Design of an NO photoinduced releaser xe

This article was originally published in a journal published by
Elsevier, and the attached copy is provided by Elsevier for the
author’s benefit and for the benefit of the author’s institution, for
non-commercial research and educational use including without
limitation use in instruction at your institution, sending it to specific
colleagues that you know, and providing a copy to your institution’s
administrator.
All other uses, reproduction and distribution, including without
limitation commercial reprints, selling or licensing copies or access,
or posting on open internet sites, your personal or institution’s
website or repository, are prohibited. For exceptions, permission
may be sought for such use through Elsevier’s permissions site at:
http://www.elsevier.com/locate/permissionusematerial

Journal of Colloid and Interface Science 300 (2006) 543–552
www.elsevier.com/locate/jcis

co
p

y

Design of an NO photoinduced releaser xerogel based on the controlled
nitric oxide donor trans-[Ru(NO)Cl(cyclam)](PF6 )2
(cyclam = 1,4,8,11-tetraazacyclotetradecane) ✩
Kleber Queiroz Ferreira a , José F. Schneider b , Pedro A.P. Nascente c ,
Ubirajara Pereira Rodrigues-Filho d , Elia Tfouni a,∗
a Departamento de Química, Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de São Paulo, Av. dos Bandeirantes 3900,

14040-901 Ribeirão Preto, SP, Brazil

al

b Insituto de Física de São Carlos, Universidade de São Paulo, CP 369, 13560-970 São Carlos, SP, Brazil
c Departamento de Engenharia de Materiais, Universidade Federal de São Carlos, 13565-905 São Carlos, SP, Brazil
d Instituto de Química de São Carlos, Universidade de São Paulo, CP 780, 13560-970 São Carlos, SP, Brazil

on

Received 21 February 2006; accepted 28 March 2006
Available online 6 April 2006

rs

Abstract

or
's

pe

The immobilization and properties of the nitric oxide donor trans-[Ru(NO)Cl(cyclam)](PF6 )2 , Ru–NO, entrapped in a silica matrix by the sol–
gel process is reported herein. The entrapped nitrosyl complex was characterized by spectroscopic (UV–vis, infrared (IR), X-ray photoelectron,
and 13 C and 29 Si MAS NMR) and electrochemical techniques. The entrapped species exhibit one characteristic absorption band in the UV–vis
region of the electronic spectrum at 354 nm and one IR νNO stretching band at 1865 cm−1 , as does the Ru–NO species in aqueous solution. Our
results show that trans-[Ru(NO)Cl(cyclam)](PF6 )2 can be entrapped in a SiO2 matrix with preservation of the molecular structure. However, in
a SiO2 /SiNH2 matrix, the complex undergoes a nucleophilic attack by the amine group at the nitrosonium. Irradiation of the complex, entrapped
in the SiO2 matrix, with light of 334 nm, resulted in NO release. The material was regenerated to its initial nitrosyl form by reaction with nitric
oxide.
 2006 Elsevier Inc. All rights reserved.
Keywords: Nitric oxide; Sol–gel; Xerogel; Ruthenium; Nitrosyl; Controlled; Photochemistry; Cyclam; Silica; Aminopropylsilica; Nucleophilic; Attack

1. Introduction

Au

th

In mammalian species, nitric oxide (NO) plays key roles in
almost every function [1], where high or low NO concentrations
can be either beneficial or harmful and could accompany numerous pathological states [1]. For this reason, there has been a
growing interest in NO donors and scavengers aiming at therapeutic applications [2–17]. Ruthenium nitrosyl complexes have
shown to be very promising NO donors [5,6,9,10,12,14,17–25],
and some of them have shown biological activity [6,22,24–27].
✩
Taken in part from K.Q. Ferreira, Ph.D. thesis, Departamento de Química
da Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de
São Paulo, 2004.
* Corresponding author. Fax: +55 16 3602 4838.
E-mail address: eltfouni@usp.br (E. Tfouni).

0021-9797/$ – see front matter  2006 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcis.2006.03.081

In these quite stable complexes, the coordinated NO has a nitrosonium character, and it can be released photochemically or
via one-electron reduction [3,12,17–20,22,26,28].
Our laboratories have directed efforts toward the synthesis
of ruthenium complexes as NO donors and scavengers [6,14,23,
24,26]. The trans-[Ru(NO)Cl(cyclam)](PF6 )2 complex releases
NO photochemically or upon reduction [14,29,30], and it is
less toxic than nitroprusside, a well-known vasodilator [6,9,14].
Furthermore, it is as effective at reducing blood pressure as nitroprusside, but with a longer effect [6]. The blood pressure
effects were interpreted in terms of the reactivities of the complexes involved in NO release. The longer blood pressure reduction effect of trans-[Ru(NO)Cl(cyclam)]2+ was interpreted
as a result of the much lower rate of NO release from trans[Ru(NO)Cl(cyclam)]2+ than from similar tetraammine nitrosyl

K.Q. Ferreira et al. / Journal of Colloid and Interface Science 300 (2006) 543–552

al

co
p

y

UV and visible region. Sanchez et al. have already reviewed the
advantages and challenges of using hybrid xerogels for optical applications, and they clearly showed the feasibility of their
use in such applications [41]. The first attempt to immobilize a
ruthenium complex with potential ability to act as NO donor
was achieved by Franco et al. They used the chemisorption
of trans-[Ru(NH3 )4 (SO2 )(H2 O)]2+ on 3-(L-imidazolyl)propyl
organomodified silica gel to prepare a ruthenium complex
modified silica gel [–Si(CH2 )3 imN-Ru(NH3 )4 SO2 ] [42]. This
method has the advantage of leading to a chemical bond between the ruthenium complex and the silica gel, possibly leading to a more stable material from the recycling point of view.
However, the ruthenium loading in these materials is low even
for relatively small complexes like the ruthenium ammine compounds because they are not able to diffuse inside the inner
silica pores. Therefore, other methodologies should be used for
the preparation of heavily loaded ruthenium nitrosyl silicas.
As suggested before, the immobilization by sol–gel entrapment/occlusion in silica matrices can be a better choice [43–46].
The mild characteristics offered by the sol–gel process allow
the introduction of inorganic complexes inside an inorganic network [47]. The sol–gel methodology has so far been used in
the context of inorganic catalysts, as part of the matrix [48],

as supports for dispersed metal particles [49], and for copolymerization with suitable silicon-containing ligands [50]. The
introduction of a host molecule is obtained by adding its solution to the polymerizing mixture. When the polymerization is
complete, the dopant molecules are entangled in the inorganic
polymeric network. The nature of the entrapment is still not
fully understood, and it is really remarkable to see how many
applications of the entrapment have been developed, without a
full understanding of the process at the molecular level [51].
The entrapment of ruthenium nitrosyls can conceivably lead to
changes in kinetic properties, such as rate of release of NO,
which would probably lower than in solution. Similarly, Avnir
and Frenkel-Mullerad [52] recently studied the unusual stabilization of alkaline and acid phosphatases occluded in xerogels.
Remarkably, the enzymes kept their activity even at pH as low
as 0.9. The explanation proposed for such a high stability took
into consideration the porous microenvironment in xerogels at a
molecular level. The restricted space inside these pores seems to
challenge the classical meaning of thermodynamic parameters
like pH, and a nanoscopic view of the interactions inside the
pores among the surface groups, i.e., silanols, adsorbed water
and entrapped species, seems to be more appropriate. Therefore, the study of chemical reaction on largely restricted media
(LRM) needs a different approach from that used in bulk solution chemistry.

