Deprotonation/protonation of coordinated secondary thioamide units of pincer ruthenium complexes: Modulation of voltammetric and spectroscopic characterization of the pincer complexes

Article abstract of DOI:10.1039/c0dt01283a

New pincer ruthenium complexes, [Ru(SCS)(tpy)]PF₆ (1) (SCS = 2,6-bis(benzylaminothiocarbonyl)phenyl), tpy = 2,2′:6′,2′'- terpyridyl) and [Ru(SNS)(tpy)]PF₆ (2) (SNS = 2,5- bis(benzylaminothiocarbonyl)pyrrolyl), having κ³SCS and κ³SNS pincer ligands with two secondary thioamide units were synthesized by the reactions of [RuCl₃(tpy)] with N,N'-dibenzyl-1,3-benzenedicarbothioamide (L1) and N,N'-dibenzyl-2,5-1H- pyrroledicarbothioamide (L2), respectively, and their chemical and electrochemical properties were elucidated. The structure of 1 was determined by X-ray crystallography. The complexes 1 and 2 showed a two-step deprotonation reaction by treatment with 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU), and the addition of DBU led to a shift of the metal-centered redox couples to a lower potential by 720 and 550 mV, respectively. The di-deprotonated complexes were also studied by ¹H-NMR and UV-vis spectroscopy. The addition of methanesulfonic acid (MSA) to the di-deprotonated complexes enabled the recovery of 1 and 2, indicating that the thioamide moiety underwent a reversible deprotonation-protonation process, which resulted in regulating the redox potentials of the metal center. The Pourbaix diagram of 1 revealed that 1 underwent a one-proton/one-electron transfer process in the pH range of 5.83-10.35, and a two-proton/one-electron process at a pH of over 10.35, indicating that the deprotonation/protonation process of the complexes is related to proton-coupled electron transfer (PCET). The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c0dt01283a

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Pincers and other hemilabile ligands
Guest Editors Dr Bert Klein Gebbink and Gerard van Koten
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PERSPECTIVE:
Cleavage of unreactive bonds with pincer metal complexes
Martin Albrecht and Monika M. Lindner
Dalton Trans., 2011, DOI: 10.1039/C1DT10339C
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Pincer Ru and Os complexes as efficient catalysts for racemization and deuteration of alcohols
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Dalton Trans., 2011, DOI: 10.1039/C1DT10498E
Mechanistic analysis of trans C–N reductive elimination from a square-planar macrocyclic aryl-
copper(III) complex
Lauren M. Huffman and Shannon S. Stahl
Dalton Trans., 2011, DOI: 10.1039/C1DT10463B
CSC-pincer versus pseudo-pincer complexes of palladium(II): a comparative study on
complexation and catalytic activities of NHC complexes
Dan Yuan, Haoyun Tang, Linfei Xiao and Han Vinh Huynh
Dalton Trans., 2011, DOI: 10.1039/C1DT10269A
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PAPER
Deprotonation/protonation of coordinated secondary thioamide units of
pincer ruthenium complexes: Modulation of voltammetric and spectroscopic
characterization of the pincer complexes†
Takuya Teratani,a Take-aki Koizumi,*a Takakazu Yamamoto,a Koji Tanakab and Takaki Kanbara*c,d
Received 25th September 2010, Accepted 2nd March 2011
DOI: 10.1039/c0dt01283a
New pincer ruthenium complexes, [Ru(SCS)(tpy)]PF6 (1) (SCS =
2,6-bis(benzylaminothiocarbonyl)phenyl), tpy = 2,2¢:6¢,2¢’-terpyridyl) and [Ru(SNS)(tpy)]PF6 (2)
3
3
(SNS = 2,5-bis(benzylaminothiocarbonyl)pyrrolyl), having k SCS and k SNS pincer ligands with two
secondary thioamide units were synthesized by the reactions of [RuCl3(tpy)] with
N,N’-dibenzyl-1,3-benzenedicarbothioamide (L1) and N,N’-dibenzyl-2,5-1H-pyrroledicarbothioamide
(L2), respectively, and their chemical and electrochemical properties were elucidated. The structure of 1
was determined by X-ray crystallography. The complexes 1 and 2 showed a two-step deprotonation
reaction by treatment with 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU), and the addition of DBU led to a
shift of the metal-centered redox couples to a lower potential by 720 and 550 mV, respectively. The
di-deprotonated complexes were also studied by 1H-NMR and UV-vis spectroscopy. The addition of
methanesulfonic acid (MSA) to the di-deprotonated complexes enabled the recovery of 1 and 2,
indicating that the thioamide moiety underwent a reversible deprotonation-protonation process, which
resulted in regulating the redox potentials of the metal center. The Pourbaix diagram of 1 revealed that
1 underwent a one-proton/one-electron transfer process in the pH range of 5.83–10.35, and a
two-proton/one-electron process at a pH of over 10.35, indicating that the deprotonation/protonation
process of the complexes is related to proton-coupled electron transfer (PCET).
