in
75 mL of methanol. Sodium hydroxide (330 mg, 8.25 mmol) was
added in excess to the solution. The solution was stirred at
25 ◦C for 30 min. The solvent was removed by rotary evaporation.
The remaining white precipitate was dissolved in 75 mL acetone.
The compound 4-methylbenzyl bromide (800 mg, 4.32 mmol) was
added to the solution. The solution was refluxed for approximately
4 h. The white precipitate was isolated by vacuum filtration and
rinsed with diethyl ether. The precipitate was air dried for 1 h.
The precipitate was dissolved in 35 mL of methanol. Concentrated
HCl was added dropwise (approximately 20 mL) until the product
precipitated as a white solid. The solid was isolated by vacuum fil-
tration and rinsed with water and air dried. Yield: 903 mg (91%). 1
H
NMR (d6-acetone, 300 MHz) ı: 8.00 (d, 1 H), 7.49 (m, 2 H), 7.35 d, 2
H), 7.22 (t, 1 H), 7.15 (d, 2 H), 4.18 (s, 2 H), 2.29 (s, 3 H).
2-(4-Methyl-benzylsulfinyl)-benzoic acid, OSOBnCH3 (2). The
ligand OS-BnCH3 (397 mg, 1.55 mmol) was dissolved in 60 mL
of acetone. In another 20 mL of acetone 1 equivalent of m-

B.L. Porter et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 341–346
343
2.7. Synthesis of [Ru(bpy)2(OSO-BnCl)](PF6)·0.75H2O
[Ru(bpy)2(OSOBnOCH3)](PF6)·0.75H2O (15). 15 was prepared
following the procedure as described above for complex 3 using
97 mg [Ru(bpy)2Cl2]·2H2O and 62 mg of 14. Yield: 152 mg (94%).
UV–vis (MeOH) ꢀmax(ε) = 402 nm (7100) S-bonded, 350 nm (8600)
and 495 nm (8600) O-bonded. E◦ꢀ Ru3+/2+ vs. Ag/Ag+ = 0.89 V S-
bonded, 0.53 V O-bonded. 1H NMR (d6-acetone, 500 MHz) ı: 9.52
(d, 1 H), 9.02 (d, 1 H), 8.89 (d, 1 H), 8.68 (t, 2 H), 8.62 (d, 1 H), 8.53 (t,
1 H), 8.24 (t, 1 H), 8.15 (m, 3 H), 8.08 (t, 1 H), 7.97 (d, 1 H), 7.63 (m,
2 H), 7.50 (m, 3 H), 7.40 (t, 1 H), 7.31 (d, 1 H), 6.56 (m, 4 H), 4.21 (d,
1 H), 3.90 (d, 1 H), 3.71 (s, 3 H). Elemental Analysis: Calculated for
[Ru(C10H8N2)2(C15H13O4S)]PF6·0.75H2O: Calculated: C: 48.80%, H:
3.58%, O: 8.82%, N: 6.51%. Found: C: 48.58%, H: 3.43%, O: 8.54%, N:
6.46%.
2-(4-Chloro-benzylsulfanyl)-benzoic acid, OSBnCl (7). 7 was pre-
pared following the procedure as described above for complex 1
using 400 mg thiosalicylic acid and 586 mg of 4-chlorobenzyl bro-
mide. Yield: 509 mg (70%). 1H NMR (d6-acetone, 300 MHz) ı: 8.00
(d, 1 H), 7.49 (m, 4 H), 7.37 (d, 2 H), 7.24 (m, 1 H), 4.23 (s, 2 H).
2-(4-Chloro-benzylsulfinyl)-benzoic acid, OSOBnCl (8). 8 was pre-
pared following the procedure as described above for complex 2
starting with complex 345 mg of 7. Yield: 313 mg (85%). 1H NMR
(d6-acetone, 300 MHz) ı: 8.19 (d, 1 H), 7.80 (d, 1 H), 7.68 (t, 1 H),
7.59 (t, 1 H), 7.29 (d, 2 H), 7.17 (d, 2 H), 4.45 (d, 1 H), 3.90 (d, 1 H).
