Photo-induced nitric oxide release from chlorobis(2,2′-bipyridine)nitrosylruthenium(II) in aqueous solution

Article abstract of DOI:10.1016/S0020-1693(01)00370-X

We report here a photochemical study of cis-[RuCl(bpy)₂(NO)](PF₆)₂ in aqueous solution with flash-photolysis at 355 nm light. The spectroelectrochemistry and pH effect were also determined. NO was measured in situ with an NO sensor. Light intensities were determined by ferrioxalate actinometry. The quantum yields for NO release were determined with a laser flash-photolysis apparatus (Continuum Q-switched Nd-YAG laser). The photochemistry of cis-[RuCl(bpy)₂(NO)](PF₆)₂ in aqueous solution released NO with laser at 355 nm. The mechanism of the photochemical pathway of cis-[RuCl(bpy)₂(NO)](PF₆)₂ was similar in aqueous and non-aqueous solution, in contrast to previously published results. We found that the best description for the photolysis product of cis-[RuCl(bpy)₂(NO)]²⁺ in aqueous solution at pH 5.7 was cis-[Ru(III)Cl(bpy)₂(OH)]⁺.

Full text of DOI:10.1016/S0020-1693(01)00370-X

www.elsevier.nl/locate/ica
Inorganica Chimica Acta 316 (2001) 7–12
Photo-induced nitric oxide release from
chlorobis(2,2%-bipyridine)nitrosylruthenium(II) in aqueous solution
Vanessa Togniolo a, Roberto Santana da Silva a,*, Antonio Claudio Tedesco b
a Faculdade de Cieˆncias Farmaceˆuticas de Ribeira˜o Preto, Uni6ersidade de Sa˜o Paulo, A6. do Cafe´ S/N, 14040-903 Ribeira˜o Preto, SP, Brazil
b Departamento de Qu´ımica, Faculdade de Filosofia, Cieˆncias e Letras de Ribeira˜o Preto, Uni6ersidade de Sa˜o Paulo, A6. dos Bandeirantes,
3900, 14040-901 Ribeira˜o Preto, SP, Brazil
Received 2 September 2000; accepted 16 January 2001
Abstract
We report here a photochemical study of cis-[RuCl(bpy)2(NO)](PF6)2 in aqueous solution with flash-photolysis at 355 nm light.
The spectroelectrochemistry and pH effect were also determined. NO was measured in situ with an NO sensor. Light intensities
were determined by ferrioxalate actinometry. The quantum yields for NO release were determined with a laser flash-photolysis
apparatus (Continuum Q-switched Nd-YAG laser). The photochemistry of cis-[RuCl(bpy)2(NO)](PF6)2 in aqueous solution
released NO with laser at 355 nm. The mechanism of the photochemical pathway of cis-[RuCl(bpy)2(NO)](PF6)2 was similar in
aqueous and non-aqueous solution, in contrast to previously published results. We found that the best description for the
photolysis product of cis-[RuCl(bpy)2(NO)]2+ in aqueous solution at pH 5.7 was cis-[Ru(III)Cl(bpy)2(OH)]+. © 2001 Elsevier
Science B.V. All rights reserved.
Keywords: Photochemistry; Ruthenium complexes; Nitrosyl complexes; Nitric oxide complexes
tion of nitrosyl ruthenium complexes, we have studied
the photochemistry of cis-[RuCl(bpy)2(NO)](PF6)2 in
1. Introduction
aqueous solution. Some physical chemistry [16–21] and
Nitric oxide (NO) has been implicated in many phys-
photochemistry [20,22] properties of this complex have
iological and pathophysiological processes [1–6] and
been described. The light irradiation at 313 nm of
has been shown to have many bioregulatory functions
including cardiovascular control, neuronal signaling,
and as an agent for defense mechanisms against mi-
croorganisms and tumors [2,7]. These recent discoveries
have stimulated interest in the chemistry and biochem-
istry of NO and derivatives such as metal nitrosyl
complexes. Therefore, it is not surprising that the use of
cis-[RuCl(bpy)2(NO)]+ in acetonitrile (acn) [20] essen-
tially produces cis-[RuCl(bpy)2(acn)]2+ while the
product obtained by irradiation with 254–332 nm light
[22] in aqueous solution was cis-[RuCl(bpy)2(NO2)]+.
We report here a study of the photochemistry with
flash-photolysis of cis-[RuCl(bpy)2(NO)]+ in aqueous
solution and we propose a different mechanism to
metalonitrosyl complexes as agents potentially capable
account for the observations made with an NO sensor,
of releasing NO in vivo has recently become a very
spectroelectrochemistry and by the pH effect.
active area of research [8–11]. One strategy would be to
employ a precursor that displays relatively low thermal
reactivity but is photochemically active to yield NO
2. Experimental
when subjected to electronic excitation [12–15].
For a better understanding of the photoreactions
resulting from metal-to-ligand charge transfer excita-
2.1. Apparatus
The ultraviolet–visible (UV–Vis) spectra were
recorded on a Hitachi U-3501 and Genesys-2 apparatus
* Corresponding author. Fax: +55-16-633 1092.
E-mail address: silva@usp.br (R. Santana da Silva).
0020-1693/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0020-1693(01)00370-X