catalytic activity.6,7 As an extension of this research, we here
Introduction
report results of the modulation of the electronic properties of the
metal center of the following Ru(II) complexes by deprotonation-
protonation reactions of the –NH– groups in the secondary
thioamide ligand.
The study of secondary thioamides has been the subject of
recent interest,1 because the N–H proton of secondary thioamides
exhibits strong hydrogen donor ability,2 whereas the sulfur atom of
thioamide is dominant as a Lewis base donor for soft transition-
metal coordination.3 The characteristics of thioamide are reflected
by the results of a number of recent applied studies of anion
reception4 and transition-metal-ion coordination chemistry.5–7
Bowman–James’s group and our group previously reported that
thioamide-based pincer complexes are photoluminescent and have
aChemical Resources Laboratory, Tokyo Institute of Technology, 4259
Nagatsuta, Midori-ku, Yokohama, Kanagawa, 226-8503, Japan
The secondary thioamide group is in equilibrium with its amino-
thione and imino-thiol tautomers as shown in Scheme 1,3d and
exhibits stronger acidity than the corresponding amide group.2
Consequently, when secondary thioamides are used as ligands
in metal complexes, they are easily deprotonated to give their
thionate anionic form, which enhances the donor capability of the
sulfur atom via the N-to-S backbone.3d,e,7e
This situation prompted us to utilize the secondary thioamide
group as not only a coordination site but also a reactive site
on the ligand of the pincer complex. Actually, the modula-
tion of the photochemical properties of the pincer platinum
bInstitute for Molecular Science, Department of Life and Coordination-
Complex Molecular Science, 5-1 Higashiyama, Myodaiji, Okazaki, Aichi,
444-8787, Japan
cTsukuba Research Center for Interdisciplinary Materials Science (TIMS),
University of Tsukuba, 1-1-1 Tennodai, Tsukuba, 305-8573, Japan
dGraduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1
Tennodai, Tsukuba, 305-8573, Japan
† Electronic supplementary information (ESI) available: The ORTEP
drawing of 2, CV curves of 5 and 5 with DBU (0.5–5.0 mol equiv), and
1H-NMR spectra of 1, 1 with 2 mol equiv of NEt3 and that of DBU,
Fig. S1–S3 and Table S1. CCDC reference numbers 789412 & 789413. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c0dt01283a
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©

Scheme 1 Equilibria and structures of a secondary thioamide group in neutral and basic solutions.
and palladium complexes could be achieved upon exposure
monocationic and that the Ru centers of the complexes were
divalent.
Fig. 1 shows the ORTEP drawing of the cationic part of 1, and
selected bond lengths and angles of 1 are summarized in Table
1. As shown in Fig. 1, 1 has a distorted octahedral geometry
similar to that of [Ru(tpy)2]2+,11 and the Ru1–C1 bond length
of 1 is in the range of those previously reported for pincer Ru
complexes.9b,12 Sums of the bond angles around the N1 and N2
atoms are 359.7 and 359.8◦, respectively, which indicate essentially
planar structures.
to chemical stimuli.7e,g In this paper, we report new pincer
ruthenium complexes, 1 and 2, with two coordinated secondary
thioamide units. The electronic properties of the complexes were
expected to be modulated by the acid–base environment of the
media because many elegant reports on the acid–base properties
of multi-nitrogen ligated ruthenium complexes8 as well as on
the voltammetric characterizations of various pincer ruthenium
complexes have been published.9 The spectroscopic character-
ization and molecular structures of the complexes are also
presented.