[Ru(bpy)2(OSOBnCl)](PF6)·0.75H2O (9). 9 was prepared follow-
ing the procedure as described above for complex 3 using 186 mg
[Ru(bpy)2Cl2]·2H2O and 119 mg of 8. Yield: 249 mg (81%). UV–vis
(MeOH) ꢀmax(ε) = 396 nm (7100) S-bonded, 350 nm (9100) and
495 nm (9100) O-bonded. E◦ꢀ Ru3+/2+ vs. Ag/Ag+ = 0.92 V S-bonded,
0.53 V O-bonded. 1H NMR (d6-acetone, 500 MHz) ı: 9.52 (d, 1 H),
9.05 (d, 1 H), 8.90 (d, 1 H), 8.72 (t, 2 H), 8.65 (d, 1 H), 8.58 (t, 1 H),
8.27 (t, 1 H), 8.18 (m, 3 H), 8.15 (t, 1 H), 7.94 (d, 1 H), 7.67 (m, 2 H),
7.59 (t, 1 H), 7.50 (m, 2 H), 7.41 (t, 1 H), 7.32 (d, 1 H), 7.13 (d, 2 H),
6.70 (d, 2 H), 4.32 (d, 1 H), 4.03 (d, 1 H). Elemental Analysis: Cal-
culated for [Ru(C10H8N2)2(C14H10O3SCl)]PF6·0.75H2O: Calculated:
C: 47.17%, H: 3.21%, O: 6.93%, N: 6.47%. Found: C: 46.85%, H: 3.44%,
O: 6.60%, N: 6.54%.
2.10. Synthesis of [Ru(bpy)2(OSO-BnNO2)](PF6)·0.75H2O
2-(4-Nitro-benzylsulfanyl)-benzoic acid, OSBnNO2 (16). 16 was
prepared following the procedure as described above for complex
1 using 200 mg thiosalicylic acid and 280 mg of 4-nitrobenzyl bro-
mide. Yield: 298 mg (78%). 1H NMR (d6-acetone, 300 MHz) ı: 8.21
(d, 2 H), 8.01 (d, 1 H), 7.77 (d, 2 H), 7.49 (m, 2 H), 7.25 (m, 1 H), 4.40
(s, 2 H).
2-(4-Nitro-benzylsulfinyl)-benzoic acid, OSOBnNO2 (17). 17 was
prepared following the procedure as described above for complex
2 starting with 148 mg of 16. Yield: 144 mg (92%). 1H NMR (d6-
acetone, 300 MHz) ı: 8.20 (d, 1 H), 8.12 (d, 2 H), 7.60-7.73 (m, 3 H),
7.38 (d, 2 H), 4.59 (d, 1 H), 4.14 (d, 1 H).
2.8. Synthesis of [Ru(bpy)2(OSO-BnF)](PF6)·0.5H2O
[Ru(bpy)2(OSOBnNO2)](PF6)·0.75H2O (18). 18 was prepared fol-
lowing the procedure as described above for complex 3 starting
with 101 mg [Ru(bpy)2Cl2]·2H2O and 65 mg of 17. Yield: 90 mg
(53%). UV–vis (MeOH) ꢀmax(ε) = 391 nm (7400) S-bonded, 345 nm
(9500) and 496 nm (8900) O-bonded. E◦ꢀ Ru3+/2+ vs. Ag/Ag+ = 0.94 V
S-bonded, 0.53 V O-bonded. 1H NMR (d6-acetone, 500 MHz) ı: 9.52
(d, 1 H), 9.00 (d, 1 H), 8.92 (d, 1 H), 8.73 (t, 2 H), 8.68 (d, 1 H), 8.56
(t, 1 H), 8.28 (t, 1 H), 8.15 (m, 4 H), 8.00 (m, 3 H), 7.68 (m, 2 H),
7.60 (t, 1 H), 7.53 (t, 1 H), 7.48 (m, 2 H), 7.34 (d, 1 H), 6.97 (d, 2
H), 4.54 (d, 1 H), 4.25 (d, 1 H). Elemental Analysis: Calculated for
[Ru(C10H8N2)2(C14H10O5SN)]PF6·0.75H2O: Calculated: C: 46.60%,
H: 3.17%, O: 10.50%, N: 7.99%. Found: C: 46.33%, H: 3.03%, O: 10.23%,
N: 8.06%.