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V. Togniolo et al. / Inorganica Chimica Acta 316 (2001) 7–12
from Spectronic. Infrared (IR) spectra were recorded
on a protege´ 460 series FT-IR spectrometer. Cyclic
voltammetry (CV) was performed with a Microqu´ımica
model MQPG 01 potentiostat coupled to a computer
employing a conventional three electrode cell with a
platinum disk working electrode, a platinum wire auxil-
iary electrode, and an AgCl/Ag reference electrode.
Some electrochemical studies were also conducted using
a glassy carbon working electrode. Solutions contained
1×10−3 M of the ruthenium complexes. The pH mea-
surements were made using a 430 pH meter from
Corning. The potential measurements were made using
a platinum combined with an AgCl/Ag electrode cou-
pled to a DMPG pH meter from Digimed. Preparative
photochemistry was carried out using a photoreactor
equipped with a 250-W lamp and irradiated for 2 h.
Quantum yields were determined using a laser flash-
quartz sample solution cell was thermostated at 25.09
0.1°C. The sample solution was stirred continuously
during the flash-photolysis experiment.
Quantum yields (bt) were measured spectroscopi-
cally according to published procedures [25,26]. The bt
values were plotted versus time (t in s) and the plot was
extrapolated to t=0. These plots proved to be linear,
with a negative slope, for the first 10% part of the
reaction. The extrapolated spectroscopic quantum yield
at t=0 was taken as bNO for the photoaquation of NO
from cis-[RuCl(bpy)2(NO)]+. Evaluation of bNO at
t=0 eliminates possible complications resulting from
secondary photolysis of the primary reaction product
[25,27].
2.4. NO measurements with the ISO-NO NO-meter
and the DUO-18 acquisition board
photolysis apparatus consisting of
a Continuum
Q-switched Nd-YAG laser (Continuum, Santa Clara,
CA) with excitation provided by the third harmonic at
u=355 nm. The pulse length was 8 ns, the beam
diameter incident on the sample was 6 mm, and the
repetition rate was 10 Hz. The pulse energy was typi-
cally 5 mJ as measured with a Field Master powermeter
with an L-30V head. NO release was measured with an
ISO-NOP NO meter from Word Precision Instruments.
The calibration curve was constructed as specified in
the instruction manual.
The sensitivity of this apparatus ranges from 1 nM to
20 mM, with a 2 mm sensor which directly detects NO
concentration by an amperometric technique. As de-
scribed in Section 2, the setup was calibrated daily as
specified in the instructions manual. During the laser
flash-photolysis measurements the electrode was posi-
tioned outside the light path to avoid any photoelectric
interference. The output of the sensor was recorded
with an IBM-PC computer linked to a DUO-18 acqui-
sition board from WPI.
2.2. Chemicals and reagents
3. Results
The recrystallized complex salts cis-[Ru(bpy)2Cl2]·
2H2O and cis-[RuCl(bpy)2(NO)](PF6)2 (bpy=2,2%-
bipyridine) were prepared by published procedures
[6,7]. The cis-[RuCl(bpy)2(NO)](PF6)2 complex was
characterized by IR (cm−1) w(NO) 1940. UV–Vis (323
nm, log m=4.09; 294 nm, log m=4.23 in acetonitrile [7];
and 323 nm, log m=3.95; 294 nm, log m=4.21 in
aqueous solution) and CV (E1/2= −0.215 and Epc=
−1.02 V versus ferrocinium/ferrocene in 0.1 M tetra
n-butylammonium hexafluorophosphate dissolved in
acn). Anal. Found: C, 31.27; N, 9.14; H, 1.74. Calc. for
C20H16N5OClF12P2Ru: C, 31.23; N, 9.11; H, 2.08%.
[Ru(NH3)6]2+ was obtained by reduction of
[Ru(NH3)6]Cl3 in aqueous solution, purchased from
Aldrich, with Zn/Hg under an argon atmosphere and
used as reducing agent [23]. The cis-[RuCl(bpy)2-
(H2O)]+ complex was obtained in situ by a published
procedure [24]. In the photochemical experiment the
buffer solution was kept constant with Na2HPO4/
NaH2PO4 at pH 5.7 and ionic strength of 0.1 M.
The quantum yields reported are for photolysis at
355 nm. The results were obtained based on the elec-
tronic spectrum of the cis-[RuCl(bpy)2(NO)]2+ com-
plex that has been previously discussed [19]. The
photoreactivity of the nitrosyl complex was accompa-
nied by a change in UV–Vis spectrum, with a decrease
in absorption at 329 nm, an increase in absorption at
294 nm, and the appearance of a new band at 436 nm
(Fig. 1).
The NO release from cis-[RuCl(bpy)2(NO)]2+ during
photolysis at 355 nm was quantitatively analyzed. The
observed quantum yield obtained spectroscopically was
0.9890.12 mol einstein−1 at 436 nm for product for-
mation. The description of NO release was supported
by in situ NO detection when aqueous cis-[Ru-
Cl(bpy)2(NO)]2+ solutions were photolyzed (Fig. 2).
The photolysis product of cis-[RuCl(bpy)2(NO)]2+
was considered to be cis-[RuCl(bpy)2(OH)]+, an assign-
ment consistent with the acid effect on the UV–Vis
spectrum of the complex (Fig. 3).
2.3. Spectroscopic quantum yield measurements
NO release from cis-[RuCl(bpy)2(NO)]2+ can also be
obtained by electrochemical or chemical reduction. The
product obtained for the reduction process in aqueous
solution was attributed to the formation of cis-[Ru-
Light intensities were determined by ferrioxalate acti-
nometry after each photolysis run. The 1-cm pathlength