Results and discussion
Preparation and characterization of Ru-pincer complexes
The Ru complexes 1 and 2 were prepared by the reaction of
AgPF6-treated [RuCl3(tpy)] with L1 and L2, respectively, in 2-
methoxyethanol under N2 as shown in Scheme 2. Alcohols
often serve as a reducing reagent for Ru compounds,10 and
2-methoxyethanol used as the solvent also seems to work as
the reducing reagent. The complexes obtained are stable in
air, and they were characterized by NMR spectroscopy and
ESI-MS spectroscopy. In the 1H-NMR spectra of 1 and 2,
the N–H proton signals were observed at d 9.22 and 8.87,
respectively; the complexation caused a downfield shift of the
signal by 0.14 and 0.01 ppm from the corresponding free ligands,
respectively.
Fig. 1 Molecular structure of 1 with thermal ellipsoids drawn at the 50%
probability level. Hydrogen atoms except H(1) and H(2), a PF6 anion,
and a solvated diethyl ether molecule are omitted for simplicity.
-
The ESI-MS spectra of 1 and 2 showed parent peaks at m/z =
710 and 699, respectively, indicating that these complexes were
Scheme 2 Synthetic routes to complexes 1 and 2.
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◦
˚
Table 1 Selected bond lengths (A) and angles ( ) of 1
Table 3 Electrochemical data of Ru complexes in the presence of base
1/2/Va
Ru1–S1
Ru1–C1
Ru1–N4
C7–S1
2.3707(19)
2.027(8)
2.056(6)
1.730(8)
1.346(8)
82.6(2)
Ru1–S2
Ru1–N3
Ru1–N5
2.376(2)
2.100(7)
2.112(8)
1.736(8)
1.357(10)
82.3(2)
172.4(2)
163.96(6)
91.43(16)
93.28(18)
89.06(18)
156.2(2)
E
Complex
Ru(III)/Ru(II)
Ligand0/-
C8–S2
1
0.028
-2.365
C7–N1
C3–N2
b
c
1+NEt3 (3)
-0.226
-0.692
0.088
-0.122
-0.460
-0.055
—
C1–Ru1–S1
C1–Ru1–N3
C1–Ru1–N5
S1–Ru1–N3
S1–Ru1–N5
S2–Ru1–N4
N3–Ru1–N4
N4–Ru1–N5
C1–Ru1–S2
C1–Ru1–N4
S1–Ru1–S2
S1–Ru1–N4
S2–Ru1–N3
S2–Ru1–N5
N3–Ru1–N5
1+DBUb (4)
—
c
97.4(3)
2
-2.326d
106.4(3)
93.80(17)
90.26(17)
104.10(16)
78.3(2)
b
c
2+NEt3
—
—
2+DBUb
c
5
-1.992d,-2.333d
a Measured in an acetonitrile solution of [(n-Bu)4N][PF6] (0.1 M). Poten-
tials in V vs. Fc+/Fc. b Addition of 2.00 mol equiv of base. c Not measured.
d Irreversible reduction peak potential(s).
78.2(2)
Table 2 Electrochemical data of Ru complexes
1/2/Va
The influence of deprotonation of the coordinated pincer ligand
on the redox potential of the Ru center was examined by CV with
the addition of a controlled amount of the base (Table 3). Fig. 2
shows the CV curves of 1, 1 with 2.00 equiv of NEt3 (pKa = 18.82 in
CH3CN), and 1 with 2.00 equiv of DBU (pKa = 24.34 in CH3CN)14
in the region of +0.1 to -1.2 V (vs. Fc+/Fc). When NEt3 is added to
1, the observed currents based on the Ru(III)/Ru(II) redox couple
of 1 at E1/2 = +0.028 V is gradually decreased and a new redox
couple appears at E1/2 = -0.226 V. Upon addition of 2.00 equiv of
NEt3, the redox couple at E1/2 = +0.028 V disappears completely
(Fig. 2(b)). The newly appearing redox couple is considered to be
associated with the Ru(III)/Ru(II) response of 3, which is a mono-
deprotonated form of 1 as shown in Scheme 4. In contrast, the
addition of DBU led to the consecutive deprotonation of 1. When
DBU was added to 1 (> 2 mol equiv), the original Ru(III)/Ru(II)
redox couple decreased in amount and two new redox couples at
E1/2 = -0.226 V and -0.692 V (vs. Fc+/Fc) appeared. After the
E
Complex
Ru(III)/Ru(II)
Ligand0/-
1
0.028
0.088
-0.178
0.167
-2.365
-2.326b
-2.031
-1.946
2
[Ru(NCN)(tpy)]+c
[Ru(PCP)(tpy)]+c
a Measured in an acetonitrile solution of [(n-Bu)4N][PF6] (0.1 M). Poten-
tials in V vs. Fc+/Fc. b Irreversible reduction peak potential. c Ref. 9b
Electrochemical properties
Electrochemical data of 1 and 2 are summarized in Table 2. The
cyclic voltammogram (CV) of 1 exhibited three reversible redox
couples at E1/2 = +0.825, +0.028 and -2.365 V (vs. Fc+/Fc) in
acetonitrile under N2. These are assigned to the metal-centered
Ru(IV)/Ru(III) and Ru(III)/Ru(II) couples and a ligand-localized
couple, respectively.