In this context and considering that ruthenium nitrosyl complexes allow the possibility of tuning the NO donor properties
[6,14,23,24,26,30], we have extended our investigations to their
immobilization for possible therapeutic applications. In this regard, initial studies on the immobilization of the ruthenium
nitrosyl complex, [Ru(salen)(OH2 )(NO)]+ (salen = N,N ′ -bis(salicylidene)ethylenediaminato), impregnated into a silica sol–
gel have indicated that it releases NO under irradiation with
light and it can also be regenerated [53].

Au

th

or
's

pe

rs

ruthenium complexes [6,9,14]. Aiming at extending the range
of potential applications, there is an interest in designing carriers or supporters for such complexes. Conceivably, the immobilization of these complexes could result in materials that may

be used in association with optical fibbers to provide the opportunity for controlled NO release at specific target sites using
laser photoexcitation [31,32].
In this regard, novel strategies using NO donors other than
metal nitrosyl complexes have also been investigated. Nitric
oxide-releasing diazeniumdiolates are successfully being immobilized in polymers, silica-gel, and metal surfaces, aiming
at biological applications [33]. Recently, the preparation, characterization, and preliminary biomedical application of various
nitric oxide (NO)-releasing fumed silica particles with amine
groups has been reported [34]. These amine groups were then
converted into the corresponding N-diazeniumdiolate groups
via reaction with NO(g) at high pressure in the presence of
methoxide bases. The N-diazeniumdiolate moieties attached to
the silica surface underwent a primarily proton-driven dissociation to NO under physiological conditions, and they also underwent slow thermal dissociation to NO. These resulting NOreleasing fumed silica particles could be embedded into polymer films to create thromboresistant coatings, via NO release at
fluxes that mimic healthy endothelial cells (EC), making them
a very interesting system. The NO-addition efficiency for this
direct reaction, however, was found to be 12% in an acetonitrile
suspension of Sil-2N [6] particles. This NO-loading capacity is
lower than the one observed for various free amines, which lead
to a typical yield of 30–90% [35]. The immobilization of diazeniumdiolates in sol–gel to yield NO releasing materials has also
been reported [36]. More recently, a ruthenium salen nitrosyl
complex has been copolymerized with ethyleneglycol dimethylacrylate to form a material which is photolabile for NO release

[37]. Toma et al. have also recently reported ruthenium hexaacetate clusters incorporated in polyvinyl alcohol films that are
sensitive to daylight [38].
The use of ruthenium nitrosyl immobilized species as NO
donors has some advantages in relation to other species, since
ruthenium complexes can deliver and recover NO at milder conditions than those associated with the diazeniumdiolate system.
Moreover, it should be noted that the NO delivery from ruthenium amine (or ammine) nitrosyl complexes can be tuned photochemically, through their different UV–vis spectra or by their
different reduction potentials [3,12,14,17,20,21,24,26,39,40],
and thus, the choice of complex can be made based on the
target and conditions. These NO donors attached to solid state
matrices can be achieved by two different approaches: (a) grafting or physical adsorption of the complex on a matrix already
prepared; (b) occlusion, where the complex is mixed with precursors of the matrix which is formed around the complex. The
matrix must be inert toward the complex and the NO released,
and it should also be chemically stable. Furthermore, in order
to keep the photochemical NO release, the matrix should not
absorb in the same wavelength range of the complex. Although
an organic matrix [37] can be envisaged, a xerogel matrix has
the advantage of exhibiting higher chemical and physical inertia, as well as displaying lower or zero absorbance in the near

on

544

K.Q. Ferreira et al. / Journal of Colloid and Interface Science 300 (2006) 543–552

In this paper, we describe the immobilization and characterization of the controlled NO donor trans-[Ru(NO)Cl(cyclam)](PF6 )2 entrapped in xerogels containing tetraethylorthosilicate
(TEOS), and 3-aminopropyltriethoxysilane (3-APTS), by the
sol–gel process. The reaction of 3-aminopropyltriethoxysilane
with the coordinated nitrosonium of the complex, the photochemical release of NO from the SiO2 material, and the regeneration of the ruthenium nitrosyl complex entrapped in the
matrix are also described.

proportions of complex, aminopropyltrietoxysilane and tetraetoxysilane. The 13 C NMR spectroscopy studies were carried
out in the solid state and in solution by bubbling NO(g) in
aminopropyltrietoxysilane dissolved in CDCl3 . Also, 13 C NMR
spectra of the free complex and in the presence of aminopropyltrietoxysilane were obtained.

co
p

y

2.2.2.1. Electrochemical measurements Cyclic voltammetry
and differential pulse voltammetry experiments were taken with
a model 273 PARC potentiostat/galvanostat, using a conventional three-electrode cell consisting of a modified carbon paste,
an Ag/AgCl, and a platinum wire as the working, reference and
auxiliary electrodes, respectively. The voltammetric spectra of
the complex on the matrix (carbon paste mixture) was hindered
by the work potential range of the carbon paste (−1.1 to 1.1 V
versus Ag/AgCl) [55]. The measurements were carried out at
′
values for the redox process of the nitrosyl ligand
25 ◦ C. E1/2
′
values
in the immobilized complex were determined. The E1/2
were the arithmetic means of the Epa and Epc values.

2. Experimental

pe

rs

Trans-[RuCl(tfms)(cyclam)](tfms) (Rutfms), trans-[RuCl2 (cyclam)]Cl (RuCl), and trans-[Ru(NO)Cl(cyclam)](PF6 )2
(RuNO) were synthesized by using a published procedure [14].