V. Togniolo et al. / Inorganica Chimica Acta 316 (2001) 7–12
9
Cl(bpy)2(H2O)]+, inferred by comparison to the synthe-
sized complex [24]. The spectroelectrochemistry of cis-
[RuCl(bpy)2(NO)]2+ in aqueous solution clearly
showed the formation of cis-[RuCl(bpy)2(H2O)]+ after
the application of a potential of −0.80 V versus AgCl/
Ag electrode (Fig. 4).
in non-aqueous solution due the low solubility of this
complex in aqueous medium. Since we are interested in
ruthenium nitrosyl complexes, we decided to study
some physical chemistry and photochemistry properties
of cis-[RuCl(bpy)2(NO)]2+ in aqueous medium. The
355 nm light irradiation of cis-[RuCl(bpy)2(NO)]2+ in a
buffer solution at pH 5.7 produced NO release (Fig. 1).
Apparently the photochemical results observed in the
present study differ from those published before [22],
probably due to different experimental conditions. The
assumption that cis-[RuCl(bpy)2(NO2)] is a photolytic
product of cis-[RuCl(bpy)2(NO)]2+ was taken as an
explanation for the photochemical reaction, when the
4. Discussion
The physical chemistry, kinetics [16–21] and photo-
chemistry [20,22] of cis-[RuCl(bpy)2(NO)]2+ have been
the subject of several studies, most of them carried out
Fig. 1. Electronic spectra of 1.0×10−3 M cis-[RuCl(bpy)2(NO)]2+ at pH 5.7 phosphate buffer solution during flash-photolysis with 355 nm light
irradiation.
Fig. 2. Chronoamperogram of NO released by photolysis of 1×10−3 M cis-[RuCl(bpy)2(NO)]2+ at pH 5.7 phosphate buffer solution using 355
nm light irradiation.