addition of 2.00 mol equiv of DBU, the redox couple at E1/2
=
Owing to the s-donor character of the pincer ligand,9d,13 the
metal-centered oxidation of 1 (Ru(III)/Ru(II); E1/2 = +0.028 V)
occurs at a lower oxidation potential than those of conventional
Ru complexes such as [Ru(bpy)3]2+ (E1/2 = +0.88 V vs. Fc+/Fc)
and [Ru(tpy)2]2+ (E1/2 = +0.92 V vs. Fc+/Fc).7,12 The Ru(III)/Ru(II)
response of 2 was observed at E1/2 = +0.088 V, which is 0.06 V
higher than that of 1. This result indicates that the electron-
donating ability of the pincer ligand of 1 is higher than that of
2.14 Moreover, from the comparison of other ruthenium complexes
shown in Table 2, the redox potential of Ru(III)/Ru(II) of 1 is higher
than that of [Ru(NCN)(tpy)]+ (NCN = [C6H3(CH2NMe2)2-2,6]-)9b
(cf. Scheme 3) and lower than that of [Ru(PCP)(tpy)]+ (PCP =
[C6H3(CH2PPh2)2-2,6]-),9b suggesting that the donation/back-
donation ability of the thiocarbonyl group in the thioamide moiety
is between those of amine and phosphine.
-0.226 V disappeared completely and only one redox couple at
E1/2 = -0.692 V was observed. The latter redox couple is assigned
to the Ru(III)/Ru(II) redox couple of the di-deprotonated complex
4 as shown in Scheme 4. When 2.00 equiv of methanesulfonic
Fig. 2 Changes in the cyclic voltammogram of 1 (1 mM) caused by
addition of base in CH3CN containing [(n-Bu)4N][PF6] (0.1 M) under N2
at sweep rate of 100 mV s-1: (a) 1, (b) 1 with 2 mol equiv of NEt3, and (c)
1 with 2 mol equiv of DBU.
Scheme 3 Structures of (a) [Ru(NCN)(tpy)]+ and (b) [Ru(PCP)(tpy)]+.
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Scheme 4 Deprotonation-protonation reaction of 1.
Table 4 Selected calculated singlet excited-state transitions for 1
Wavelength Oscillator Assignment (% of major transition
acid (MSA) was added to 4, the redox couple observed at E1/2
-0.692 V (vs. Fc+/Fc) disappeared and the redox couple at E1/2
=
=
-0.028 V (vs. Fc+/Fc) reappeared. This result indicates that the
di-deprotonated complex 4 is converted smoothly to 1 by the
addition of the protic acid, and the clean conversion between the
protonated and deprotonated forms regulates the redox potential
of the Ru(III)/Ru(II) couple by 720 mV, which is much larger than
the shift in the redox potential reported for the Ru complexes
bearing multi-nitrogen ligands.8 The metal-centered redox couples
of 1, 3, and 4 were essentially unchanged in repeated scans under
N2, and no electrochemical response occurring at the ligand or the
added base was observed in the scan range.
Complex (nm)
strength
contributing to the band)
1
474
401
0.0967
0.0759
HOMO - 2 → LUMO (55%)
HOMO - 1 → LUMO + 3 (71%)
of MSA to the DBU-treated solution caused the signals of 1 to
recover completely.
The ESI-MS spectrum of DBU-treated 1 showed a parent peak
at m/z = 708 in the negative-ionization mode, and no peaks for a
dinuclear complex or larger clusters were found in the mass spectra
under basic conditions.
A similar reversible deprotonation-protonation was observed
in 2, and the difference in redox potential between 2 and di-
deprotonated 2 was 550 mV. As a control experiment, a pincer
Ru complex bearing two tertiary thioamide groups, 5 (shown in
Scheme 5), was prepared and its redox potentials under conditions
of added base were observed.