2-(4-Fluoro-benzylsulfanyl)-benzoic acid, OSBnF (10). 10 was pre-
pared following the procedure as described above for complex
1 starting with 151 mg of thiosalicylic acid and 121 ␮L of 4-
fluorobenzyl bromide. Yield: 149 mg (58%). 1H NMR (d6-acetone,
300 MHz) ı: 8.01 (d, 1 H), 7.50 (dd, 4 H), 7.24 (m, 1 H), 7.10 (t, 2 H),
4.23 (s, 2 H).
2-(4-Fluoro-benzylsulfinyl)-benzoic acid, OSOBnF (11). 11 was
prepared following the procedure as described above for complex
2 starting with 125 mg of complex 10. Yield: 70 mg (53%). 1H NMR
(d6-acetone, 300 MHz) ı: 8.19 (d, 1 H), 7.81 (d, 1 H), 7.73 (t, 1 H),
7.63 (t, 1 H), 7.19 (m, 2 H), 7.02 (m, 2 H), 4.43 (d, 1 H), 3.89 (d, 1 H).
[Ru(bpy)2(OSOBnF)](PF6)·0.5H2O (12). 12 was prepared follow-
ing the procedure as described above for complex 3 using 102 mg
[Ru(bpy)2Cl2]·2H2O and 60 mg of 11. Yield: 104 mg (76%). UV–vis
(MeOH) ꢀmax(ε) = 398 nm (6500) S-bonded, 350 nm (8300) and
496 nm (8400) O-bonded. E◦ꢀ Ru3+/2+ vs. Ag/Ag+ = 0.90 V S-bonded,
0.54 V O-bonded. 1H NMR (d6-acetone, 500 MHz) ı: 9.53 (d, 1 H),
9.00 (d, 1 H), 8.90 (d, 1 H), 8.69 (d, 2 H), 8.60 (d, 1 H), 8.55 (t, 1 H),
8.25 (t, 1 H), 8.15 (m, 3 H), 8.11 (t, 1 H), 7.90 (d, 1 H), 7.63 (m, 2
H), 7.50 (m, 3 H), 7.39 (t, 1 H), 7.25, (d, 1 H), 6.87 (t, 2 H), 6.70 (t,
2 H), 4.30 (d, 1 H), 4.11 (d, 1 H). Elemental Analysis: Calculated for
[Ru(C10H8N2)2(C14H10O3SF)]PF6·0.5H2O: Calculated: C: 48.34%, H:
3.23%, O: 6.63%, N: 6.63%, S: 3.80%. Found: C: 48.42%, H: 3.09%, O:
6.31%, N: 6.57%, S: 3.73%.
3. Results and discussion
Shown in Table 1 are selected data obtained from the study of
these compounds and the Hammett parameters for each of the
R groups specified in Scheme 1. We have chosen to employ the
substituent parameters determined from the ionization of benzoic
acid [31]. The 1H NMR spectrum reveals resonances ascribed to
the bipyridine and phenyl aromatic ring protons as well as the
diastereotopic methylene protons, explicitly depicted in Scheme 1.
This latter resonance is responsive to the identity of the R group.
Electron-withdrawing groups (NO2, CF3, Cl and F) shift this reso-
nance downfield relative to electron-donating groups (OCH3, CH3
2.9. Synthesis of [Ru(bpy)2(OSO-BnOCH3)](PF6)·0.75H2O
Table 1
2-(4-Methoxy-benzylsulfanyl)-benzoic acid, OSBnOCH3 (13). 13
was prepared following the procedure as described above for
complex 1 using 435 mg thiosalicylic acid and 490 ␮L of 4-
methoxybenzyl bromide. Yield: 620 mg (80%). 1H NMR (d6-acetone,
300 MHz) ı: 8.00 (d, 1 H), 7.50 (m, 2 H), 7.38 (d, 2 H), 7.22 (t, 1 H),
6.89 (d, 2 H), 4.17 (s, 2 H), 3.78 (s, 3 H).