2.2.2.2. Electronic, vibrational, and NMR spectra Electronic
absorption spectra of immobilized complex were recorded using a Hewlett–Packard model 8452A recording spectrophotometer. Since the refraction index of carbon tetrachloride and
the silica matrix are nearly the same [56], the supported complex on the silica gel surface was immersed in a spectra grade
carbon tetrachloride and the spectra of the suspension were
obtained using a quartz cell of 1 mm path length. Diffuse reflectance infrared spectra were obtained on a MB-102 Bomem
spectrophotometer.
13 C and 29 Si solid-state high resolution NMR spectra were
obtained on a Varian Unity INOVA 400 (9.4 Tesla) spectrometer
and a CP/MAS 7 mm Varian probe. The magic angle spinning
technique (MAS) was used with a spinning frequency of 5 kHz.
29 Si NMR spectra were measured by applying a single radio
frequency π/2 pulse of 4.5 µs. A recycle time of 400 s was
sufficient to ensure complete magnetization recovery for all the
silicon species. For the 13 C spectra, the cross-polarization (CP)
1 H–13 C NMR technique was applied with a π/2 1 H pulse of
4 µs, contact time of 1 ms and recycle time of 5 s. Tetramethylsilane was used as reference for the obtention of 13 C and 29 Si
chemical shifts.

on

2.2. Complex syntheses

al

2.1. Chemicals and reagents
Ruthenium trichloride (RuCl3 ·nH2 O) (Strem) was the starting material for the synthesis of the ruthenium complexes.
Acetone, acrylonitrile, chloroform, and ethanol were purified
according to procedures published in the literature [54]. Doubly distilled water was used throughout. Tetraethylorthosilicate
(TEOS) and 3-aminopropyltriethoxysilane (3-APTS) (Aldrich)
were used without further purification. All other materials were
reagent grade and were used without further purification.

545

Au

th

or
's

2.2.1. Preparation of the entrapped complex
trans-[Ru(NO)Cl(cyclam)](PF6 )2
The ruthenium complex (25 mg; 3.8 × 10−5 mol) was dissolved in a hydrolytic solution containing TEOS (2.5 mL) in
ethanol:water (4:1 v/v) and 0.1 M HCl (0.4 mL). TEOS hydrolysis resulted in the formation of silanol groups (Si–OH) [51].
These silanol moieties reacted further among them to form
siloxane (Si–O–Si) oligomers in a condensation reaction, leading to the formation of a colloidal suspension (sol). Finally,
the solvents were removed from the interconnected porous network during a three-day drying process at 50 ◦ C, leading to
2.7 g of a dry vitreous material, the xerogel SiO2 /RuNO. The
xerogel with amino groups, SiO2 /SiNH2 /RuNO, was obtained
using the desired amount of Ru complex and TEOS (1.7 mL,
8.5 mmol) mixed with 3-APTS (0.83 mL, 4.0 mmol) instead
of the pure TEOS solution. The xerogels were characterized
by optical and scanning electronic microscopy; X-ray photoelectron, UV–visible, infrared, and 29 Si and 13 C MAS NMR
spectroscopies; and electrochemical techniques.
2.2.2. Reaction of trans-[Ru(NO)Cl(cyclam)](PF6 )2 with
3-aminopropyltrietoxysilane
The reaction of the complex with aminopropyltrietoxysilane
was monitored by three spectroscopic techniques: (1) diffuse
reflectance infrared spectroscopy, using the decrease of the intensity of the νNO band at 1865 cm−1 , and the νNO /νSiO peaks
areas ratios; (2) electronic, using the increase of the absorbance
of the absorption band at 484 nm; (3) 13 C NMR, using various

2.2.2.3. Scanning electron microscopy The xerogel was
spread over a double-side carbon tape and the sample was
coated with a carbon film using a BALTECMed 020 sputter
unit. Scanning electron microscopy of the xerogel was recorded
using a LEO 440 microscopy equipped with an Oxford EDS detector.
2.2.2.4. X-ray photoelectron spectroscopy The powder samples were prepared by spreading the powder over a carbon
double-side adhesive tape. The charge correction was done using the internal C–Hx C 1s peak at 285 eV and the Si 2p peak
of SiO2 at 103.4 eV [57]. A take-of angle of 90◦ was used.
The pressure in the analyzer chamber was in the 5 × 10−7 to
1 × 10−6 Pa range. A non-monochromatic MgKα radiation

546

(b)

co
p

(a)

y

K.Q. Ferreira et al. / Journal of Colloid and Interface Science 300 (2006) 543–552

Fig. 1. (a) Topographic, backscattered electron, image of trans-[Ru(NO)Cl(cyclam)](PF6 )2 entrapped in the SiO2 matrix. (b) EDAX mapping based on
the Si κα X-ray emission line.

(1253.6 eV; 180 W), as the X-ray power source, and a Kratos
XSAM spectrometer were used. The Xp-spectra were fitted to
a Gaussian–Lorentzian set of peaks as described before [58].

al
on

pe

rs

2.2.2.5. Photochemistry Monochromatic irradiations at 334
nm were carried out using a 150 W Xenon lamp in a model
6253, Oriel Universal Arc Lamp Source. The irradiation wavelength was selected with an Oriel interference filter for photolysis at the appropriate wavelengths. The interference filters had
an average band pass of 10 nm and the collimated beam intensities ranged from 1 × 10−9 to 4 × 10−8 einstein−1 cm−2 , as
determined by ferrioxalate actinometry. A sample of the xerogel in CCl4 was placed in the cuvette and deaerated by bubbling
argon before irradiation. The progress of the photoreactions was
monitored spectrophotometrically on a MB Bomem 102 FTIR
Spectrometer, using a ZnSe ATR crystal, or on HP8452A diode
array spectrophotometers, in the cases of in situ vibrational and
electronic spectroscopy, respectively.

Fig. 2. Electronic spectrum in CCl4 of trans-[Ru(NO)Cl(cyclam)](PF6 )2 entrapped in the SiO2 matrix (100 mg of SiO2 /RuNO in 10 mL).

3. Results and discussion

or
's

3.1. Characterization of the material

Au

th

The material containing trans-[Ru(NO)Cl(cyclam)](PF6 )2
entrapped in the SiO2 matrix exhibits a homogeneous color
and smooth surface as judged by optical microscopy. The homogeneous distribution of the Ru–NO complex was verified
by scanning electronic spectroscopy (SEM) using the EDS detector to generate an elemental mapping of Si, and Cl X-ray
emission. Unfortunately, it was not possible to collect enough
signal to study the distribution of trans-[Ru(NO)Cl(cyclam)]2+ ,
so we had to use the Cl κα line to do so. The topographic image and EDAX Si and Cl mapping of the SiO2 /RuNO xerogel,
which is similar to that of SiO2 /SiNH2 /RuNO, are shown in
Fig. 1. The Si mapping (Fig. 1b) confirms the siliceous nature
of the xerogel. The Cl mapping, not shown, shows an homogeneous distribution for the Ru complex in the material.
3.2. Elemental analysis
The concentration of ruthenium in the xerogels, as estimated from the Cl X-ray emission, is 0.6 × 10−2 mmol g−1 ,
corresponding to 43% of the theoretical maximum value of

Fig. 3. Electronic spectrum in CCl4 of trans-[Ru(NO)Cl(cyclam)](PF6 )2 entrapped in the SiO2 /SiNH2 matrix (100 mg of SiO2 /SiNH2 /RuNO in 10 mL).
Solid line: experimental. Dotted line: spectrum fitted with Gaussian components.