In the UV-vis absorption spectrum of 1 in CH3CN, broadened
absorption bands at lmax = 420 nm and lmax = 495 nm with a
shoulder peak at 550 nm were observed. Assignments of the p–
p* transition and the MLCT absorption bands were made by
time-dependent density functional theory (TD-DFT) calculations
of the complex.15 Table 4 shows the oscillator strengths and
corresponding assignments of the primary electronic transitions.
Three-dimensional plots of the HOMOs and LUMOs of 1
and their molecular orbitals (MOs) are shown in Fig. 3 using
GaussView 4.1.16 From the TD-DFT calculation, 1 has two strong
MLCT transitions originating from HOMO - 2 to LUMO at 474
nm and from HOMO - 1 to LUMO + 3 at 401 nm. For 1, the
LUMO consisting of p* orbitals is from the thioamide ligand,
while LUMO + 3 is from the tpy ligand. The HOMO - 1 and
HOMO - 2 are pure d orbitals of the central metal. Two transitions
at lmax = 474 nm and 401 nm are assigned to the metal-to-ligand
charge transfer (MLCT) contributed by the thioamide ligand
and tpy unit, respectively. These assignments are in agreement
with other previously reported Ru(II) complexes containing tpy
ligands.8,9
The UV-vis spectrum of 1 is changed by the addition of DBU,
as shown in Fig. 4. The increase in the amount of DBU caused
a decrease in the area of the original peak of 1, and the area
of a new absorption band peak at lmax = 565 nm increased with
an isosbestic point at 532 nm. The absorbance of the new peak
at lmax = 565 nm saturates at a DBU/1 ratio of 2.00, indicating
that the deprotonation of 1 proceeds consecutively to give the di-
deprotonated complex 4. The addition of MSA to the solution of
4 led to an immediate recovery of 1. When excess NEt3 was used as
a base instead of DBU, the 1H NMR, ESI-MS, and UV-vis spectra
of 1 were almost unchanged, even though the CV of 1 changed
under the same conditions. These inconsistent results suggest that
Scheme 5 Molecular structure of 5.
As a result, 5 was electrochemically stable under basic condi-
tions (see Fig. S1†), indicating that the incorporation of secondary
thioamide groups in 1 and 2 is necessary to control the redox
potentials of the complex by acid/base treatment.
Spectroscopic study of 1 with added base
The reversible deprotonation-protonation behavior of 1 was also
1
monitored by H NMR, ESI-MS, and UV-vis spectroscopy. The
1H NMR spectra of 1, 1 with 2.00 mol equiv of NEt3, and 1 with
2.00 mol equiv of DBU are shown in Fig. S2.† When excess DBU
was added to 1, the N–H proton signal of 1 at d 9.22 completely
disappeared, and significant upfield shifts of the aromatic signals
were observed. The newly generated compound is considered to be
the di-deprotonated complex 4. The addition of an excess amount
8882 | Dalton Trans., 2011, 40, 8879–8886
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Fig. 3 Calculated (a) HOMO - 1, (b) HOMO - 2, (c) LUMO, and (d) LUMO + 3 orbitals of 1.
deprotonation-protonation of the N–H group of coordinated
secondary amides can control the redox potential of the ruthenium
center markedly by approximately 500 mV. The redox potentials of
1 in pH-controlled solutions (solvent: MeCN-Robinson-Britton
buffer (1 : 1 v/v)) were monitored by CV. Fig. 5 shows the pH-
dependent CV curves of 1 in the pH range of 2.18–12.15. With
an increase in pH, the E1/2 potential of the Ru(III)/Ru(II) is
shifted successively to negative potentials. This result indicates
that the deprotonation reaction of 1 takes place via PCET. The
Fig. 4 Changes in the absorption spectrum of 1 (2.5 ¥ 10-5 M) caused
by addition of DBU in CH3CN under N2. The inset shows the range of
400–700 nm.
the mono-deprotonation of 1 by the addition of NEt3 proceeded
by the electro-oxidation of the central Ru atom.
Study of proton-coupled electron transfer (PCET) process in 1
In an electrochemical reaction mechanism, the simultaneous
transfer of electrons and protons is called PCET.8a From the
experiments using NEt3 the deprotonation/protonation of N–
H protons is considered to take place via PCET. Many com-
plexes containing ionizable protons in ligands were reported to
undergo PCET. Kojima et al. revealed the PCET reaction of
the Ru complex containing two amide groups;8e the reversible
Fig.