2-(4-Methoxy-benzylsulfinyl)-benzoic acid, OSOBnOCH3 (14). 14
was prepared following the procedure as described above for com-
plex 2 starting with 124 mg of 13. Yield: 114 mg (87%). 1H NMR
(d6-acetone, 300 MHz) ı: 8.18 (d, 1 H), 7.89 (d, 1 H), 7.74 (t, 1 H),
7.63 (t, 1 H), 7.14 (d, 2 H), 6.83 (d, 2 H), 4.40 (d, 1 H), 3.76 (d, 4 H).
R group, Hammett parameter (ꢁp), chemical shift (ı; ppm), absorption maxima
(ꢀ; nm), Reduction Potentials (E◦ꢀ; V), and isomerization quantum yield (˚S→O) for
[Ru(bpy)2(OSOBnR)]+.
ꢀ
ꢀ
R
ꢁp
0.78
0.54
0.23
0.06
0
ı
ꢀSmax
ꢀOmax
ES◦
EO◦
˚
S→O
NO2
CF3
Cl
F
H
4.40
4.33
4.18
4.16
4.13
4.10
4.06
391
392
396
398
396
399
402
496
496
495
496
496
496
495
0.94
0.93
0.92
0.90
0.90
0.89
0.89
0.53
0.53
0.53
0.54
0.53
0.52
0.53
0.34 0.05
0.13 0.01
0.16 0.01
0.17 0.01
0.22 0.02
0.13 0.01
0.15 0.01
CH3
OCH3
−0.17
−0.27

344
B.L. Porter et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 341–346
Table 2
Crystal data and structure refinement for [Ru(bpy)2(OSOBnNO2)](PF6).
R = NO2
N
N
Identification code
Ru-NO2
N
S
N
CF3
Cl
F
H
CH3
RuII
RuIII
N
O-
N
O-
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
C35H30F6N5O6PRuS
894.74
100(2) K
0.71073 A
Triclinic
P-1
a = 10.229(4) A ˛ = 61.608(5)
b = 13.785(5) A ˇ = 86.977(6)
c = 14.072(5) A ꢂ = 88.524(6)
1743.1(11) A
O
O
-eŠ
N
N
O
S
H2C
R
O
˚
H2C
OCH3
◦
◦
◦
˚
˚
˚
3
Unit cell dimensions
R
˚
Volume
Z
2
Scheme 1. Compounds employed and reaction investigated in this study.
Density (calculated)
Absorption coefficient
F(0 0 0)
1.705 Mg/m3
0.643 mm−1
904
0.17 mm × 0.11 mm × 0.09 mm
and H). The 1H NMR and elemental analyses of each of the com-
pounds are consistent with the respective chemical formulae.
Crystal size
Theta range for data collection 1.65–27.00−
Shown in Fig.
1
is the molecular structure of
Index ranges
−13 < = h < = 12, −17 < = k < = 17, −17 < = l < = 17
[Ru(bpy)2(OSOBnNO2)]+ with selected crystallographic data
presented in Table 2. The Ru–N bond distances range from 2.047(3)
Reflections collected
Independent reflections
14473
7458 [R(int) = 0.0412]
Completeness to theta = 27.00− 98.1%
˚
to 2.087(3) A and are typical of ruthenium bipyridine distances
Absorption correction
Max. and min. transmission
Refinement method
SADABS
˚
[32]. The Ru–S and S–O bond distances are 2.208(1) and 1.478(2) A,
0.9444 and 0.8985
Full-matrix least-squares on F2
7458/0/552
respectively. The S–O bond distance is in accord with a great
number of ruthenium sulfoxide complexes and is significantly
shorter than that observed for uncoordinated dimethylsulfoxide
Data/restraints/parameters
Goodness-of-fit on F2
0.941
Final R indices [I > 2sigma(I)]
R indices (all data)
Largest diff. peak and hole
R1 = 0.0442, wR2 = 0.0941
R1 = 0.0611, wR2 = 0.0966
0.813 and −1.469e−3
˚
(1.513(5) A) [33–36]. Similarly, the Ru–S bond distance is consis-
tent with a number of ruthenium sulfoxide structures [20,33,34]. In
addition, the structure shows an apparent ␲-stacking interaction
between the benzyl ring and the neighboring bipyridine ring.
