Control of NO release by light irradiation from nitrosyl-ruthenium complexes containing polypyridyl ligands

Article abstract of DOI:10.1016/S0020-1693(03)00271-8

The series of nitrosyl complexes cis-[RuL(bpy)₂NO]³⁺ (bpy=bipyridine; L=pyridine, 4-picoline, 4-acetylpyridine) were prepared and studied by laser plash-photolysis. All the studied complexes showed nitric oxide (NO) release under light irradiation at 355 nm account by NO measured made by a NO sensor. The quantum yields for NO release was determined with a laser flash-photolysis apparatus (Continuum Q-switched Nd-YAG laser). The major product of the irradiation of cis-[RuL(bpy)₂(NO)]³⁺ is an intermediate which was suggested to be cis-[Ru^IIIL(bpy) ₂(NO?)]³⁺ that in aqueous solution release NO to produce cis-[RuL(bpy)₂(OH)]²⁺.

Full text of DOI:10.1016/S0020-1693(03)00271-8

Inorganica Chimica Acta 355 (2003) 191ꢀ196
/
www.elsevier.com/locate/ica
Control of NO release by light irradiation from nitrosylꢀ
/
ruthenium
complexes containing polypyridyl ligands
Marilia Gama Sauaia a, Fabiana de Souza Oliveira b, Antonio Claudio Tedesco a,b
Roberto Santana da Silva a,b,
,
*
a Departamento de Qu´ımica da Faculdade de Filosofia Cieˆncias e Letras de Ribeirao Preto, Universidade de Sao Paulo, Av. dos Bandeirantes 3900,
˜
14040-901 Ribeirao Preto, SP, Brazil
˜
b Faculdade de Cieˆncias Farmaceˆuticas de Ribeirao Preto, Universidade de Sao Paulo, Av. do Cafe´ s/n, 14040-903 Ribeirao Preto, SP, Brazil
˜
˜
Received 26 February 2003; accepted 25 April 2003
Abstract
The series of nitrosyl complexes cis-[RuL(bpy)2NO]3ꢁ (bpyꢂ
/
bipyridine; Lꢂpyridine, 4-picoline, 4-acetylpyridine) were
/
prepared and studied by laser plash-photolysis. All the studied complexes showed nitric oxide (NO) release under light irradiation at
355 nm account by NO measured made by a NO sensor. The quantum yields for NO release was determined with a laser flash-
photolysis apparatus (Continuum Q-switched Nd-YAG laser). The major product of the irradiation of cis-[RuL(bpy)2(NO)]3ꢁ is
an intermediate which was suggested to be cis-[RuIIIL(bpy)2(NO+)]3ꢁ that in aqueous solution release NO to produce
cis-[RuL(bpy)2(OH)]2ꢁ
# 2003 Elsevier B.V. All rights reserved.
.
Keywords: Nitric oxide; Nitrosylꢀruthenium complexes; Metalonitrosyl
/
1. Introduction
agents that release NO in a controlled manner in living
systems [6] and could also be potentially developed as
Nitric oxide (NO) has emerged as an important
molecule involved in initiating or regulating numerous
biological processes. The endogenous mediator charac-
ter of NO has been implicated in the range from
regulation of cardiovascular function to participation
therapeutic agents [7ꢀ9].
/
Most of the known NO donor systems involve the
formation of NO by either simple dissociation or by a
complex series of reaction, often requiring metabolism
in the cell [10]. Therefore it’s important to produce
compounds that can be trigged to release NO when
subjected to any external or internal stimulation [11].
One of the possibilities is to use light irradiation in the
appropriated wavelength on metalonitrosyl complexes,
which are compounds potentially capable of release NO
[12]. In this way, a special class of metal complexes,
in memory [1ꢀ4]. Since the initial discovery that the
/
endothelium-derived relaxing factor is NO [5], the main
goal of many scientific researches has been focused on
elucidating the mechanisms that regulate NO synthesis
and understanding the physiological and pathophysio-
logical consequences of biological NO production.
These multiple effects of NO in biological systems
have resulted in intense investigation into the mechan-
isms of NO-mediated events. Because of its short half-
life in biological systems and the inherent challenges of
studying its role in the body, there is a great number of
which has received considerable attention, is ironꢀ
/
sulfurꢀnitrosyl cluster anions that could release NO
/
thermally or photochemically [13]. The photochemical
experiment conducted with classical compounds as
[Fe4S3(NO)7]ꢃ showed relatively small quantum yield
for the NO production. Another group of light sensitive
compounds is the chloro nitrosylꢀruthenium complexes
/
[14]. The [RuCl3NO] and K2[RuCl5NO] are thermally
stable but photolabile, releasing NO under UV light
exposure. The quantum yield for those ruthenium
* Corresponding author. Tel.: ꢁ
1092.
E-mail address: silva@usp.br (R. Santana da Silva).
/
55-166-02 4428; fax: ꢁ55-166-33
/
0020-1693/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0020-1693(03)00271-8

