Synthesis, characterization and structure of ruthenium(II) phosphine complexes with N-heterocyclic thiolate ligands

Article abstract of DOI:10.1016/j.ica.2005.05.042

The [Ru(dppb)(mbt)₂], mbt = 2-mercaptobenzothiazole, complex, isolated from the reaction of the mer-[RuCl₃(dppb)(OH₂)] complex with the 2,2′-dithiobis(benzothiazole), mbts, was characterized by spectroscopic and electrochemical techniques (E_1/2 = 0.78 V versus NHE) and its structure was determined by crystal X-ray analysis. The structural analysis suggests that the S-S bond of the mbts ligand is cleaved, thus forming two four-membered chelate rings coordinated to the ruthenium through the N,S-donor atoms of the mbt reduced ligands, with an average bite angle of 67.295(11)°. The ¹H and ¹³C NMR signals observed for mbts ligand coordinated to the metal center, the changes in the vibrational spectra, and the appearance of a MLCT band in the electronic spectrum of the complex point for the reduced state of the ruthenium metal center, Ru ^II. These reducing processes are suggested to be due to the methanol interference, which is observed to be strongly affected by N-methylmorpholine. The cytochrome c electrochemistry was analyzed by using the SAMs formed by the mbts and the [Ru(dppb)(mbt)₂] complex on gold, with only the former presenting electroactivity.

Full text of DOI:10.1016/j.ica.2005.05.042

Inorganica Chimica Acta 359 (2006) 391–400
www.elsevier.com/locate/ica
Synthesis, characterization and structure of ruthenium(II)
phosphine complexes with N-heterocyclic thiolate ligands
a
a
a
Solange O. Pinheiro , Jackson R. de Sousa , Marcelo O. Santiago ,
a
a
b
c
Idalina M.M. Carvalho , Ana L.R. Silva , Alzir A. Batista , Eduardo E. Castellano ,
c
a
a,
*
´
´
Javier Ellena , Icaro S. Moreira , Izaura C.N. Diogenes
a
ˆ ˆ
´
Departamento de Quı´mica Organica e Inorganica, Universidade Federal do Ceara, Cx Postal 12200, 60455-760 Fortaleza, CE, Brazil
b
Departamento de Quı´mica, Universidade Federal de Sa˜o Carlos, Cx Postal 676, 13565-905 Sa˜o Carlos, SP, Brazil
Instituto de Fı´sica de Sa˜o Carlos, Universidade de Sa˜o Paulo, Cx Postal 780, 13560-250 Sa˜o Carlos, SP, Brazil
c
Received 21 December 2004; received in revised form 11 April 2005; accepted 5 May 2005
Available online 20 December 2005
Abstract
The [Ru(dppb)(mbt)2], mbt = 2-mercaptobenzothiazole, complex, isolated from the reaction of the mer-[RuCl3(dppb)(OH2)] complex
with the 2,20-dithiobis(benzothiazole), mbts, was characterized by spectroscopic and electrochemical techniques (E1/2 = 0.78 V versus
NHE) and its structure was determined by crystal X-ray analysis. The structural analysis suggests that the S–S bond of the mbts ligand
is cleaved, thus forming two four-membered chelate rings coordinated to the ruthenium through the N,S-donor atoms of the mbt reduced
ligands, with an average bite angle of 67.295(11)ꢀ. The 1H and 13C NMR signals observed for mbts ligand coordinated to the metal cen-
ter, the changes in the vibrational spectra, and the appearance of a MLCT band in the electronic spectrum of the complex point for the
reduced state of the ruthenium metal center, RuII. These reducing processes are suggested to be due to the methanol interference, which is
observed to be strongly affected by N-methylmorpholine. The cytochrome c electrochemistry was analyzed by using the SAMs formed by
the mbts and the [Ru(dppb)(mbt)2] complex on gold, with only the former presenting electroactivity.
ꢁ 2005 Elsevier B.V. All rights reserved.
Keywords: Ruthenium phosphine compounds, N-Heterocyclic thiolate ligands; Chemically modified electrode; SAM; SERS; Cyt c
1. Introduction
rocatalysis or chemical sensing involving the pendant moi-
eties of a given complex.
The p-back-bonding capability of the Ru(II) complexes
together with the singular properties of phosphine ligands
[1] constitute interesting aspects concerning the possibility
of make thin inorganic films with potential coordination
chemistry on surface [2]. Many studies have focused on
altering the physical-chemistry properties of these materials
by introducing different substituents or metal centers with
different backbone capacities. This allows tuning of the
electronic properties which may have applications in elect-
Self-assembled monolayers (SAMs) formed by organ-
othiol species, which are ordered and highly packed, are
usually used as sensors in a general sense. For instance,
the redox process of cytochrome c (cyt c) metalloprotein
has been electrochemically studied [3–5] by using gold elec-
trode modified by the organothiol ligand 4-mercaptopyri-
dine (pyS). In a series of paper [6–8], it has been
demonstrated that the p-back-bonding interaction capabil-
ity of the [MII(CN)5]3ꢀ (M = Fe and Ru) metal center
enhances the interaction between the gold atoms and the
pyS sulfur head group portion. Another example that
may be reported here is the direct electrochemistry of folic
acid at a 2-mercaptobenzothiazole (mbt) SAM on gold
*
Corresponding author. Tel.: +55 0318532889844; fax: +55 03185328
89978.
´
E-mail address: izaura@dqoi.ufc.br (I.C.N. Diogenes).
0020-1693/$ - see front matter ꢁ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.05.042

392
S.O. Pinheiro et al. / Inorganica Chimica Acta 359 (2006) 391–400
perchlorate (TBAP), from Fluka, was recrystallized twice
5'
6'
from absolute ethanol and dried under vacuum. The
Suprapur H2SO4 (Merck) was used as received. Horse
heart cytochrome c (type VI, 99%, Aldrich Co.) was puri-
fied as described in the literature [13].
4'
7'
7'a
N
4'a
P
1'
S
Cl
S
SH
The [Ru(dppb)(mbt)2] complex was isolated from the
reaction of mer-[RuCl3(dppb)(OH2)] with the mbts ligand
following the procedure described in the literature for
similar complexes [10,14]. A 29.99 mg sample (0.046
mmol) of the mer-[RuCl3(dppb)(OH2)] complex was dis-
solved in 2 mL of methanol, under argon atmosphere.
In another flask, an equimolar quantity of the mbts ligand
was dissolved in CH2Cl2, in anaerobic conditions. After
total dissolution of both reagents, the mer-[RuCl3(dppb)-
(OH2)] complex solution was slowly transferred to the
flask containing the ligand and a few drops (160 lL) of
N-methylmorpholine was added. This last compound
was used to confer a basic medium reaction aiming to
facilitate the mbts coordination to the metal center. The
resulting solution developed a pale green color and was
allowed to stand for 4 h in the absence of light, under stir-
ring and argon flow. The green precipitate formed during
the reaction was collected by filtration, washed with hex-
ane, dried, and stored under vacuum in the absence of
light. Yield: 86%. Anal. Calc. for [Ru(dppb)(mbt)2]: C,
26.09; H, 2.63, N, 18.27. Found: C, 26.31; H, 2.59; N,
18.58%.