1.4 × 10−2 mmol g−1 . It was not possible to estimate the N
content by EDS.
3.3. Electronic and infrared spectra
Figs. 2–4 show the electronic and vibrational spectra of
trans-[Ru(NO)Cl(cyclam)](PF6 )2 entrapped in the SiO2 and
SiO2 /SiNH2 matrices.
The electronic and vibrational spectra of trans-[Ru(NO)Cl(cyclam)](PF6 )2 entrapped in the silica matrices indicate the
presence of coordinated nitrosonium (NO+ ). The electronic
spectrum of the free complex in aqueous solution displays two
broad absorption bands in the near UV–visible region. One band
is centered around 352 nm (ε = 190 cm−1 M−1 ); the other,
much weaker (ε = 54 cm−1 M−1 ), is located around 452 nm.
The UV–vis spectra for the complex entrapped in the two different matrices are fairly similar to that of the free complex
in solution. The absorption band at 352 nm for the free complex is shifted to 355 and 350 nm in the case of the SiO2
and SiO2 /SiNH2 matrices, respectively. The band at 452 nm

547

co
p

y

K.Q. Ferreira et al. / Journal of Colloid and Interface Science 300 (2006) 543–552

Fig. 5. 29 Si NMR spectrum of trans-[Ru(NO)Cl(cyclam)](PF6 )2 entrapped in
the SiO2 matrix, SiO2 /RuNO. Dotted curves: least-square fittings to the observed peaks.

Au

th

or
's

on

pe

rs

is barely seen in the matrices, and by fitting the spectra an absorption band can be located at 484 nm for the SiO2 /SiNH2
matrix. The absence of the former band and the presence of the
latter indicate the absence of the NO+ group. However, it has
been observed that nitrosonium can undergo nucleophilic attack
from an amine in solution [59–61]. As a matter of fact, a relatively more intense band appears at 484 nm in the spectrum of a
mixture of the complex and 3-aminopropyltrietoxysilane during
gel formation as a result of such reaction (see ahead). Thus, the
relatively weak absorption at 484 nm in the SiO2 /SiNH2 matrix may indicate that there is also some amount of a modified
complex in the matrix. This is consistent with the color change
from yellow to orange in this matrix and with the infrared results.
The νNO band in the infrared spectra of ruthenium nitrosyl
complexes is medium-dependent and can split as a result of
solid-state effects [30]. For trans-[Ru(NO)Cl(cyclam)](PF6 )2 ,
this band appears at 1875 cm−1 in KBr pellet [14,26,30]. In nujol mulls this band is split into two bands, 1881 and 1867 cm−1 ,
and in acetonitrile and water it appears as a single absorption at
1889 and 1899 cm−1 , respectively [30]. For the complex entrapped in the SiO2 and SiO2 /SiNH2 matrices, the νNO band
appears at 1870 and 1854 cm−1 , respectively. These differences
could also be conceivably due to possible different microenvironments in the matrix, as already observed for other systems
[62–64]. The infrared spectrum of pure silica displays a weak
band near 1870 cm−1 assigned to a combination of fundamental silica skeleton vibrations, which may also contribute to absorption in this region in the entrapped complex. However, for
SiO2 /SiNH2 , the νNO band decreases in intensity and there is
an increase in the absorption at 1600 cm−1 . These changes are
consistent with a nucleophilic attack of the –NH2 group on the
coordinated nitrosonium.

readily attributed to silicon species with different condensation degrees: Q4 , Q3 and Q2 , respectively [65]. As usual, Qn
indicates the degree of condensation of a given SiO4 tetrahedron, being n the number of O involved in Si–O–Si bridges.
The silicon species ratios can be obtained from the integrated
intensities of the NMR lines. A least square fitting of Gauss
(Q4 ) and Lorentz (Q3 and Q2 ) functions was carried out to
quantify these intensities. From these results, we can conclude
that 55% of the silicon present in the matrix correspond to
Q4 species, where the polymerization occurred in three dimensions, not leaving bound silanol groups. Also, 43% correspond
to Q3 (only one silanol group) and less than 2% to Q2 (geminated silanol).
These observations have led us to think that the condensation of the hydrolyzed tetraethylorthosilicate was not hindered
by the presence of the complex in the sol medium, resulting
in a highly interconnected three-dimensional network of SiO2 .
This is very important for the final properties of the material, since the presence of large numbers of geminal groups or
even of hydrolyzed tetraethylorthosilicate would mean a loss in
the thermal and mechanical stability of the xerogel or would
even prevent its formation. Also, the presence of 43% of isolate silanol groups (Q3 ) is typical of high surface energy and
high polar surface. So, we could expect a polar environment
surrounding the complex, similar to that felt by the complex in
a polar solvent like ethanol [66]. This polar environment could
stabilize charge dissociation reaction pathways and facilitate induced NO dissociation from the entrapped complex. A similar
matrix effect was observed for pentacyanoferrates anchored on
organomodified silica gel [67] and for molybdenum carbonyls
in the intrazeolyte cavity of the NaY zeolite [68].
Figs. 6a and 6b show the 13 C CP-MAS NMR spectra of
trans-[Ru(NO)Cl(cyclam)](PF6 )2 entrapped in the SiO2 matrix and in the SiO2 /SiNH2 matrix, respectively. The 13 C MAS
NMR spectrum of SiO2 /RuNO display peaks in the 10–70 ppm
region, as observed for the RuNO complex in solution. These
peaks are due to the –CH2 carbon of the cyclam ring. However,
besides the peaks at 10–70 ppm region, the 13 C MAS NMR
spectrum of SiO2 /SiNH2 /RuNO displays a peak at 177 ppm,
whose origin is not clear (see ahead). This is consistent with
the UV–vis and infrared results, which give evidence of a pos-

al

Fig. 4. Vibrational spectrum of trans-[Ru(NO)Cl(cyclam)](PF6 )2 entrapped in
the SiO2 matrix (SiO2 /RuNO).