192
M.G. Sauaia et al. / Inorganica Chimica Acta 355 (2003) 191ꢀ196
/
complexes on the irradiation in the range 300ꢀ
/
350 nm
2.2. Chemicals and reagents
RuCl3×3H2O, 2,2?-bipyridine (bpy), py, 4-pic, 4-acpy
were purchased as high purity reagents from Aldrich
Chemicals and potassium ferrioxalate was purchased
from Fisher Scientific Co. and used as supplied. Doubly
distilled H2O was used for all experiments. The recrys-
varies from 0.06 to 0.012 mol einsteinꢃ1. In the course
of probing photochemical NO releaser, a series of
nitrosyl porphyrin complexes has been synthesized and
/
the photochemical pathway described [15ꢀ18]. Among
/
them Hoshino and co-workers reported that NO com-
plexes of Co(II) and Mn(II) meso-tetraphenylporphyr-
ins undergo efficient photodenitrosylation [15,16].
Similar observation has been made by Ford and co-
workers involving [RuII(P)(NO)(NO2)]2ꢁ [17,18] where
P is octaethylporphyrin or tetra(m-tolyl)porphyrin li-
tallized complex salt cis-[RuL(bpy)2(NO)](PF6)3 (Lꢂ
pic, py and 4-acpy) was prepared as previously pub-
lished [20]. The complex salt cis-[RuL(b-
py)2(H2O)](PF6)2 (Lꢂ4-pic, py and 4-acpy) was
prepared as previously described for cis-[Ru(b-
py)2(py)(H2O)](PF6)2 [21]. The cis-[RuL(b-
py)2(NO)](PF6)3 complex was characterized by IR for
Lꢂ
4-pic (nNO 1944 cmꢃ1), Lꢂpy (nNO 1947 cmꢃ1),
Lꢂ Vis for Lꢂ4-
4-acpy (nNO 1943 cmꢃ1) and by UVꢀ
pic (332 nm, log oꢂ3.67; 300 nm log oꢂ4.09), py (334
nm, log oꢂ3.70; 300 nm, log oꢂ4.17), 4-acpy (336 nm,
log oꢂ3.77; 300 nm log oꢂ4.29) in 0.1 M HCl. In the
photochemical experiment the buffer solution was kept
constant with NaTFA/HTFA (TFAꢂtrifluoroacetate)
/4-
/
gand. For those nitrosyl nitritoꢀruthenium complexes
/
they found at least two main pathways involving NO
and NO2ꢃ photolabilisation.
Despite numerous investigations of nitrosyl ruthe-
nium photochemistry, many questions concerning the
electronic effect under photoreactivity are still not
completely understood. The trans-[RuL(NH3)4NO]3ꢁ
for example, showed low NO photolabilisation [19],
although our studies conducted with cis-
[RuCl(bpy)2NO]2ꢁ showed higher quantum yield,
around 0.98 mol einsteinꢃ1 [8]. We believe that the
electronic effect under the nitrosyl group should affect
the NO reactivity. To answer these and many other
/
/
/
/
/
/
/
,
/
/
/
/
/
at pH 2.0 and ionic strength of 0.1 M adjusted with KCl.
2.3. NO measurements with the ISO-NO NO-meter and
the DUO-18 acquisition board
questions we have synthesized
a series of cis-
[RuL(bpy)2NO]3ꢁ complex where L could be pyridine
(py), 4-picoline (4-pic) and 4-acetylpyridine (4-acpy) and
systematically studied the photoreactivity using laser
flash-photolysis coupled to a NO sensor. The p-acceptor
character of the L ligand was taken as main factor in the
nitrosyl compounds, responsible for the observed photo-
reactivity of cis-[RuL(bpy)2NO]3ꢁ. The mechanism
pathway we have been proposed was mainly based on
the experimental observations on the spectroscopy
properties and the chronoamperometry studies using
NO sensor.
NO release was measured with an ISO-NOP NO
meter from World Precision Instruments. 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.
The calibration curve was constructed by dilutions of
NO stock solution. NO gas was purchased from
Oxigeˆnio do Brasil and passed through a 10 M KOH
solution to remove any trace of NO2ꢃ specie. NO stock
solution was prepared by degassing aqueous TFA buffer
solution (pH 2.0) following by the introduction of NO
as previously described [22]. The NO concentrations
were calculated according to reported molar fraction
solubility of NO (1.9 mM at 25 8C) [23], that was
confirmed by titration with KMnO4 as previously
described [24].
2. Experimental
2.1. Apparatus
The ultravioletꢀ
/
visible (UVꢀ
/
Vis) spectra were re-
The 1-cm pathlength quartz cell containing the sample
corded on a Hitachi U-3501 and Genesys-2 apparatus
from Spectronic. Infrared (IR) spectra were recorded on
a protege´ 460 series FT-IR spectrometer. The pH
measurements were made using a 430 pH meter from
Corning. The photolysis was carried out using a laser
flash-photolysis apparatus consisting of a Continuum
Q-switched Nd-YAG laser (Continuum, Santa Clara,
CA) with excitation provided by the third harmonic at
solution was routinely thermostated at 25.090.1 8C.
/
The sample solution was stirred continuously during the
flash-photolysis experiment and the NO measurements
were made with the electrode positioned 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 acquisition board from
WPI.
lꢂ355 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 typically
10 mJ pulseꢃ1 as measured with a Field Master power-
meter with an L-30V head.
2.4. Quantum yield measurements
Light intensities were determined by the classical
potassium ferrioxalate actinometry procedure before