The [Ru(dppb)(dmbts)2] compound was obtained from
the reaction of the [RuCl2(dppb)(PPh3)] complex with the
dmbts ligand. A 40.32 mg sample (0.25 mmol) of this
ligand was added to a solution of CH2Cl2/CH3OH (80/
20) containing 86.07 mg (0.10 mmol) of the [RuCl2(dppb)-
(PPh3)] complex. The reaction was allowed to proceed for
3 h, under stirring and argon flow, at room temperature.
The resulting yellow solution was concentrated to near
1 mL volume followed by the addition of 10 mL of diethyl
ether. The yellow precipitate formed was collected, washed
with diethyl ether, and dried under vacuum in the absence
of light. Yield: 86%. Anal. Calc. for [Ru(dppb)(dmbts)2]: C,
56.65; H, 5.70; N, 3.30. Found: C, 57.30; H, 5.80; N,
3.35%.
N
H2O
2'
3'
dmbts
S
S
Ru
Cl
HS
2
Cl
N3
N
1
S
P
S
4a
7a
7
4
6
5
mbt
mbts
mer-[RuCl (dppb)(OH )]
3 2
Chart 1. Planar representations of the dmbts, mbt, mbts ligands and the
mer-[RuCl3(dppb)(OH2)] complex.
electrode (mbt/SAM/Au) [9]. The good performance of
this SAM in determining folic acid in solution was assigned
to the mbt ability in strongly binding to gold forming a clo-
sely packed monolayer that enhances the inherent electron
transfer rate equilibrium constant.
Aiming to evaluate the electronic properties of the
metal center upon the attachment of a conjugated substi-
tuent and develop a system with a potential application to
act as a mimetic biosensor and to be used in the catalysis
field, we have been characterizing and studying the
properties of the compound isolated from the reaction
of the 2,20-dithiobis(benzothiazole) (mbts) ligand with
the mer-[RuIIICl3(dppb)(OH2)] complex, where dppb = 1,
4-bis(diphenylphosphine)butane. For correlation purposes,
the complex isolated from the reaction of [RuIICl2(dppb)-
(PPh3)] with the piperidinedithiocarbamate (dmbts) ligand
was also characterized. The planar representations of
these compounds are illustrated in Chart 1.
2. Experimental
2.1. Materials
The electrochemical data obtained for the cyt c with
gold electrodes modified with the [Ru(dppb)(mbt)2] com-
plex and mbts ligand were correlated with those acquired
with the gold surface modified with the [Ru(CN)5(pyS)]4ꢀ
complex [7], where pyS = 4-mercaptopyridine. Because of
this, the [Ru(CN)5(pyS)]4ꢀ complex was synthesized
according to the literature procedures [7].
The water used throughout was purified by a Milli-Q
system (Millipore Co.). The mer-[RuCl3(dppb)(OH2)]
and [RuCl2(dppb)(PPh3)] complexes were synthesized
according to the literature [10,11]. Sodium piper-
idinedithiocarbamate, C6H10S2NNa (dmbts), was synthe-
sized according to the method reported in the literature
[12]. The 2,20-dithiobis(benzothiazole) organothiol ligand,
potassium hydroxide, potassium phosphate (KH2PO4)
and N-methylmorpholine (Aldrich) were used without
previous purification. Hexane, ethanol, diethyl ether and
methanol solvents were purchased from Merck and used
as received. Dichloromethane of HPLC grade (Merck)
was treated by refluxing over CaH2 and distilled twice
before its electrochemical usage. Tetrabutylammonium
2.2. Apparatus
Crystallographic data were performed with graphite
˚
monochromated Mo Ka (k = 0.71073 A) radiation on
an Enraf-Nonius Kappa-CCD diffractometer. Data were
collected up to 50ꢀ in 2h, with a redundancy of 4. The
final unit cell parameters were based on all reflections.
Data collections were made using the COLLECT program

S.O. Pinheiro et al. / Inorganica Chimica Acta 359 (2006) 391–400
393
[15]; integration and scaling of the reflections were per-
formed with the HKL Denzo-Scalepack system of pro-
grams [16]. Absorption corrections were carried out
using the multi-scan method [17]. The structures were
solved by direct methods with SHELXS-97 [18]. The model
was refined by full-matrix least-squares on F2 by means
of SHELXL-97 [18]. All the hydrogen atoms were located
on stereochemical grounds, stereochemically positioned
and refined with the riding model [19]. The data collec-
tions and experimental details for the complexes are sum-
marized in Table 1.
ments performed in nonaqueous medium. For these exper-
iments, a reference Ag/AgCl electrode, prepared only at the
working day for about 1 h before the experiments begin-
ning, was used as reference electrode. The potentials
reported in this study, however, were all converted to the
normal hydrogen electrode (NHE), based on the ferro-
cene/ferrocenium (Fc+/0
) redox process, which was
observed at 0.64 V in CH2Cl2. A conventional three-electrode
glass cell with a platinum disk (0.0314 cm2 of geometri-
cal area) and foil as working and auxiliary electrodes,
respectively, was used for the complexes and ligand
characterizations.
The electronic spectra of the complexes and ligands were
acquired with a Hitachi model U-2000 spectrophotometer.
The transmission infrared spectra of the compounds dis-
persed in KBr were obtained by using a Perkin–Elmer
The electrochemical experiments with cyt c were carried
out by using a three-electrode configuration cell where the
working electrode was polycrystalline gold surface modi-
fied with the mbts ligand, and with [Ru(CN)5(pyS)]4ꢀ
and [Ru(dppb)(mbt)2] complexes. The 0.1 M phosphate
(KH2PO4) buffer aqueous solution, pH 7.0, was used as
electrolyte, at room temperature. Before the experiments,
the cyt c solutions were stored at 4 ꢀC in order to avoid
protein denaturizing process [20]. A BAS AgjAgCljCl
(3.5 M KCl) and a gold flag were used as reference and
auxiliary electrodes, respectively. The surface modification
procedure was made by immersing the gold electrodes in a
saturated aqueous solution of the ligand or the complex for
at least 2 h. For comparative purpose, the modification of a
gold surface with the [Ru(CN)5(pyS)]4ꢀ complex (Rupy-
SAu) was also performed by immersing the electrode in a
20 mM complex aqueous solution for 15 min as reported
in the literature [7].