3.4. Magic angle spinning nuclear magnetic resonance
3.4.1. 13 C and 29 Si solid-state NMR
The 29 Si NMR spectrum of the SiO2 /RuNO matrix is shown
in Fig. 5. Three broad NMR peaks centered at −111.0 ppm,
−102.0 ppm and −91.5 ppm can be observed. They can be

K.Q. Ferreira et al. / Journal of Colloid and Interface Science 300 (2006) 543–552

co
p

y

548

Fig. 8. In situ vibrational spectrum of a mixture of trans-[Ru(NO)Cl(cyclam)](PF6 )2 (1 × 10−3 mol L−1 ) and 3-aminopropyltrietoxysilane
(0.1 mol L−1 ) in acetonitrile. Time between successive spectra is 5 min.

al

Fig. 6. (a) Representative 13 C CP-MAS NMR spectrum of trans-[Ru(NO)Cl(cyclam)](PF6 )2 entrapped in the SiO2 matrix, SiO2 /RuNO. (b) 13 C CP-MAS
NMR spectrum of trans-[Ru(NO)Cl(cyclam)](PF6 )2 entrapped in the SiO2 /
SiNH2 matrix.

pe

rs

on

to the ruthenium ion, with an estimated half-life of 10 min.
Furthermore, the 13 C NMR spectrum in d3 -acetonitrile of the
product of the reaction of trans-[Ru(NO)Cl(cyclam)](PF6 )2 and
3-aminopropyltrietoxysilane displays a peak at 180 ppm, which
is not present in the 13 C NMR spectrum obtained in solution
by bubbling NO(g) in 3-aminopropyltrietoxysilane dissolved
in CDCl3 or in the 13 C NMR spectrum of the free complex
in acetonitrile-d3 . These results are consistent with the reaction between trans-[Ru(NO)Cl(cyclam)](PF6 )2 and the amine
group, and also with the infrared and UV–visible spectral data
of the complex immobilized in the SiO2 /SiNH2 matrix. The
residual absorption peak at 1875 cm−1 in the infrared spectrum can be attributed to an incomplete reaction. However,
the reaction product of this reaction is not clear. The reaction of some nitrosyl complexes with aromatic amines has
been reported to be a diazotation reaction [59–61], resulting in infrared bands around 2000 cm−1 and UV–vis bands
around 300 nm in the spectra of the products. In the case of
trans-[Ru(NO)Cl(cyclam)](PF6 )2 , there are no infrared bands
at ∼2000 cm−1 in the xerogel and the band at 484 nm present
in the electronic spectrum of the free complex in solution is not
observed in the xerogel either, possibly because only a partial
reaction occurs.

or
's

Fig. 7. In situ electronic spectrum of a mixture of trans-[Ru(NO)Cl(cyclam)](PF6 )2 (1 × 10−3 mol L−1 ) and 3-aminopropyltrietoxysilane
(0.1 mol L−1 ) in acetonitrile. Spectra taken after 5, 10, 15, 60, 90, 100 and
130 min of reaction.

th

sible nucleophilic attack of the –NH2 group on the coordinated
nitrosonium [59–61]. This process occurs in the ammino modified xerogel but not in the SiO2 xerogel, because of the nucleophilic character of the –NH2 groups.

Au

3.4.2. Nucleophilic attack of 3-aminopropyltrietoxysilane to
trans-[Ru(NO)Cl(cyclam)](PF6 )2
Figs. 7 and 8 show the in situ UV–vis and infrared absorption spectra of the reaction between the trans-[Ru(NO)Cl(cyclam)]2+ complex and 3-aminopropyltrietoxysilane. The
spectroscopic monitoring of this reaction shows alterations in
the electronic and vibrational spectra of the complex as indicated by an increase in the absorbance of the bands at 350
and 484 nm (Fig. 7) due to the reaction products, and a decrease in the intensity of the stretching band of the coordinated NO at 1875 cm−1 , accompanied by an increase around
1600 cm−1 . These spectral changes can be attributed to the nucleophilic attack of the –NH2 group to the NO+ coordinated

3.5. X-ray photoelectron spectroscopy
In order to study the xerogels by XPS, it was necessary to
first characterize the non-entrapped Ru-cyclam compounds by
the same spectroscopy. The Ru 2p3/2 , N 1s, Cl 2p3/2 , and the
separation between N 1s (NO) and O 1s (NO) of the pure trans[Ru(NO)Cl(cyclam)](PF6 )2 and trans-[RuCl2 (cyclam)]Cl compounds are shown in Table 1. Those of the entrapped complexes
are shown in Table 2. The (N–O) difference observed for the
complex is about the same as that observed in other M–NO
complexes [69,70].
From Table 1, it is possible to see that the Ru 2p3/2 peaks
of Ru–Cl have higher binding energy than those of trans-

549

K.Q. Ferreira et al. / Journal of Colloid and Interface Science 300 (2006) 543–552

Table 1
XPS binding energies in eV for the ruthenium nitrosyl complexes synthesized herein and for complexes reported in the literature
Ru 3p3/2

N 1s
(cyclam)

Trans-[RuCl2 (cyclam)]+
Trans-[RuCl(NO)(cyclam)]2+
[Ru(NO)2 (Pφ3 )2 ]4+
[Ru(NO)Cl5 ]2−

463.1
462.5
464.5
464.6

398.7
399.4

N 1s
(NO)

Cl 2p3/2
(bound)
196.5
198.6

400.9
400.6
402.8

(N 1s–O 1s)

Ref.
a
a

131.9
132.0

[69]
[70]

y

Sample

co
p

a This work.

Table 2
XPS binding energies in eV for the xerogel matrices (SiO2 and SiO2 /SiNH2 ) and for the xerogel matrices containing the entrapped complexes (SiO2 /RuNO,
SiO2 /SiNH2 /RuNO)
Sample

Si 2p

Silica gel

103.4

C 1s

N 1s

Trans-[Ru(NO)Cl(cyclam)]2+ (RuNO)

399.4
400.9

101.6
103.4

283.3
284.2
285.6
287.5

SiO2 /RuNO

101.4
103.4

284.0
285.2

SiO2 /SiNH2 /RuNO (2:1)

101.5
103.4

SiO2 /SiNH2 /RuNO (1:1)

101.6
103.4

or
's

a This work.

th

281.6

(N 1s–O 1s)NO

131.9

Ref.
[57,69]
a

a

530.2
531.5
532.7
534.2

a

398.9
400.8

530.5
531.9
533.1
534.5

397.6
399.2
400.8

530.0
531.5
532.8
(39)
534.0

282.1

a

284.8
286.3

398.2
399.8
402.0

530.2
531.7
533.1
534.8

283.1

a

284.8
286.2

399.0
400.8

530.1
531.3
532.6
534.1

283.5

a

[Ru(NO)Cl(cyclam)](PF6 )2 . This higher binding energy can be
interpreted as a result of a lower electronic density on the Ru–Cl
complex. This is in agreement with the oxidation state of the
Ru atoms in the different complexes. In Ru–Cl the Ru atom is
formally +3, and in the trans-[Ru(NO)Cl(cyclam)](PF6 )2 complex it is +2. It is also possible to see that the Cl 2p3/2 and
N 1s (cyclam) peaks in the trans-[Ru(NO)Cl(cyclam)](PF6 )2
complex are at higher binding energies than those for the trans[RuCl2 (cyclam)]Cl complex. Therefore, it seems that the coordinated nitrosonium, NO+ , is drifting electronic density from
the chloro ligand in trans and from the nitrogen of the cyclam. This explanation is in agreement with the previously
reported decrease in the pKa of the coordinated water in trans[Ru(NH3 )4 (NO)(H2 O)]3+ compared to [Ru(NH3 )5 (H2 O)]3+
[23,26]. Indeed, the authors explained this decrease in pKa in
terms of electronic density drifting from water to NO through
the H2 O–Ru–NO+ axis [23,26]. So, the chloro ligand in trans-