5
pH-dependent redox couple of Ru(III)/Ru(II) for 1 in a
CH3CN/Britton–Robinson buffer (1 : 1 v/v) solution.
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Dalton Trans., 2011, 40, 8879–8886 | 8883
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resultant Pourbaix diagram8a–c,e is shown in Fig. 6. Plots in
Fig. 6 illustrate two different PCET processes taking place. The
gradient of the linear relationship between pH and E1/2 for the
Ru(III)/Ru(II) redox couple is determined to be - 56.4 V/pH in
the pH range of 5.83–10.35, which relates well to the expected
value for a one-electron/one-proton process,8a,8g as shown in
Scheme 6(a).
Conclusions
We have synthesized new SCS- and SNS-pincer Ru complexes
containing secondary thioamide units in the pincer ligand and
characterized their chemical and electrochemical properties. The
electrochemical behavior of the complexes suggests that the
electron donating ability of the benzene-centered pincer ligand
is higher than that of the pyrrole-centered one, which is similar
to the behavior of SCS- and SNS-pincer Ni complexes. Depro-
tonation/protonation reaction of N–H protons in 1 and 2 using
acid/base can regulate the redox potentials of the complexes in
the range of 550–720 mV. The redox regulations of 1 and 2 with
acid/base suggest that the catalytic activity of reactions using a
redox process could be adjusted.17 The PCET process of 1 was
examined by pH-dependent cyclic voltammetry. In the MeCN-
buffer solution, one-electron/one-proton transfer is exhibited in
the pH range of 5.83–10.35, and one-electron/two-proton transfer
is observed at pH > 10.35.
Experimental Section
General methods
1H-NMR spectra were recorded on a JNM EX-300 or an
EX-270 spectrometer. Chemical shifts (d, ppm) were reported
with reference to TMS. UV-vis spectra were measured with
a Shimadzu UV-2550 UV-visible spectrophotometer. Elemental
analyses were carried out with a Yanaco MT-5 CHN autorecorder.
Electrochemical measurements were performed with a BAS ALS
1200A automatic polarization system. A conventional three-
electrode configuration was used, with glassy carbon working
electrode (BAS electrode) and a platinum wire auxiliary electrode
(The Nilaco Corp., special order) and 0.01 M AgNO3/Ag as
reference (BAS RE-5). Cyclic voltammograms were recorded at
a scan rate of 100 mV s-1: Fc+/Fc = +115 mV vs. 0.10 M
AgNO3/Ag, and +425 mV vs. SCE. The Pourbaix diagram
was obtained by measurements of E1/2 values through pH
titration by saturated NaOH aqueous solutions in a CH3CN/
Fig.
6
Pourbaix
diagram
of
the
titration
of
1
in
CH3CN/Britton–Robinson buffer (1 : 1 v/v). Each region represents the
following species: (A) [Ru(II)LH2]+; (B) [Ru(III)LH2]2+; (C) [Ru(III)LH]+;
(D) [Ru(III)L]0.
The slope increases to -117.3 mV/pH at a pH of over 10.35,
and the slope is close to that expected for a one-electron/two-
proton transfer, as shown in Scheme 6(b). Under these con-
ditions, we could not observe the [Ru(II)LH]0–[Ru(III)L]0 or
[Ru(II)L]-–[Ru(III)L]0 process in the pH range of 2.18–12.15. These
results suggest that the reversible deprotonation/protonation
process of secondary thioamides in the pincer ligand can con-
trol the redox potential of the Ru center in the range of ca.
500 mV.
Scheme 6 Proton-coupled electron transfer reaction of 1 in two different pH ranges.
8884 | Dalton Trans., 2011, 40, 8879–8886 This journal is The Royal Society of Chemistry 2011
©

Britton–Robinson buffer (1 : 1 v/v) mixture at room tempera-
ture. The apparent pHs of this mixture are referred to as pH.
ESI-MS spectra were obtained with a Waters-Micro-massLCT.