1
instrument model Spectrum 1000. H and 13C NMR nor-
mal and two-dimension COSY 1H–1H and HMQC
1H–13C spectra were recorded on Bruker AVANCE 500
spectrometer and referenced to the residual proton solvent
resonances (CDCl3, d 7.27). 31P{1H} NMR spectra were
obtained on a T BRUKER DRX400 spectrometer at
298K, using H3PO4 85% as external reference for
31P{1H} (161 MHz). Electrochemical experiments were
performed with an electrochemical analyzer BAS 100W
from Bioanalytical System at 25 0.2ꢀC. TBAP was used
as supporting electrolyte for all electrochemical measure-
Table 1
Crystallographic data and structure refinement for [Ru(dppb)(mbt)2] and
[Ru(dppb)(dmbts)2] complexes
The ex situ surface enhanced Raman scattering (SERS)
spectra of the SAMs were acquired by using a Renishaw
Raman Imaging Microscope System 3000 equipped with a
charge coupled device (CCD) detector, and an Olympus
(BTH2) with 50· objective to focus the laser beam on
the sample in a backscattering configuration. As exciting
radiation, k0, the 632.8 nm line from a He–Ne (Spectra-
Physics) laser was used. The gold substrates used for
spectra SERS acquisition were activated by the oxida-
tion–reduction cycles (ORC) procedure in 0.1 M KCl as
described by Gao et al. [21], without the active species
in solution. The activation of the gold surface for SERS
spectra acquisition was made by using a PAR 273
potentiostat.
The polishing procedure of the gold surfaces employed
in different experiments cited above was made as described
by Qu et al. [22]. These electrodes were mechanically pol-
ished with alumina paste of different grades to a mirror fin-
ish, rinsed and sonicated (10 min) with Milli-Q water.
Then, the electrode was immersed in a freshly prepared
‘‘piranha solution’’ (3:1 concentrated H2SO4/30% H2O2 –
Caution: the ‘‘piranha solution’’ is a strong oxidant solu-
tion that reacts violently with organic compounds), rinsed
exhaustively with water and sonicated again. The cleanness
was evaluated by comparison of the i versus E curve
obtained in a 0.5 M H2SO4 solution with the well-estab-
lished curve for a clean gold surface [23].
Compound
[Ru(dppb)(mbt)2]
[Ru(dppb)(dmbts)2]
Empirical formula
[C42H36N2P2S4Ru]
Æ CH3CH2OCH2CH3
934.10
293(2)
triclinic
[C40H48N2P2S4Ru]
Æ CH2Cl2
932.98
120(2)
triclinic
Formula weight
Temperature (K)
Crystal system
Crystal size (mm3)
Space group
Unit cell dimensions
0.08 · 0.06 · 0.04
0.12 · 0.06 · 0.04
ꢀ
ꢀ
P1
P1
˚
a (A)
11.3178(9)
12.849(1)
16.508(1)
104.652(6)
107.832(5)
92.206(5)
2193.4(3)
2
2.1946(4)
12.5932(5)
15.1898(7)
95.887(2)
108.169(2)
104.120(2)
2108.6(2)
2
˚
b (A)
˚
c (A)
a (ꢀ)
b (ꢀ)
c (ꢀ)
3
˚
V (A )
Z
Dcalc (Mg/m3)
Absorption
coefficient (mmꢀ1
F(000)
1.414
0.658
1.469
0.805
)
964
2.93–25.00ꢀ
964
2.43–25.00ꢀ
h Range for
data collection (ꢀ)
Reflections collected
Independent
reflections (Rint
Final R indices [I > 2r(I)]
13570
7712 (0.0662)
13762
7372 (0.0818)
)
R1 = 0.0541,
wR2 = 0.0989
R1 = 0.1214,
wR2 = 0.1189
R1 = 0.0575,
wR2 = 0.1505
R1 = 0.0849,
wR2 = 0.1656
R indices (all data)

394
S.O. Pinheiro et al. / Inorganica Chimica Acta 359 (2006) 391–400
3. Results and discussion
3.1. Crystal and molecular structure
Recrystallization of the [Ru(dppb)(mbt)2] and [Ru-
(dppb)(dmbts)2] complexes from dichloromethane/diethyl
ether solutions yielded pale green and yellow crystals,
respectively, suitable for single crystal X-ray analyses. Both
ꢀ
complexes crystallize in the P1 triclinic space group with
the ruthenium environments adopting a distorted octahe-
dral geometry in which the nitrogen and sulfur atoms,
respectively, are trans to the phosphorous atoms. Selected
˚
bond lengths (A) and angles (ꢀ) are displayed in Table 2
for these crystals.
The ORTEP views of the structures are shown in Fig. 1.
The molecular structure for the [Ru(dppb)(mbt)2] complex,
Fig. 1A, reveals two units of the 2-mercaptobenzothiazole
ligand coordinated to the ruthenium metal center as a
bidentate N,S-donor, forming two four-membered chelate
ring with an average bite angle of 67.295(11)ꢀ. It is clear
from the structure illustrated in Fig. 1A that for each
mbt ligand bonded to ruthenium, there is a non-coordi-
nated sulfur atom in the ring and only one sulfur atom
coordinated to the metal center. This suggests that upon
coordination to ruthenium, the S–S bond is broken
through reduction of the mbts ligand. For the [Ru(dppb)-
(dmbts)2] complex, the X-ray analysis points also for a
structure with bidentate S,S-donor covalently bonded to
the ruthenium metal center (Fig. 1B), with an average bite
angle of 71.515(4)ꢀ.
Table 2
˚
Selected bond lengths (A) and angles (ꢀ) for [Ru(dppb)(mbt)2],
[Ru(dppb)(dmbts)2] and complexes with estimated deviations in
Fig. 1. ORTEP [19] diagrams for the asymmetric units of (A)
[Ru(dppb)(mbt)2] and (B) [Ru(dppb)(dmbts)2] complexes, showing the
atoms labelling and the 50% probability ellipsoids.
parentheses
˚
Bond length (A)
[Ru(dppb)(mbt)2]
Ru–N(1)
Ru–N(2)
Ru–P(1)
Ru–P(2)
[Ru(dppb)(dmbts)2]
Ru–S(12)
Ru–S(22)
Ru–P(1)
Ru–P(2)
2.143(4)
2.167(4)
2.2697(15)
2.2733(14)
2.4430(14)
2.4559(14)
2.4407(13)
2.4497(12)
2.2837(13)
2.2959(12)
2.4101(13)
2.4136(13)
As no other reducing agent was inserted in the reac-
tional medium, the reducing of the S–S bridge of the mbts
ligand, as well as the redox process of the ruthenium metal
center (RuIII ! RuII), is proposed to be due to the metha-
nol interference. In fact, the literature report for some syn-
thetic methods of ruthenium complexes, that the reduction
of Ru(III) to Ru(II) occurs by organic solvents [24]. An
interesting experimental observation that must be
addressed here is the fact that the synthesis of the
[Ru(dppb)(mbt)2] complex is strongly facilitated in the
presence of N-methylmorpholine. Similar procedure was
made with no N-methylmorpholine in solution and the
compound isolated was found to be not pure and com-
pletely different.