Indeed, the centroid-to-centroid distance for these two rings is
merization of the bound sulfoxide. Visible irradiation of the ground
state S-isomer yields the corresponding ground state O-isomer.
Table 1 shows that the MLCT absorption maxima for the O-bonded
isomers (ꢀOmax) are largely invariant with changes in R. Thus, we
conclude that the R group has little effect on the ruthenium d␲
HOMO when the sulfoxide is O-bonded. This conclusion necessi-
tates a significant change in orbital overlap between Ru–Ssulfoxide
and Ru–Osulfoxide groups.
Cyclic voltammograms of these complexes are consistent with
electron-transfer triggered S → O isomerization of the sulfoxide
(see Supplementary Data) [13,14,16]. The reaction is depicted
as one-step in Scheme 1. Shown in Fig. 2 is a representative
voltammogram for [Ru(bpy)2(OSOBnF)]+. The first scan features a
one-electron oxidation near +0.95 V vs. Ag/Ag+. The cathodic scan
reveals a quasi-reversible wave assigned to the Ru3+/2+ S-bonded
˚
∼3.43 A.
The visible absorption spectrum of each of these complexes
is dominated by a moderately intense (ε ∼ 103 M−1 cm−1) Ru
d␲ → bpy ␲* Metal-to-Ligand Charge-Transfer (MLCT) transition.
Sufficient data now exists to suggest that the promoting orbital (Ru
d␲) has significant sulfur character [37–39]. Table 1 shows that the
S-bonded absorption maxima (ꢀSmax) are responsive to the iden-
tity of R. This change in the MLCT absorption maximum affirms our
assertion that the promoting orbital contains some sulfur character
[39]. It is not surprising that there are deviations from this trend
as absorption maxima represent the difference in energy between
excited- and ground-state surfaces.
In accord with similar compounds, [17] these complexes feature
photochromic behavior associated with phototriggered S → O iso-
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
Potential (V)
Fig. 1. Molecular structure of [Ru(bpy)2(OSOBnNO2)]+. Atoms are thermal ellipsoids
Fig. 2. Cyclic voltammogram of [Ru(bpy)2(OSOBnF)]+. Scan rate 0.1 V/s,
rendered at 50% probability. Hydrogen atoms have been omitted for clarity.
WE:platinum, CE:platinum, RE:Ag/AgPF6).

B.L. Porter et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 341–346
345
as a function of R group. It is evident that the greatest and small-
est values are found for the NO2 and OCH3 substituted complexes,
respectively. However, the complexes between these two extremes
do not reveal a monotonic or linear correlation. Thus, while the
R group affects metal-based properties (absorption maxima and
reduction potential), it does not appear to effect isomerization
quantum yields in accord with a linear free energy relationship.
While speculative, a possible explanation for the lack of a lin-
ear free energy relationship is that the isomerization mechanism
has changed significantly within these complexes. Such a change
may involve stabilization of the transition state to form an inter-
mediate or involve a dramatic shifting of the transition state along
the reaction coordinate. The obvious transition state for this reac-
tion is an Á2-bound sulfoxide. At present, we are not aware of any
mononuclear Á2-bound sulfoxide ligands, but there are three bin-
uclear structures featuring an unusual, Ru–S–O–Ru bridging mode
from dimethylsulfoxide [44–46] as well as a metastable Á2-bound
SO2-ruthenium complex formed from irradiation of the S-bonded
complex [47,48].