Au

Ru 3d

al

SiO2 /SiNH2

532.8

on

284.7
286.4

rs

101.9
103.4

pe

SiO2

O 1s

[Ru(NO)Cl(cyclam)](PF6 )2 is probably acting as a σ - and π Lewis base toward Ru and, therefore, it transfers electronic
density to the Ru–NO moiety.
The Ru 3d5/2 and N 1s XP-peaks of SiO2 /RuNO are quite
similar to those of the bulk trans-[Ru(NO)Cl(cyclam)]2+ , as
shown in Table 2. These findings agree with the FT-IR results.
The N 1s XP-peaks of SiO2 /SiNH2 display an asymmetric
peak fitted with 2 Gaussian–Lorentzian components assigned to
NH2 and NH+
3 species at 398.9 and 400.8 eV, respectively [71].
The ratio between these species is ca. 3:1.
The Ru 3d5/2 XPS peaks obtained for the trans-[Ru(NO)Cl(cyclam)](PF6 )2 complex are at a lower energy than those
observed for all the encapsulated complexes (Table 2), suggesting an electron-withdrawing effect for the xerogel matrix.
A greater difference is observed for the Ru 3d5/2 binding energy of the complex encapsulated in the SiO2 /SiNH2 /RuNO
matrix. The binding energies for this complex lie at ca. 283.1

550

K.Q. Ferreira et al. / Journal of Colloid and Interface Science 300 (2006) 543–552

[41] and other [Ru(NH3 )4 (X)(NO)]n+ [8,20,72] and trans[RuCl(NO)([15]ane)]2+ [73] complexes in solution, which renders the respective Ru(III) aqua species and NO.
Despite the undoubted infrared results, confirmation of the
photolabilization of NO from the entrapped complex in the
case of SiO2 /RuNO was achieved by a trapping technique using a quartz cuvette topped with a glass reservoir. The xerogel
was placed in the cuvette and 3 mL of a 7.4 × 10−5 mol L−1
solution of the trapping agent, trans-[RuCl(cyclam)(OH2 )]2+ ,
was placed in the upper reservoir under argon atmosphere. The
trans-[RuCl(cyclam)(OH2 )]2+ complex reacts with NO (kon =
0.2 M−1 s−1 at pH 1 and 25 ◦ C) [29] to form the corresponding
nitrosyl complex trans-[RuCl(NO)(cyclam)]2+ . Upon photolysis of the solid material in the cuvette, the released NO was
thus trapped by trans-[RuCl(cyclam)(OH2 )]2+ to give trans[RuCl(NO)(cyclam)]2+ as evidenced by the new absorption
band at 266 nm, typical of the latter species (Fig. 11).

and 283.5 eV for the xerogel made with 3-APTS and trans[Ru(NO)Cl(cyclam)](PF6 )2 complex with molar ratios of 2:1
and 1:1. This higher shift seems to corroborate the UV–vis,
FTIR, and 13 C NMR results for these xerogels.
3.6. Electrochemistry

al

co
p

y

The immobilization of trans-[Ru(NO)Cl(cyclam)](PF6 )2 in
the SiO2 matrix does not significantly affect the redox potentials, as in the case of trans-[Ru(NH3 )4 (imN)(SO2 )] [42].
In the potential range studied (−1.0–1.0 V), where the materials are not electro-active, the cyclic voltammetry of trans[Ru(NO)Cl(cyclam)](PF6 )2 entrapped in SiO2 shows the occurrence of two electrochemical processes (vs Ag/AgCl): Epc1 at
′
−0.39 V, Epc2 at −0.30 V (E1/2
= −0.34 V; E = 0.09 V;
Ipa /Ipc = 0.8). The Epc1 value is close to that of the complex
in solution (−0.33 V vs Ag/AgCl) [14], and is assigned likewise to the reduction of the nitrosonium ligand (NO+/0 ) in the
immobilized complex.

Fig. 10. UV–vis spectra of SiO2 /RuNO (a) before irradiation at 334 nm and
(b) after 60 min of irradiation.

Au

th

or
's

pe

rs

In view of the previous results, we focused our attention
on SiO2 /RuNO. Exposure of SiO2 /RuNO, in CCl4 , in deaerated conditions, to irradiation with light of 334 nm results
in decrease in the intensity of the νNO band at 1870 cm−1 ,
(Fig. 9). This decrease undoubtedly suggests the photochemical labilization of NO. Similar changes in the infrared spectrum had already been observed with a 10−4 mol L−1 trans[RuCl(NO)(cyclam)](PF6 )2 solution at pH 7 (0.1 mol L−1 ,
phosphate buffer) [29]. Furthermore, after photolysis, the color
of the solid material changed from yellow to green, with a
broad absorption increase in the 300–340 nm region of the
UV–vis spectrum, which is consistent with the formation of
a RuIII (cyclam) complex photoproduct in the intra-pores of
the xerogel, thus yielding an NO depleted material, SiO2 /Ru
(Fig. 10), as observed for trans-[RuCl(NO)(cyclam)](PF6 )2

on

3.7. Photochemical studies

Fig. 9. Vibrational spectra, in the mid-infrared region, of SiO2 /RuNO under
irradiation at 334 nm. Arrows indicate the decrease in the NO peak as a function
of the irradiation time. Elapsed time between each line is 60 min.

Fig. 11. UV–vis spectra of the NO sequestering complex, trans-[RuCl(cyclam)(OH2 )]2+ , during the NO trapping experiment, as a function of time of irradiation of SiO2 /RuNO.

K.Q. Ferreira et al. / Journal of Colloid and Interface Science 300 (2006) 543–552

551

co
p

y

attack by the amine group at the nitrosonium, thus indicating
that the use of amine functionalized silicas for metal nitrosyl
complexes should be avoided. Like the complex in solution, irradiation of the complex entrapped in a SiO2 matrix with light
of 334 nm results in the release of NO, as evidenced by changes
in the UV–vis and IR spectra of the solid as well as by trapping
the released NO. This material was regenerated to the nitrosyl form by reaction with nitric oxide. Thus, this system has
the potential to serve as a model to design regenerable precursors for photochemical NO delivery to various biological
targets, and it also contributes to the design of materials coatings such as stents. Experiments concerning further materials
characteristics as well as quantitative data concerning chemical
and photochemical release of NO and chemical regeneration of
the material are under schedule in our lab.
Fig. 12. Infrared spectral changes during reaction between the photolysis product, at 334 nm, of trans-[Ru(NO)Cl(cyclam)](PF6 )2 entrapped in SiO2 and
NO(g). (a) Before reaction (in Web version green solid) and (b) after reaction
(in Web version yellow solid).

on

al

The authors thank the Brazilian agencies FAPESP, CNPq,
and CAPES for financial support, Prof. Thiery Gacoin for
helpful suggestions and Dr. Cynthia Maria de Campos Prado
Manso, for the English revision of the manuscript.
Supplementary material
The online version of this article contains additional supplementary material.
Please visit DOI: 10.1016/j.jcis.2006.03.081.

pe

rs

The NO depleted xerogel (SiO2 /Ru) could be reloaded to the
nitrosyl form, SiO2 /RuNO, by reaction with bubbling NO(g) ,
at 25 ◦ C, for 60 min in deaerated conditions. The regeneration
of SiO2 /RuNO was confirmed by observing the reappearance
of its typical yellow color and the νNO stretching band at
1870 cm−1 in the FTIR spectrum (Fig. 12). Therefore, a loading–depleting–reloading cycle can be delineated as illustrated
in Fig. 13.