The deviation from the ideal octahedral geometry can be
seen by the angles between P(1)–Ru–P(2) 95.23(6)ꢀ, N(2)–
Ru–S(21) 66.90(11)ꢀ; N(1)–Ru–S(11) 67.69(11)ꢀ for the
[Ru(dppb)(mbt)2] complex and P(1)–Ru–P(2) 94.28(5)ꢀ;
S(11)–Ru–S(12) 71.49(4)ꢀ; S(21)–Ru–S(22) 71.54(4)ꢀ for
the [Ru(dppb)(dmbts)2] complex. Attention must be paid
Ru–S(11)
Ru–S(21)
Ru–S(11)
Ru–S(21)
Bond angle (ꢀ)
RudppbMBTS
N(1)–Ru–N(2)
N(1)–Ru–P(1)
N(2)–Ru–P(1)
N(1)–Ru–P(2)
N(2)–Ru–P(2)
P(1)–Ru–P(2)
N(1)–Ru–S(11)
N(2)–Ru–S(11)
P(1)–Ru–S(11)
P(2)–Ru–S(11)
N(1)–Ru–S(21)
N(2)–Ru–S(21)
P(1)–Ru–S(21)
P(2)–Ru–S(21)
S(11)–Ru–S(21)
RudppbDMBTS
S(21)–Ru–S(22)
S(11)–Ru–S(22)
S(11)–Ru–S(12)
S(21)–Ru–S(12)
P(1)–Ru–S(22)
P(1)–Ru–S(11)
P(1)–Ru–S(12)
P(1)–Ru–S(21)
P(1)–Ru–P(2)
P(2)–Ru–S(22)
P(2)–Ru–S(12)
P(2)–Ru–S(21)
P(2)–Ru–S(11)
S(12)–Ru–S(22)
S(11)–Ru–S(21)
82.20(15)
91.06(11)
171.48(11)
166.14(11)
92.49(11)
95.23(6)
67.69(11)
94.53(11)
87.67(5)
100.18(5)
96.64(11)
66.90(11)
108.99(5)
92.98(5)
71.54(4)
95.28(4)
71.49(4)
88.46(4)
87.21(4)
102.18(5)
170.08(4)
96.03(5)
94.28(5)
176.30(4)
93.10(4)
104.91(4)
87.73(4)
85.83(4)
157.02(5)
157.80(5)

S.O. Pinheiro et al. / Inorganica Chimica Acta 359 (2006) 391–400
395
for the N–Ru–S and S–Ru–S angles of [Ru(dppb)(mbt)2]
and [Ru(dppb)(dmbts)2] complexes, respectively. Almost
sure, the geometrical arrangement of the mbt or dmbts
moieties in the [Ru(dppb)(mbt)2] or [Ru(dppb)(dmbts)2]
structures must be dictated by the position of the sulfur
atoms within the ligands. In fact, since the non-coordinated
sulfur atoms are trans to each other in the mbt moieties, the
repulsion between them is reduced with a smaller N–Ru–S
pounds in the sense that the nitrogen atom is better p
acceptor than the sulfur species, which is better r donor.
Additionally, the X-ray structural characterizations of
the [Ru(dppb)(mbt)2] and [Ru(dppb)(dmbts)2] complexes
present a diethyl ether (CH3CH2OCH2CH3) and a dichlo-
romethane (CH2Cl2) unit inserted in the respective struc-
tures. The presence of these species in the structures of
[Ru(dppb)(mbt)2] and [Ru(dppb)(dmbts)2] complexes must
be assigned to the crystallization and synthetic procedure,
respectively.
angle, getting
a more stabilized structure for the
[Ru(dppb)(mbt)2] complex. The same can be suggested
for the coordinated sulfur atoms. Here, again, the trans
position should probably occur for minimize the electronic
density repulsion between the four sulfur atoms in both the
complexes. For the [Ru(dppb)(dmbts)2] complex, the
unusually small S(21)–Ru–S(22) angle (71.54(4)ꢀ) compar-
atively to other ruthenium phosphine compounds with
mercapto ligands [25], strongly reinforces the conclusion
about the repulsion between the coordinated and non-
coordinated sulfur atoms of a given ligand.
3.2. NMR
The 13C and 1H NMR chemical shifts in CDCl3
obtained for the [Ru(dppb)(mbt)2] complex are displayed
in Table 3, in addition with coupling data, when it were
observed. The assignments proposed were based on two-
1
1
dimension COSY H–1H and HMQC H–13C spectra and
in comparative form with the H and 13C spectra acquired
1
All Ru–ligand distances obtained for [Ru(dppb)(mbt)2]
and [Ru(dppb)(dmbts)2] complexes are comparable with
those found in other ruthenium phosphine compounds with
ligands containing sulfur and/or nitrogen atoms [25–28].
Surprisingly, the Ru–P bond distances are similar in both
for the free ligand in the same experimental conditions.
1
The H and 13C NMR spectra of the [Ru(dppb)(mbt)2]
complex in CDCl3 solution present high-resolution sug-
gesting that the complex may be diamagnetic, i.e., the
ruthenium atom is in its reduced state, RuII. These spectra
present, respectively, four (d = 7.17, 6.86–6.89, 6.91–6.94
and 7.24–7.26 ppm) and seven signals (d = 118.3, 120.7,
128.5, 129.2, 132.5, 150.8 and 181.9 ppm) ascribed to the
mbts ligand after coordination. By accounting for a higher
expected C–P coupling constant values for the carbons
located on the neighborhood of the P atoms, the signals
observed at 31.9, 139.3 and 138 ppm in the 13C NMR spec-
trum are assigned, respectively, to the CH2 (Chart 1, a and
d), and C1 and C10 (Chart 1, ring A and B) as described in
˚
the [Ru(dppb)(mbt)2] (2.2697(15) and 2.2733(14) A) and
˚
[Ru(dppb)(dmbts)2] (2.2837(13) and 2.2959(12) A) com-
plexes. This observation enables us to suggest that the trans
influence of the nitrogen and sulfur atoms are practically
identical. These results, however, can be better analyzed if
considering two distinct effects: (i) the higher trans influence
of the sulfur atom comparatively to the nitrogen atom that
implies in the Ru–P bonds weakening for the [Ru(dppb)-
(dmbts)2] complex and (ii) the competitive effect between
the nitrogen and phosphorus atoms (both good p acceptors)
which makes the Ru–P bond distances longer than should
be expected for the [Ru(dppb)(mbt)2] compound. Coinci-
dently, these two effects induce indeed equivalent Ru–P dis-
tances in both the complexes that are being studied.
Table 3
1H and 13C chemical shifts in CDCl3 for the [Ru(dppb)(mbt)2] complex
Groups
1H
13C
For the [Ru(dppb)(mbt)2] complex, both the Ru–N and
Ru–P bonds (Table 2) are slightly smaller comparatively
to similar pyridinic ruthenium complexes [29] (Ru–P:
CH2a and CH2d
CH2b and CH2c
1.74 d
2.75 d
31.9 t 1J
¼ 28:8
¼ 3:3
13C31
P
24.4 d 2J
13C31
P
Ring A
1,10
2,20, 6,60
3,30, 5,50
4,40
˚
2.3445(6) and 2.3427(6) A; Ru–N: 2.151(2) and
139.3 t 1J
132.9 t 2J
128.0 t 3J
125.4
¼ 38:8
13C31
13C31
13C31
P
˚
2.1538(19) A). This observation may reflect a higher
7.87–7.88 d 4H
7.17 m 4H
7.07–7.10 t 2H
¼ 8:8
¼ 8:8
P
P
energy level similarity between the Ru dp orbitals and
those of the appropriated symmetry of the mbt ligand,
which imply in a more effective p-back-bonding interac-
tion [30]. A comparative analysis among the covalent
radii of the nitrogen and phosphorus atoms shows that
Ring B
1,10
2,20, 6,60
3,30, 5,50
4,40
138.0 t 1J
132.2 t 2J
127.5 t 3J
122.4
¼ 38:8
¼ 8:8
¼ 8:8
13C31
13C31
13C31
P
P
P
7.66 d 4H
7.17 m 4H
6.99–7.02 t 2H
˚
the former is only 0.36 A smaller than the latter [31].