[RuCl3(tpy)],18 1,3-bis(benzylaminothiocarbonyl)benzene (L1),7d
2,5-bis(benzylaminothiocarbonyl)pyrrole (L2),7d and 1,3-bis-
(dimethylaminothiocarbonyl)benzene7b were prepared according
to the literature methods.
into aqueous NH4PF6. The resulting violet solid was collected by
filtration, dried in vacuo and recrystallized from ether/acetone to
give 5 (91.8 mg, 37%) as black solid (dH (270 MHz, acetonitrile-d3)
8.45 (2 H, d, J 8.1), 8.26 (2 H, d, J 8.1), 8.04 (1 H, t, J 8.1), 7.92 (2
H, d, J 7.9 Hz), 7.69 (2 H, td, J 7.8 and 1.5), 7.09 (3 H, m), 6.96
(2 H, dt, J 4.8 and 0.7), 3.57 (12 H, s).
[Ru(SCS-Bn2)(tpy)]PF6 (1)
Crystal structure determination
To a CH3OCH2CH2OH solution (20 mL) of [RuCl3(tpy)] (400 mg,
0.90 mmol) was added AgPF6 (460 mg, 1.40 mmol) and the
resulting mixture was stirred at 75 ◦C for 2 h. The resulting
off-white solid was eliminated by celite filtration. L1 (344 mg,
0.92 mmol) was added to the violet filtrate and the reaction mixture
was stirred at 75 ◦C for 12 h. The solution was concentrated to
ca. 1 mL and poured into aqueous NH4PF6. The resulting violet
solid was collected by filtration, dried in vacuo, and recrystallized
from ether/acetone to give 1 (349 mg, 44%) as deep purple needles
(Found: C, 51.56; H, 3.80; N, 7.87. Calc. for C37H30F6N5PRuS2: C,
51.99; H, 3.54; N, 8.19%); dH (300 MHz; acetonitrile-d3) 9.22 (2
H, s, NH), 8.45 (2 H, d, J 8.1), 8.23 (2 H, d, J 6.75), 8.05 (1 H, t,
J 7.6), 7.70 (2 H, td, J 7.8 and 1.6), 7.31 (1 H, t, J 7.8), 7.22–7.19
(10 H), 7.05 (2 H, t, J 6.48), 6.96 (2 H, d, J 4.8), 4.86 (4 H, d, J
5.9); m/z (ESI) 710 (M+. C37H30N5RuS2 requires 710.10).
Crystal of 1 for X-ray analysis was obtained as described
in the preparations. The suitable crystal was mounted on a
glass fiber. Data collection for 1 was performed at -160 ◦C
on a Rigaku/MSC Saturn CCD diffractometer with graphite
˚
monochromated Mo-Ka radiation (l = 0.7107 A). The data
were collected to a maximum 2q of 55◦. A total of 720
oscillation images were collected. A sweep of the data was
performed using w scans from -110◦ to 70◦ in 0.5◦ steps at
c = 45.0◦ and f = 0.0◦. The structures were solved using the
CrystalStructure software package.19 Atom scattering factors
were obtained from the literature. Refinements were performed
anisotropically for all non-hydrogen atoms by the full-matrix least-
square method. Hydrogen atoms except H1 and H2 were placed
at the calculated positions and were included in the structure
calculation without further refinement of the parameters. H1
and H2 of 1 were determined by difference Fourier mapping
and refined isotropically. The residual electron densities were
of no chemical significance. The crystal data and processing
parameters are summarized in Table 5. Crystallographic data for
the structural analysis of 1 in CIF format have been deposited
with the Cambridge Crystallographic Data Centre under CCDC
No. 789412 (1).† These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC,
12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033;
e-mail: deposit@ccdc.cam.ac.uk).
[Ru(SNS-Bn2)(tpy)]PF6 (2)
Method A. To a CH3OCH2CH2OH solution (20 mL) of
[RuCl3(tpy)] (50 mg, 0.114 mmol) was added L2 (42 mg,
0.114 mmol) and triethylamine (2 mL), and the resulting mixture
was stirred at 75 ◦C for 12 h. The solution was concentrated to ca.
1 mL, and poured into aqueous NH4PF6. The resulting deep red
solid was collected by filtration and dried in vacuo (56 mg, 40%).
Method B. To a CH3OCH2CH2OH solution (20 mL) of
[RuCl3(tpy)] (100 mg, 0.23 mmol) was added AgPF6 (115 mg,
0.35 mmol), and the resulting mixture was stirred at 75 ◦C for 2 h.
The resulting off-white solid was eliminated by celite filtration, and
L2 (83 mg, 0.23 mmol) was added to the violet filtrate. The mixture
was stirred at 75 ◦C for 12 h. The solution was concentrated to ca.