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V. Togniolo et al. / Inorganica Chimica Acta 316 (2001) 7–12
Fig. 3. Electronic spectra of 5.5×10−4 M cis-[RuCl(bpy)2(NO)]2+. Solid line, before photolysis at pH 5.7 phosphate buffer solution; dashed line,
after photolysis with UV light irradiation; dotted line, after adding H3PO4 to the photolyzed solution up to pH 1.0.
Fig. 4. Spectroelectrochemical reduction of a 5.5×10−4 M solution of cis-[RuCl(bpy)2(NO)]2+ in aqueous 0.1 M NaBF4 forming cis-[Ru-
Cl(bpy)2(H2O)]+
.
solution was irradiated with 254–334 nm light [22].
This interpretation was based on the electronic spec-
trum of cis-[RuCl(bpy)2(NO)]2+ in basic medium,
which has been reported to be similar to the electronic
spectrum obtained after irradiation of cis-[Ru-
Cl(bpy)2(NO)]2+ [22]. In the flash-photolysis experi-
ment described here we found a compound which
showed an electronic spectrum with a band in a region
similar to that of cis-[RuCl(bpy)2(NO2)]. For a com-
plete description of the photochemical pathway we
studied the pH effect under the electronic spectrum of
cis-[RuCl(bpy)2(NO)]2+ and compared the results ob-
tained with those of the flash-photolysis experiment. In
fact the electronic spectrum of the cis-[Ru-
Cl(bpy)2(NO2)] complex can be obtained by the reac-
tion with base. The spectral changes accompanying this
reaction are shown in Fig. 5. The acid–base equi-
librium in the nitrosyl–nitro intercoversion of cis-
[RuCl(bpy)2NO)]2+ can be written as shown in
Scheme 1.
Closely similar results were previously reported for
the cis-[Ru(NH3)(bpy)2(NO)]2+ complex [28]. The
spectrophotometric titration curve of the cis-[Ru-
Cl(bpy)2(NO)]2+ complex showed that above pH 6.0
the nitro form was present in significant amounts at
equilibrium. The overall electronic spectrum of the
product showed bands at 296, 324 and 436 nm (Fig. 5),
which was different from the electronic spectrum ob-

V. Togniolo et al. / Inorganica Chimica Acta 316 (2001) 7–12
11
Fig. 5. Absorption spectra of 5.5×10−4 M cis-[RuCl(bpy)2(NO)]2+ in aqueous solution for the following pH values: 3.5, 6.0, 9.0, 10.5, and 12.0.
5. Conclusions
tained for the flash-photolysis product of cis-[Ru-
Cl(bpy)2(NO)]2+ (Fig. 1). The assumption of NO re-
lease for the photolytic product of cis-[RuCl-
(bpy)2(NO)]2+ observed in the present study can also
be inferred based on the spectroelectrochemistry experi-
ment (Fig. 4). The reduction of cis-[RuCl(bpy)2-
(NO)]2+ caused NO release that probably occurred as
shown in Scheme 2.
The UV–Vis spectrum of the reduction product of
cis-[RuCl(bpy)2(NO)]2+ is consistent with the attribu-
tion of cis-[RuCl(bpy)2(H2O)]+, which was also ob-
tained from cis-[RuCl2(bpy)2] as starting material [24].
The reduction of the photolysis product, which was
considered to be cis-[RuCl(bpy)2(OH)]+, was essen-
tially cis-[RuCl(bpy)2(H2O)]+, which is consistent with
the pKa of ruthenium(III) species in comparison to
ruthenium(II) species [29].
The photochemistry of cis-[RuCl(bpy)2(NO)]2+ in
aqueous solution indeed released NO when the com-
pound was irradiated with laser at 355 nm. The in situ
monitoring of NO during the laser shot can be used to
demonstrate this photochemical reaction. The photo-
chemical pathway of cis-[RuCl(bpy)2(NO)]2+ by flash-
photolysis can be written in a way similar to that for
cis-[RuCl(bpy)2(NO)]2+ in non-aqueous solution [20],
considering the release of NO.
Scheme 1.
Based on the photolysis studies, we can infer the
photochemical pathway shown in Scheme 3.
The photolysis of cis-[RuCl(bpy)2(H2O)]+ and cis-
[RuCl(bpy)2(H2O)]2+ carried out in a preparative way
did not show any change in the electronic spectrum of
the starting material after 2 h of irradiation.
The in situ NO monitoring during photolysis (Fig. 2)
is an indubitable way to prove the photochemical path-
way [30,31]. The signal recorded by the NO sensor rose
quickly when photolysis was initiated, then decreased
when the light was turned off owing to NO consump-
tion via various pathways, principally autooxidation
[32]. A similar experiment was run with the blank
solution. The absence of the signal shows that the result
observed and illustrated in Fig. 2 is consistent with NO
release.
Scheme 2.
Scheme 3.

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V. Togniolo et al. / Inorganica Chimica Acta 316 (2001) 7–12
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