Upon coordination, however, the difference between the
lengths of the Ru–N and Ru–P bonds is only about
mbts coordinated
2,20
181.9
128.5
150.8
118.3
120.7
129.2
132.5
˚
0.12 A (see Table 2 for the [Ru(dppb)(mbt)2] complex).
4,40
7.17 m 2H
This observation hints that the bond strength between
the ruthenium and these atoms are very strong and that
the nitrogen is indeed as good p acceptor as the phospho-
rus atom is. The Ru p-back-bonding interaction should be
reflected in the electrochemical properties of both com-
4a,4a0
5,50
6.86–6.89 t 2H
6.91–6.94 t 2H
7.24–7.26 d 2H
6,60
7,70
7a,7a0

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S.O. Pinheiro et al. / Inorganica Chimica Acta 359 (2006) 391–400
Table 3. All these signals are observed in a slightly higher
field when the mbts species is free of coordination. This
downshift observed after interaction with the metal center
points for the electronic delocalization capability of the
mbts molecule. However, the signal assigned to the 2 and
20 carbons of the mbts species (Chart 1) presents an unex-
pected more pronounced high shift from 168.3 ppm when it
is free of interaction to 181.9 ppm upon coordination to the
mer-[RuCl3(dppb)(OH2)] metallic complex. This shift
might be explained if considering a strong deshielding of
these atoms, which may happen as a consequence of an
withdraw effect. The sulfur or nitrogen atoms that are close
to the 2 and 20 carbons of the mbts species, at once, can
induce this effect. Nevertheless, this strong shift is most
probably to occur as the consequence of a bond character
change in the neighborhood of the 2 and 20 carbons. In
fact, the structure determined by X-ray diffraction
(Fig. 1A) points for the formation of a chelate complex
as resultant of the mbts S–S bond broken which meaning-
fully affects the bond character in the vicinity of these
atoms.
The ring breathing is observed as a strong signal at
1008 cmꢀ1 in the IR spectrum of the mbts ligand, as
expected for aromatic species [34,35]. In the
[Ru(dppb)(mbt)2] spectrum, however, it appears around
996 cmꢀ1. This downshift may be supported by the high
aromaticity of the ring due the delocalization of the bond-
ing electrons around the conjugated ring system [39].
According to Taube [40], an enhancement in the electronic
density of a molecule that is coordinated to a given metal
center may occur if the metal is in its own reduced state
and if the ligand considered holds an appropriated set of
LUMO orbitals that enable it to act as a p acid. As pointed
out by the X-ray and NMR results, the ruthenium metal
center is very probably in the 2+ state and, in such case,
one must expect for its p-back-bonding interaction capability
with the p appropriated mbt orbitals. In fact, the N-hetero-
cyclic class of ligands is currently cited by the literature
[41,42] as presenting a good p acceptor character that facil-
itates the p-back-bonding interaction of metals such as Fe,
Ru and Os.
The bands observed from 1610 to 1660 cmꢀ1 are tenta-
tively assigned to the C@C coupled with C@N stretch
modes m(C@C + C@N). For some similar compounds,
the literature points for a m(C@C + C@N) frequency down-
shift also based on aromaticity degree increment [39]. With
this consideration in mind, the observation of the shift
from 1660 cmꢀ1 in the IR of the mbts to 1651 cmꢀ1 in
the IR spectrum of the [Ru(dppb)(mbt)2] complex is indic-
ative of an enhancement of the electronic density on the
ligand as a consequence of the p-back-bonding effect of
the ruthenium metal center.
The 31P{1H} spectrum of the [Ru(dppb)(mbt)2] complex
showed only a single signal, as expected for a symmetrical
arrangement of the two phosphorous atoms [27,32]. The
observation of this signal at 52 ppm suggests a strong r
donor character for the mbt ligand based on the data
reported by MacFarlane et al. [33].
All the resonance data taken together point for the
reduced state of the ruthenium metal center in the
[Ru(dppb)(mbt)2] isolated compound. Aiming to validate
the ruthenium oxidation state in this case, the synthesis
was performed by using the [RuCl2(dppb)(PPh3)] complex
instead of the mer-[RuCl3(dppb)(OH2)] compound as start-
ing material. The 31P{1H} spectrum of the product thus
isolated presents, also, a single signal at 52 ppm. This result
confidently confirms that the ruthenium atom is in its
reduced state, RuII, in the [Ru(dppb)(mbt)2] complex.
3.4. Electronic analysis
The electronic spectrum of the [Ru(dppb)(mbt)2] com-
plex in CH2Cl2 solution presents three bands at 240
(e = 1.59 · 104 Mꢀ1 cmꢀ1), 350 (e = 1.59 · 104 Mꢀ1 cmꢀ1
)
and 685 nm (e = 262 Mꢀ1 cmꢀ1), besides
a shoulder
3.3. Vibrational analysis
around 276 nm. The two bands and the shoulder observed
at higher energies are presented too in the electronic spec-
trum of the mer-[RuCl3(dppb)OH2] complex in CH2Cl2
solution [10,43]. Additionally, the absorption observed at
240 nm must involve the pp* pp mbt intraligand transi-
tions once the bands at 230 and 273 nm are observed in the
electronic spectrum of the mbts ligand free of coordination
in CH2Cl2 solution.
For the [Ru(dppb)(mbt)2] complex, the absorption at
685 nm is assigned to a charge-transfer transition because
of its sensitivity toward solvent changes, i.e., in acetonitrile
it is observed at 677 nm. Based on the qualitative molecular
orbital diagram for compounds of approximately octahe-
dral geometry [44,45], this electronic transition is believed
to occur from the ruthenium dp to the mbt pp* orbitals,
which characterizes a metal-to-ligand charge-transfer tran-
sition. Low extinction coefficients for charge-transfer tran-
sitions were also observed for a series of [Ru(NH3)5L]3+
type complexes, where L = dithioether ligands [46]. This
The assignments of the signals observed in the Raman
and infrared (IR) vibrational spectra of the [Ru(dppb)-
(mbt)2] complex were made in a qualitative form by
comparison with similar data reported in the literature
[34–37] for related compounds and with the signals
observed in the vibrational spectra of the mbts and mer-
[RuCl3(dppb)(OH2)] start materials.
Apart from the vibrations at 505, 670, 695, 702 and
744 cmꢀ1 due to the Ru(PPh3)2 moiety [36,37], strong sig-
nals observed from 400 to 750 cmꢀ1 in the IR and Raman
vibrational spectra of the [Ru(dppb)(mbt)2] complex are
assigned to the C–S stretch frequencies, m(C–S) [34]. The
absence of the signal at 536 cmꢀ1, assigned to the S–S
stretch frequency m(S–S) [38], in the Raman spectrum of
the [Ru(dppb)(mbt)2] complex compared with the spectrum
of the mbts ligand free of coordination, indicates that the
S–S bridge was really broken upon coordination.