Acknowledgments

4. Summary

[1] M.J. Ignarro, Nitric Oxide: Biology and Pathobiology, Academic Press,
New York, 2000.
[2] H. Kunkely, A. Vogler, Inorg. Chim. Acta 346 (2003) 275.
[3] M.G. Sauaia, R.G. de Lima, A.C. Tedesco, R.S. da Silva, J. Am. Chem.
Soc. 125 (2003) 14718.

Au

th

or
's

Our results show that the controlled NO donor trans[Ru(NO)Cl(cyclam)](PF6 )2 can be entrapped in a SiO2 matrix
with preservation of its molecular structure and properties. In
a SiO2 /SiNH2 matrix, the complex undergoes a nucleophilic

References

Fig. 13. Schematic representation of a reversible NO photolabilization and NO loading in SiO2 /RuNO materials. The colors are representative of chromatic transitions between the NO loaded and the NO-depleted material.

K.Q. Ferreira et al. / Journal of Colloid and Interface Science 300 (2006) 543–552

al

co
p

y

[35] J.A. Hrabie, J.R. Klose, D.A. Wink, L.K. Keefer, J. Org. Chem. 58 (1993)
1472.
[36] S.M. Marxer, A.R. Rothrock, B.J. Nablo, M.E. Robbins, M.H. Schoenfisch, Chem. Mater. 15 (2003) 4193.
[37] J.T. Mitchell-Koch, T.M. Reed, A.S. Borovik, Angew. Chem. Int. Ed. 43
(2004) 2806.
[38] H.E. Toma, A.M. Alexiou, A.L.B. Formiga, M. Nakamura, S. Dovidauskas, M.N. Eberlin, D.M. Tomazela, Inorg. Chim. Acta 358 (2005)
2891.
[39] J.C. Toledo, H.A.S. Silva, M. Scarpellini, V. Mori, A.J. Camargo, M.
Bertotti, D.W. Franco, Eur. J. Inorg. Chem. 9 (2004) 1879.
[40] J.M. Slocik, M.S. Ward, R.E. Shepherd, Inorg. Chim. Acta 317 (2001)
290.
[41] C. Sanchez, B. Lebeau, F. Chaput, J.P. Boilot, Adv. Mater. 15 (2003) 1969.
[42] S.M.C. Neiva, J.A.V. Santos, J.C. Moreira, Y. Gushikem, H. Vargas, D.W.
Franco, Langmuir 9 (1993) 2982.
[43] J. Blum, A. Rosenfeld, N. Polak, O. Israelson, H. Schumann, D. Avnir,
J. Mol. Catal. A Chem. 107 (1996) 217.
[44] Y. Kurusu, D.C. Neckers, J. Org. Chem. 56 (1991) 1981.
[45] A. Rosenfeld, J. Blum, D. Avnir, J. Catal. 164 (1996) 363.
[46] C. Sanchez, F. Ribot, B. Lebeau, J. Mater. Chem. 9 (1999) 35.
[47] H. Schmidt, A. Kaiser, H. Patzelt, H. Scholze, J. Phys. 43 (1982) 275.
[48] D. Bianchi, M. Lacroix, G.M. Pajonk, S.J. Teichner, J. Catal. 68 (1981)
411.
[49] T. Lopez, P. Bosch, M. Asomoza, R. Gomez, J. Catal. 133 (1992) 247.
[50] U. Schubert, New J. Chem. 18 (1994) 1049.
[51] D. Avnir, Acc. Chem. Res. 28 (1995) 328.
[52] H. Frenkel-Mullerad, D. Avnir, J. Am. Chem. Soc. 127 (2005) 8077.
[53] J. Bordini, P.C. Ford, E. Tfouni, Chem. Commun. (2005) 4169.
[54] A.I. Vogel, Química Orgânica, Ao Livro Técnico, Rio de Janeiro, 1980.
[55] K. Kalcher, J.M. Kauffmann, J. Wang, I. Svancara, K. Vytras, C. Neuhold,
Z. Yang, Electroanalysis 7 (1995) 5.
[56] Y. Gushikem, C.R.M. Peixoto, U.P. Rodrigues, L.T. Kubota, E. Stadler,
J. Colloid Interface Sci. 184 (1996) 236.
[57] S.S. Chao, Y. Takagi, G. Lucovsky, P. Pai, R.C. Custer, J.E. Tyler, J.E.
Keem, Appl. Surf. Sci. 26 (1986) 575.
[58] A. Proctor, D.M. Hercules, Appl. Spectrosc. 38 (1984) 505.
[59] S. Sinha, A.K. Banerjee, B.K. Ghosh, Trans. Met. Chem. 23 (1998) 151.
[60] F. Doctorovich, C. Trápani, Tetrahedron Lett. 40 (1999) 4635.
[61] W.L. Bowden, W.F. Little, T.J. Meyer, J. Am. Chem. Soc. 99 (1977) 4340.
[62] Z.J. Wu, H. Joo, I.S. Ahn, J.H. Kim, C.K. Kim, K. Lee, J. Non-Cryst.
Solids 342 (2004) 46.
[63] M.S. Ahola, E.S. Sailynoja, M.H. Raitavuo, M.M. Vaahtio, J.I. Salonen,
A.U.O. Yli-Urpo, Biomaterials 22 (2001) 2163.
[64] M. Zayat, R. Pardo, D. Levy, J. Mater. Chem. 13 (2003) 2899.
[65] G. Engelhardt, E.D. Michel, High Resolution Solid-State NMR of Silicates and Zeolites, Wiley, Norwich, 1987.
[66] C. Rottman, G. Grader, D. Avnir, Chem. Mater. 13 (2001) 3631.
[67] U.P. Rodrigues, Y. Gushikem, F.Y. Fujiwara, E. Stadler, V. Drago, Struct.
Chem. 5 (1994) 129.
[68] H.O. Pastore, G.A. Ozin, A.J. Pöe, J. Am. Chem. Soc. 115 (1993) 1215.
[69] D.T. Clark, I.S. Woolsey, S.D. Robinson, K.R. Laing, J.N. Wingfield, Inorg. Chem. 16 (1977) 1201.
[70] V.I. Nefedov, Y.V. Salyn, A.P. Sadovsky, L. Beyer, J. Electron Spectrosc.
Relat. Phenom. 12 (1977) 121.
[71] J.L. Magalhaes, L.M. Moreira, U.P. Rodrigues, M.J. Giz, M.A. Pereira da
Silva, R. Landers, R.C.G. Vinhas, P.A.P. Nascente, Surf. Interface Anal. 33
(2002) 293.
[72] E. Tfouni, Coord. Chem. Rev. 196 (2000) 281.
[73] D. Bonaventura, F.D. Oliveira, V. Togniolo, A.C. Tedesco, R.S. da Silva,
L.M. Bendhack, Nitric Oxide Biol. Chem. 10 (2004) 83.