1 mL and poured into aqueous NH4PF6. The resulting deep red
solid was collected by filtration, dried in vacuo, and recrystallized
from ether/acetone to give 2 (153 mg, 79%) as deep red needles
(Found: C, 49.44; H, 3.46; N, 9.61. Calc. for C35H29F6N6PRuS2:
C, 49.82; H, 3.46; N, 9.96%.); dH (270 MHz, acetonitrile-d3) 8.87
(2 H, s, NH), 8.39 (2 H, d, J 8.1), 8.30 (2 H, d, J 8.1), 7.95 (1 H,
t, J 8.0), 7.84 (1 H, td, J 7.7 and 1.5), 7.62 (2 H, d, J 5.3), 7.41
(2 H, t, J 5.9), 7.20 (12 H, m), 4.73 (4 H, s); m/z (ESI) 699 (M+.
C35H29N6RuS2 requires 699.09).
Table 5 Crystal data and details of the structure refinements for 1·Et2O
Formula
C41H40ON5S2RuPF6
Molecular Weight
Crystal System
Space Group
928.95
Triclinic
¯
P1(No. 2)
˚
a (A)
11.75(2)
13.87(3)
14.74(3)
77.43(2)
86.74(3)
64.02(4)
2106.4(68)
2
˚
b (A)
˚
c (A)
a (◦)
b (◦)
g (◦)
3
˚
V (A )
Z
m/cm-1
5.745
948.00
120.1
1.465
18850
8913
554
F(000)
Ru(SCS-Me4)(tpy)]PF6 (5)
T/K
Dc/g cm-3
No. total reflns
No. unique reflns
No. variables
To a CH3OCH2CH2OH solution (20 mL) of [RuCl3(tpy)] (150 mg,
0.34 mmol) was added AgPF6 (172 mg, 0.68 mmol), and the
resulting mixture was stirred at 75 ◦C for 2 h. The resulting
off-white solid was eliminated by celite filtration, and 1,3-
bis(dimethylaminothiocarbonyl)benzene was added (88.2 mg,
0.35 mmol) to the violet filtrate. The mixture was stirred at 75 ◦C
for 12 h. The solution was concentrated to ca. 1 mL and poured
a
R1
0.0726
0.1048c
b
Rw
a R1 = RꢀFo| - |Fcꢀ/R|Fo| for I > 2.0s(I) data. b Rw = R[w(Fo
-
2
2
Fc2)2/Rw(Fo2)2]1/2
.
c Weighting scheme 1/[0.0059Fo + 1.0000s(Fo2)].
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 8879–8886 | 8885
©

Computational Details
Yamamoto, Chem. Lett., 2006, 35, 558; (e) K. Okamoto, T. Yamamoto,
M. Akita, A. Wada and T. Kanbara, Organometallics, 2009, 28, 3307;
(f) T. Koizumi, T. Teratani, K. Okamoto, T. Yamamoto, Y. Shimoi
and T. Kanbara, Inorg. Chim. Acta, 2010, 363, 2474; (g) Y. Ogawa,
A. Taketoshi, J. Kuwabara, K. Okamoto, T. Fukuda and T. Kanbara,
Chem. Lett., 2010, 39, 385.
All the DFT calculations reported in this study were carried out
using the Gaussian 03 suite of programs.18 The geometries of 1
were optimized at the B3LYP/LANL2DZ level.
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1991, 30, 3843; (d) H. A. Nieuwenhuis, J. G. Haasnoot, R. Hage, J.
Reedijk, T. L. Snoeck, D. J. Stufkens and J. G. Vos, Inorg. Chem., 1991,
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Acknowledgements
The authors are grateful to Dr K. Okamoto and Dr J. Kuwabara
of our laboratory and Dr Y. Shimoi of the National Institute of
Advanced Industrial Science and Technology (AIST) for their
useful discussions and experimental support. The authors are
grateful to the Chemical Analysis Center of University of Tsukuba
for performing the X-ray crystallography, NMR spectroscopy, and
elemental analysis.
9 (a) P. Steenwinkel, D. M. Grove, N. Veldman, A. L. Spek and G.
van Koten, Organometallics, 1998, 17, 5647; (b) M. Gagliardo, H. P.
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8886 | Dalton Trans., 2011, 40, 8879–8886
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©