S.O. Pinheiro et al. / Inorganica Chimica Acta 359 (2006) 391–400
397
Table 4
ment is consistent with the vibrational discussion in which
the downshift of some relevant mbt bands after coordina-
tion was assigned to the increment of the aromaticity due
to the delocalization of the bonding electrons around the
ring system [39].
The relatively higher potential (0.74 V) observed for the
[RuCl2(dppb)(1,4-dt)] complex (Table 4) in which the sul-
fur atoms are the unique coordination sites of the ligand
is assigned [48] to the strong p back-bonding interaction
with the sulfur atoms. This observation hints that the coor-
dination to sulfur atoms comparatively to nitrogen atoms
gives raise to a stronger stabilization of the ruthenium
metal center in its reduced state. This conclusion is an addi-
tional evidence that the coordination of the mbt ligand
ought to involve one of its sulfur atoms.
UV–Vis (k and e) and RuIII/II half-wave formal potential, E1/2 (in V vs.
NHE), for some ruthenium complexes
b
Complex
k
max/nm
E1/2
Reference
(e/L molꢀ1 cmꢀ1 a
)
[Ru(dppb)(dmbts)2]
xxc
0.42
0.45
0.52
0.70
0.74
0.78
0.84
this work
[27]
[27]
[27]
[48]
[RuCl2(dppb)(4-NH2py)2]
[RuCl2(dppb)(4-Nme2py)2]
[RuCl2(dppb)(py)2]
[RuCl2(dppb)(1,4-dt)]
[Ru(dppb)(mbt)2]
xxc
680 (70)
672 (90)
530 (2000)
685 (262)
675 (105)
this work
[27]
[RuCl2(dppb)(4-CNpy)2]
a
Solvent: CH2Cl2.
b
All electrochemical data were obtained in CH2Cl2 containing 0.1 M
TBAP.
c
Data not available.
Also, the cyclic voltammogram obtained for the
[Ru(dppb)(mbt)2] complex in a more opened potential win-
dow (from ꢀ1.2 to +1.2 V versus NHE) does not present
any additional wave beyond that already commented
(E1/2 = 0.78 V). The cyclic voltammogram of the mbts
ligand free of coordination obtained in the same experi-
mental conditions of the [Ru(dppb)(mbt)2] complex, shows
two processes. A reduction at ꢀ0.72 V, which in turn dis-
plays a reoxidation peak at +0.46 V. This reoxidation pro-
cess is more clearly visualized when the electrode potential
is applied in the reverse scan. The wave at ꢀ0.72 V is
assigned to the reduction of the S–S bridge of the mbts
ligand, thus forming the 2-mercaptobenzothiazole (mbt)
molecule and that at 0.46 V to the oxidation of the mbt
to the mbts. Similar behavior was also observed for the
4,40-dithiodipyridine (pySSpy) organothiol ligand [51] in
an aqueous phosphate buffer solution (pH 7.0). For this
species, the potential observed at ꢀ0.16 V was assigned
to the pySSpy reduction to 4-mercaptopyridine (pyS),
and the pyS thus formed was reoxidized to pySSpy at
+0.69 V. Accounting for the aprotic medium in which the
mbts electrochemical measurements were acquired, the
more negative reduction potential (ꢀ0.72 V), compara-
tively to that observed for the pySSpy (ꢀ0.16 V), may be
explained based on the higher electronic density on the
mbts sulfur bridge that makes the S–S bond more robust
and so more difficult to be reduced. In reality, it has been
shown by the literature that the S–S bond is really affected
by the induction of an external electronic density [42,52].
These results reinforce the conclusions obtained from
the X-ray and vibrational results concerning the cleavage
of the mbts S–S bridge, i.e., if this bond had not be broken,
some redox process would be observed around ꢀ0.73 V.
result, in accordance with those obtained by NMR and
vibrational spectroscopies, strongly suggests that the metal
center is in the reduced state, RuII, in spite of the fact that
in the start complex, mer-[RuCl3(dppb)(OH2)], the ruthe-
nium was in the oxidized state, RuIII. Bands assigned to
the metal-to-ligand charge-transfer transitions in the visible
region are also observed for a series of ruthenium(II) phos-
phine compounds [47].
For correlation purposes, UV–Vis data assigned to the
charge-transfer transitions involving the ruthenium metal
atom of a set of ruthenium phosphine compounds are set
out in Table 4.
All absorbance data but that obtained for the
[RuCl2(dppb)(1,4-dt)] complex, are too proximate to rea-
sonably infer any degree difference in the Ru p-back-bond-
ing capability in function of a change of the N-pyridyl
ligand. For the [RuCl2(dppb)(1,4-dt)] complex, the obser-
vation of the charge-transfer transition in relatively higher
energy is explained based on a strong interaction between
the dp orbitals of ruthenium with the d orbitals of sulfur
[48,49].
3.5. Electrochemical study
Cyclic voltammetry of the [Ru(dppb)(mbt)2] complex
displays a well-defined wave assigned to the RuIII/II redox
process. This process is observed with characteristics of
high reversibility [50]. The half-wave formal potential
value, E1/2 = (Ea + Ec)/2, where Ea and Ec are the anodic
and cathodic potentials, respectively, is observed at
0.78 V. For correlation purpose, this data together with
few ones reported in the literature for some relevant ruthe-
nium phosphine complexes are compiled in Table 4.
The E1/2 values described in Table 4 enable us to ratio-
nalize the order dmbts <4-NH2py < 4-Nme2py < py < 1,4-
dt < mbt < 4-CNpy of p electron withdrawing ability in
the sense that higher the p back-bonding capability of the
metal center, higher will be the E1/2 value for its respective
redox process, i.e., the metal oxidation is made more diffi-
cult to occur. In this way, one might classify the mbt ligand
as an excellent acceptor of p electron density. This com-
3.6. SAMs electroactivity
The electroactivity of the self-assembled monolayers
(SAMs) formed on the gold surface by the mbts ligand
(mbtsAu) was analyzed by cyclic voltammetry by using
the cyt c metalloprotein as a probe molecule. The efficiency
of this SAM in the assessment of the cyt c heterogeneous
electron transfer (hET) reaction was compared with that

398
S.O. Pinheiro et al. / Inorganica Chimica Acta 359 (2006) 391–400
reported [7,8] for the SAM formed onto gold by the
[Ru(CN)5(pyS)]4ꢀ complex (RupySAu). Fig. 2 illustrates
the cyclic voltammograms of a cyt c solution in physiolog-
ical medium by using the mbtsAu and RupySAu electrodes.