Au

th

or
's

pe

rs

[4] Y. Chen, R.E. Shepherd, J. Inorg. Biochem. 68 (1997) 183.
[5] A.S. Torsoni, B.F. de Barros, J.C. Toledo, M. Haun, M.H. Krieger, E.
Tfouni, D.W. Franco, Nitric Oxide Biol. Chem. 6 (2002) 247.
[6] F.G. Marcondes, A.A. Ferro, A. Souza-Torsoni, M. Sumitani, M.J. Clarke,
D.W. Franco, E. Tfouni, M.H. Krieger, Life Sci. 70 (2002) 2735.
[7] P.C. Ford, I.M. Lorkovic, Chem. Rev. 102 (2002) 993.
[8] K. Szacilowski, W. Macyk, G. Stochel, Z. Stasicka, S. Sostero, O. Traverso, Coord. Chem. Rev. 208 (2000) 297.
[9] E. Tfouni, M. Krieger, B.R. McGarvey, D.W. Franco, Coord. Chem.
Rev. 236 (2003) 57.
[10] A.K. Patra, M.J. Rose, K.A. Murphy, M.M. Olmstead, P.K. Mascharak,
Inorg. Chem. 43 (2004) 4487.
[11] S. Wecksler, A. Mikhailovsky, P.C. Ford, J. Am. Chem. Soc. 126 (2004)
13566.
[12] R.G. de Lima, M.G. Sauaia, D. Bonaventura, A.C. Tedesco, R.F.V. Lopez,
L.M. Bendhack, R.S. da Silva, Inorg. Chim. Acta 358 (2005) 2643.
[13] P.G. Wang, M. Xian, X.P. Tang, X.J. Wu, Z. Wen, T.W. Cai, A.J. Janczuk,
Chem. Rev. 102 (2002) 1091.
[14] D.R. Lang, J.A. Davis, L.G.F. Lopes, A.A. Ferro, L.C.G. Vasconcellos,
D.W. Franco, E. Tfouni, A. Wieraszko, M.J. Clarke, Inorg. Chem. 39
(2000) 2294.
[15] S.P. Fricker, E. Slade, N.A. Powell, O.J. Vaughan, G.R. Henderson, B.A.
Murrer, I.L. Megson, S.K. Bisland, F.W. Flitney, Br. J. Pharmacol. 122
(1997) 1441.
[16] J. Bourassa, W. DeGraff, S. Kudo, D.A. Wink, J.B. Mitchell, P.C. Ford,
J. Am. Chem. Soc. 119 (1997) 2853.
[17] M.G. Sauaia, F.D.S. Oliveira, R.G. de Lima, A.D.L. Cacciari, E. Tfouni,
R.S. da Silva, Inorg. Chem. Commun. 8 (2005) 347.
[18] J. Bordini, D.L. Hughes, J.D.D. Neto, C.J. da Cunha, Inorg. Chem. 41
(2002) 5410.
[19] C.F. Works, C.J. Jocher, G.D. Bart, X.H. Bu, P.C. Ford, Inorg. Chem. 41
(2002) 3728.
[20] R.M. Carlos, A.A. Ferro, H.A.S. Silva, M.G. Gomes, S.S.S. Borges, P.C.
Ford, E. Tfouni, D.W. Franco, Inorg. Chim. Acta 357 (2004) 1381.
[21] F.D. Oliveira, V. Togniolo, T.T. Pupo, A.C. Tedesco, R.S. da Silva, Inorg.
Chem. Commun. 7 (2004) 160.
[22] J.C. Toledo, L.G.D. Lopes, A.A. Alves, L.P. da Silva, D.W. Franco,
J. Inorg. Biochem. 89 (2002) 267.
[23] C.W.B. Bezerra, S.C. da Silva, M.T.P. Gambardella, R.H.A. Santos,
L.M.A. Plicas, E. Tfouni, D.W. Franco, Inorg. Chem. 38 (1999) 5660.
[24] B.F. de Barros, J.C. Toledo, D.W. Franco, E. Tfouni, M.H. Krieger, Nitric
Oxide Biol. Chem. 7 (2002) 50.
[25] J.A. Silva, J.C. Toledo, M.E.R. Guzzi, D.W. Franco, D. Wilhelm, R.C.
Pedrosa, T.B. Creczynski-Pasa, Free Radical Biol. Med. 33 (2002) S244.
[26] B.R. McGarvey, A.A. Ferro, E. Tfouni, C.W.B. Bezerra, I. Bagatin, D.W.
Franco, Inorg. Chem. 39 (2000) 3577.
[27] D. Bonaventura, F.S. Oliveira, R.S. da Silva, L.M. Bendhack, Clin. Exp.
Pharmacol. Physiol. 32 (2005) 478.
[28] V. Mori, J.C. Toledo, H.A.S. Silva, D.W. Franco, M. Bertotti, J. Electroanal. Chem. 547 (2003) 9.
[29] K.Q. Ferreira, Ph.D. thesis, Universidade de São Paulo, 2004.
[30] E. Tfouni, K.Q. Ferreira, F.G. Doro, R.S. da Silva, Z.N. da Rocha, Coord.
Chem. Rev. 249 (2005) 405.
[31] G. Stochel, A. Wanat, E. Kulis, Z. Stasicka, Coord. Chem. Rev. 171 (1998)
203.
[32] P.C. Ford, J. Bourassa, K. Miranda, B. Lee, I. Lorkovic, S. Boggs, S. Kudo,
L. Laverman, Coord. Chem. Rev. 171 (1998) 185.
[33] L.K. Keefer, Annu. Rev. Pharmacol. Toxicol. 43 (2003) 585.
[34] H.P. Zhang, G.M. Annich, J. Miskulin, K. Stankiewicz, K. Osterholzer,
S.I. Merz, R.H. Bartlett, M.E. Meyerhoff, J. Am. Chem. Soc. 125 (2003)
5015.

on

552