0.94
0.92
0.90
0.88
NO2
CF3
Cl
H
F
OCH3
CH3
25500
25200
24900
4.4
4.3
4.2
4.1
-0.4 -0.2
0.0
0.2
0.4
0.6
0.8
4. Conclusions
Hammett Parameter,
p
ꢀ
We have prepared a family of ruthenium sulfoxide complexes
that differ only in the presence of tunable R group on the benzyl
Fig. 3. Plot of ES◦ (top, ꢀ), S-bonded absorption energy (EmS ax, ꢁ, middle), and chem-
ical shift (ı, ꢂ, bottom) of diastereotopic methylene protons vs. the Hammett
parameter ꢁp. The identity of the R group is denoted in the top plot.
ligand. The data show that the chemical shift of the methylene
ꢀ
linkage (ıCH2), the S-bonded Ru3+/2+ reduction potential (ES◦ ), and
the S-bonded MLCT absorption maximum (ꢀSmax) all change pre-
dictably with the identity of R. We do not observe a predictable
effect between the identity of R and ˚S→O. Future work will involve
the design of a new family of complexes in which the tunable R
group may be placed directly on or closer to the sulfoxide. In addi-
tion, excited state rates of isomerization in these complexes will
be measured directly and compared to determine if a linear free
energy relationship exists for these compounds.
ꢀ
couple (ES◦ ). Scanning to less positive potentials reveals another
couple assigned to the Ru3+/2+ O-bonded isomer (EO◦ ). Just as oxi-
dation triggers S → O isomerization, reduction of Ru3+ to yield Ru2+
prompts O → S isomerization. The Ru3+/2+ O-bonded couple is only
observed following oxidation of the more positive couple. The
appearance of the voltammogram is dependent upon the isomer-
ization rates, the switching potentials and the scan rate.
ꢀ
ꢀ
ꢀ
Shown in Table
1
are ES◦ and EO◦ values for isomers of
ꢀ
[Ru(bpy)2(OSO-BnR)]+. The data show that EO◦ values are invari-
Acknowledgements
ꢀ
ant with changes in the R group. In contrast, ES◦ decreases by
nearly 50 mV as the R group becomes more electron-donating.
We are grateful to Melanie Heying and Shadrick Paris for
helpful discussions. JJR thanks NSF (CHE 0809669), Ohio Uni-
versity, Condensed Matter and Surface Science (CMSS) and the
NanoBioTechnology Initiative (NBTI) for funding of this work. BLP
thanks OU for a Provost’s Undergraduate Research Fund (PURF)
award and the Manasseh-Cutler Scholar Program. BAM recognizes
NDSEG for a fellowship.
Such shifts are not uncommon in electrochemical studies of ruthe-
nium complexes [40]. For example, in studies of [(H3N)5Ru(L)]2+
,
where L is a 4-substituted pyridine, the Ru3+/2+ reduction potential
varies predictably based on the ␲-accepting ability of pyri-
dine (py). For [(H3N)5Ru(py)]2+, this couple is +0.30 V vs. NHE,
which shifts to +0.322 V, +0.375 V, and +0.394 V for p-Cl-pyridine,
p-CONH2-pyridine (isonicotinamide) and p-CF3-pyridine, respec-
tively [41–43]. However, these shifts are due to alterations of ligand
acceptor orbitals involved in bonding to ruthenium. The ␲* orbitals
Appendix A. Supplementary data
on pyridine stabilize the Ru d␲ orbital set altering E◦ꢀ accordingly.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jphotochem.2010.11.002.
ꢀ
In the present study, shifts in ES◦ are due to modifications of the
benzyl group bound to the sulfoxide. There is no apparent direct
orbital interaction between the distant R-group and ruthenium.
The voltammetric results are in accord with both the absorption
andchemical shiftdataand furtherillustratethat the Rgroup affects
the basicity of the sulfoxide group when sulfur is bound to ruthe-
nium, but not when oxygen is bound to ruthenium. The R group
influences the chemical shift of the methylene protons, the absorp-
tion maxima of the lowest energy visible transition and the Ru3+/2+
reduction potential. Indeed, plots of the methylene chemical shift
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