Although the curve obtained with the RupySAu elec-
trode is clearly better defined than that obtained for the
mbtsAu electrode, these cyclic voltammograms point for
the efficiency of the SAMs to assess the fast cyt c hET reac-
tion. For the mbtsAu electrode, however, the sequence of
scans presents a gradual reduction of the current and shape
deformation indicating the mbts desorption process. Lamp
et al. [53] observed similar behavior for the SAM formed
by pyS on gold and used a set of infrared reflection spectra
to show that this happens because of a structural conver-
sion on the substrate yielding monolayers composed of
atomic and/or oligomeric sulfur species due to the C–S
bond cleavage of this molecule. For this experimental rea-
son it was not possible to esteem the cyt c hET rate con-
stant, k0, by using the Nicholson method [54] when the
mbtsAu electrode is used. This relatively poor result may
be assigned to the conformation of the mbts molecule on
the surface as pointed out by the SERS spectrum, which
is illustrated in Fig. 3.
Comparatively to the normal Raman spectrum of the
mbts species in the solid state (Fig. 3, dot line), the absence
of the signal at 536 cmꢀ1, assigned to the m(S–S) [38], in the
SERS spectrum of the mbts SAM (Fig. 3, solid line), indi-
cates the cleavage of the S–S bridge on the gold surface.
Similar behavior was observed for the SAM formed by
pySSpy on the gold surface [51]. The intensities of the
bands in the range from 300 to 800 cmꢀ1 observed in the
mbts normal Raman due to the out-of-plane vibrational
modes of the ligand [36,55,56] are strongly reduced in the
mbtsAu surface spectrum. This result suggests that the
mbts is chemisorbed on the gold surface in a perpendicular
orientation [57] as suggested by the inset in Fig. 3. For this
conformation, the adsorbate is expected to bind the gold
surface through sulfur r interaction. In fact, this strong
affinity of thiol groups for gold surfaces is well documented
in the literature [2].
+
The gold electrode modified by the [Ru(dppb)(mbt)2]
complex (RumbtAu) electrode has not shown electroactiv-
ity toward the electrochemical response of the cyt c redox
process, even after 72 h of immersion. This observation
may be explained based on the groups that are pointing
for the solution. Accounting for the coordination through
the nitrogen and one of the bridge sulfur atoms of the mbt
ligand, the chemisorption on gold through the mbt sulfur
atoms, if occurs, make the phosphine moieties as the func-
tional terminal groups, i.e., the ones that will be in contact
0.2 µA
-0.3 -0.2 -0.1
0.0
0.1
0.2
0.3
Potential / V vs Ag/AgCl
Fig. 2. Cyclic voltammogram at 100 mV sꢀ1 of 1 mM cyt c in 100 mM
KH2PO4, pH 7, solution with the mbtsAu (solid line) and RupySAu (dot
line) electrodes. + (i = 0.0).
Au
S
S
S
S
N
N
(S-S)
250
500
750
1000
1250
1500
1750
-1
Wavenumber / cm
Fig. 3. Normal Raman (dot line) of the mbts ligand in the solid state and SERS spectra (solid line) of the mbts SAM on gold.

S.O. Pinheiro et al. / Inorganica Chimica Acta 359 (2006) 391–400
399
with the redox active species in solution. For get an effec-
tive molecular recognition with cyt c molecules, terminal
functional groups that are anionic or weakly basic for
interacting with the lysine terminal ends of the cyt c that
are positively charged in physiological medium, have been
claimed by the literature [5,20]. Probably, the phosphine
groups do not furnish an optimum geometrical arrange-
ment and/or electronic density for cyt c to be recognized.
In fact, when species that are not in a normal position in
relation to the electrode surface are used for assessment
of the cyt c hET reaction, this process is weakly or even
not observed [58].
[Ru(dppb)(dmbts)2] complexes are CCDC 251546 and
251547. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/
j.ica.2005.05.042.
References
[1] T. Naota, H. Hakaya, S.-I. Murahashi, Chem. Rev. 98 (1998) 2599.
[2] D.M. Kolb, Angew. Chem. Int. Ed. 40 (2001) 1162.
[3] F.A. Armstrong, H.A.O. Hill, N.J. Walton, Acc. Chem. Res. 21
(1988) 407.
[4] F.M. Hawkridge, I. Taniguchi, Comments Inorg. Chem. 17 (1995)
163.
[5] P.M. Allen, H.A.O. Hill, N.J. Walton, J. Electroanal. Chem. 178
(1984) 69.
4. Conclusions
´
[6] I.C.N. Diogenes, F.C. Nart, M.L.A. Temperini, I.S. Moreira,
Electroanalysis 14 (2002) 153.
The structural characterizations (X-ray and NMR)
show that the compounds synthesized from the coordina-
tion of the mbts and dmbts ligands to the mer-[RuCl3(dppb)-
(OH2)] and [RuCl2(dppb)(PPh3)] complexes, respectively,
are chelate species with two four-membered rings. For
the [Ru(dppb)(mbt)2] complex, the electrochemical, vibra-
tional and electronic data taken together point for the
reduced state of the ruthenium metal center, RuII, despite
the 3+ oxidation state of this atom in the mer-[RuCl3(dpp-
b)(OH2)] start complex. This reducing process as well as
that of the mbts to mbt molecules are suggested to be
due to the methanol interference, which is strongly affected
by the N-methylmorpholine presence in the reactional med-
ium. The stability enhancement of the ruthenium metal
atom in the reduced state in the [Ru(dppb)(mbt)2] complex
is ascribed to the p-back-bonding interaction between the
dp orbitals of this metal with the p appropriated orbitals of
the mbt ligand. Comparatively to other N-heterocyclic
ligands, the mbt ligand can be classified as a good p acceptor.
The electroactivity study of the SAM formed by the
[Ru(dppb)(mbt)2] complex in cyt c solution did not present
any redox process. This result is assigned to the conforma-
tion of the complex on gold surface that does not permit
the metalloprotein to be recognized. On the other hand,
the SAM formed by the mbts ligand on gold, for
which the SERS spectrum shows that the S–S bridge is bro-
ken on the surface thus forming a monolayer composed of
mbt units, assess the cyt c redox process in solution. How-
ever, the performance of this SAM should not be compara-
ble with that reported for the RupySAu because of the mbt
gradual desorption process.
´
[7] I.C.N. Diogenes, F.C. Nart, M.L.A. Temperini, I.S. Moreira, Inorg.
Chem. 40 (2001) 4884.
´
[8] I.C.N. Diogenes, J.R. Sousa, I.M.M. Carvalho, M.L.A. Temperini,
A.A. Tanaka, I.S. Moreira, Dalton Trans. 11 (2003) 2231.
[9] Q. Wan, N. Yang, J. Electroanal. Chem. 527 (2002) 131.
[10] L.R. Dinelli, A.A. Batista, K. Wohnrath, M.P. Araujo, S.L. Queiroz,
M.R. Bonfadini, G. Oliva, O.R. Nascimento, P.W. Cyr, K.S.
MacFarlane, B.R. James, Inorg. Chem. 38 (1999) 5341.
[11] C.E. Jung, P.E. Garrou, P.R. Hoffman, K.G. Caulton, Inorg. Chem.
23 (1984) 726.
[12] P.K. Singh, M.M. Jones, G.R. Gale, L.M. Atkins, R.A. Bulman, J.
Toxicol. Environ. Health 28 (1989) 501.
[13] D.L. Brautigan, M.S. Ferguson, E. Margoliash, Methods Enzymol.
53 (1978) 128.
[14] M.D. Hargreaves, M.F. Mahon, M.K. Whittlesey, Inorg. Chem. 41
(2002) 3137.
[15] Enraf-Nonius, COLLECT, Nonius BV, Delft, The Netherlands, 1997–
2000.
[16] Z. Otwinowski, W. Minor, HKL Denzo and Scalepack, in: C.W.
CarterJr., R.M. Sweet (Eds.), Methods in Enzymology, vol. 276,
Academic Press, New York, 1997, p. 307.
[17] R.H. Blessing, Acta. Crystallogr., Sect. A 51 (1995) 33.
[18] G.M. Sheldrick, SHELXS-97: Program for Crystal Structure Resolu-
tion, University of Go¨ttingen, Go¨ttingen, Germany, 1997.
[19] L.J. Farrugia, ORTEP3 for Windows, J. Appl. Crystallogr. 30 (1997)
565.
[20] F.A. Armstrong, Struct. Bond. 72 (1990) 137.
[21] P. Gao, D. Gosztola, L.-W.H. Leung, M.J. Weaver, J. Electroanal.
Chem. 233 (1987) 211.
[22] X. Qu, T. Lu, S. Dong, J. Mol. Catal. 102 (1995) 111.
[23] D.T. Sawyer, A. Sobkowiak, J.L. Roberts Jr., Electrochemistry for
Chemists, second ed., Wiley, New York, 1995.
[24] E.A. Seddon, K.R. Seddon, The Chemistry of Ruthenium, Elsevier,
New York, 1984.
[25] P.G. Jessop, C.-L. Lee, G. Rastar, B.R. James, C.J.L. Lock, R.
Faggiani, Inorg. Chem. 31 (1992) 4601.
[26] R. Mukherjee, Coord. Chem. Rev. 203 (2000) 151.
[27] S.L. Queiroz, A.A. Batista, G. Oliva, M.T. do, P. Gambardella,
R.H.A. Santos, K.S. MacFarlane, S.J. Rettig, B.R. James, Inorg.
Chim. Acta 267 (1998) 209.
Appendix A. Supplementary data
[28] M.K. Motlagh, N. Safari, C.B. Pamplin, B.O. Patrick, B.R. James,
Inorg. Chim. Acta 320 (2001) 184.
[29] J.G. Malecki, J.O. Dzie˛gielewski, R. Kruszynski, T.J. Bartczak,
Inorg. Chem. Commun. 6 (2003) 721.
[30] C.R. Johnson, R.E. Shepherd, Inorg. Chem. 22 (1983) 2439.
[31] J.E. Huheey, E.A. Keiter, R.L. Keiter, Inorganic Chemistry, Princi-
ples of Structure and Reactivity, fourth ed., Harper Collins College
Publisher, New York, 1993.
[32] A.A. Batista, L.A.C. Cordeiro, G. Oliva, O.R. Nascimento, Inorg.
Chim. Acta 258 (1997) 131.
Complete tables of bond lengths and angles, final atomic
coordinates and equivalent isotropic thermal parameters,
calculated hydrogen parameters, anisotropic thermal
parameters, and structure factors for the structures are
available as supplementary material. Also, tables of atomic
coordinates and bond lengths and angle were deposited
with the Cambridge Crystallographic Data Center. The
respective numbers for the [Ru(dppb)(mbt)2] and

400
S.O. Pinheiro et al. / Inorganica Chimica Acta 359 (2006) 391–400
[33] K.S. MacFarlane, A.M. Joshi, S.J. Rettig, B.R. James, Inorg. Chem.
35 (1996) 7304.
[44] P. Ford, D.F.P. Rudd, R. Gaunder, H. Taube, J. Am. Chem. Soc. 90
(1968) 1187.
[34] C.D. Dyer, J.D. Kilburn, W.F. Maddams, P.A. Walker, Spectrochim.
Acta A 47 (1991) 1225.
[35] J.H.S. Green, W. Kynaston, H.M. Paisley, Spectrochim. Acta 19
(1963) 549.
[45] E. Stuls, J.K.M. Sanders, M. Montalti, L. Prodi, N. Zaccheroni,
F.F. Biani, E. Grigiotti, P. Zanello, Inorg. Chem. 41 (2002) 5269.
[46] C.G. Kuehn, S.S. Isied, Prog. Inorg. Chem. 27 (1979) 153.
[47] F. Basuli, S.-M. Peng, S. Bhattacharya, Inorg. Chem. 40 (2001) 1126.
´
[36] S. Bhattacharya, C.G. Pierpont, Inorg. Chem. 30 (1991) 1511.
[37] P.K. Sinha, L.R. Falvello, S.M. Peng, S. Bhattacharya, Polyhedron
19 (2000) 1673.
[48] M.O. Santiago, J.R. Sousa, I.C.N. Diogenes, L.G.F. Lopes, E.
Meyer, E.E. Castellano, J. Ellena, A.A. Batista, I.S. Moreira,
Polyhedron (2005) in press.
[38] F.R. Dollish, W.G. Fateley, F.F. Bentley, Characteristic
Raman Frequencies of Organic Compounds, Wiley, New
York, 1974.
[49] C. Stein, H. Taube, Inorg. Chem. 18 (1979) 2212.
[50] A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals
and Applications, Wiley, New York, 1980.
[39] K.R.G. Devi, D.N. Sathyanarayana, E.M. Engler, J. Mol. Struct. 71
(1981) 1.
[51] I. Taniguchi, M. Iseki, H. Yamaguchi, K. Yasukouchi, J. Electroanal.
Chem. 186 (1985) 299.
[40] H. Taube, in: S.S. Isied (Ed.), Electron Transfer Reactions, vol. 253,
Advanced Chemical Series, Washington, DC, 1997, pp. 1–17.
[41] L.M. Baraldo, P. Forlano, A.R. Parise, L.D. Slep, J.A. Olabethe,
Coord. Chem. Rev. 219–221 (2001) 881.
[42] I.S. Moreira, J.B. de Lima, D.W. Franco, Coord. Chem. Rev. 196
(2000) 197.
[52] I.S. Moreira, D.W. Franco, Inorg. Chem. 33 (1994) 1607.
[53] B.D. Lamp, D. Hobara, M.D. Porter, K. Nili, T.M. Cotton,
Langmuir 13 (1997) 736.
[54] R.S. Nicholson, Anal. Chem. 37 (1965) 1351.
[55] D. Cook, Can. J. Chem. 39 (1961) 2009.
[56] E. Spinner, J. Chem. Soc. (1960) 1237.
[43] S.-X. Xiao, W.C. Trogler, D.E. Ellis, Z. Berkovitch-Yellin, J. Am.
Chem. Soc. 105 (1983) 7033.
[57] T. Sawaguchi, F. Mizutani, I. Taniguchi, Langmuir 14 (1998) 3565.
[58] S. Larsson, J. Chem. Soc., Faraday Trans. 79 (1983) 1375.