M.G. Sauaia et al. / Inorganica Chimica Acta 355 (2003) 191ꢀ
/
196
193
each photolysis experiments. The NO quantum yields
production were calculated based on NO concentrations
obtained by NO meter measurement. For most runs the
initial concentration of cis-[RuL(bpy)2(NO)](PF6)3 was
[RuL(bpy)2NO]3ꢁ complex. Clearly we have seen that
the photoreactivity of the nitrosyl complex was accom-
panied by a change in the UVꢀVis spectrum, with a
/
decrease in absorption at the 330 nm region, and an
increase in absorption at the 288 and 430 nm regions for
all studied complexes.
approximately 1.4ꢄ
10ꢃ4 M. After equilibration to the
/
cell holder temperature, photolysis was begun by
irradiating the sample for a period of time ranging
from 0 to 30 s with increments of 5 s. The calculated ft
values obtained in the range studies, were plotted versus
t. These plots were linear, with a negative slope, for the
first 40% of the reaction. The extrapolated spectroscopic
The NO release from cis-[RuL(bpy)2NO]3ꢁ during
the photolysis at 355 nm was supported in situ by NO
detection based on the chronoamperogram of NO
release showed in the Fig. 2. Quantitative NO concen-
trations could be estimated from these signals by
comparison to the NO calibration curve. The quantum
yield FNO for cis-[Ru(4-pic)(bpy)2NO]3ꢁ at 355 nm
photolysis are reported in Fig. 3. The calculated
quantum yield for NO release measured after the
irradiation of the series of cis-[RuL(bpy)2NO]3ꢁ com-
quantum yield at tꢂ
photoaquation of NO from cis-[RuL(bpy)2(NO)]3ꢁ
compounds. Evaluation of FNO at tꢂ0 eliminates
possible complications resulting from secondary photo-
lysis of primary reaction product [25ꢀ27].
/
0 was taken as FNO for the
/
/
pounds was 0.179
/
0.02, 0.169
/
0.02, 0.0790.01 for mol
/
einsteinꢃ1 4-pic, py and 4-acpy, respectively.
The primary product for the photolysis of cis-
[RuL(bpy)2NO]3ꢁ involves the formation of an inter-
mediate which the electronic spectrum has showed
dependence on time function. In order to pursue the
details of this reaction, an aqueous solution of cis-
[RuL(bpy)2NO]3ꢁ complex where L is py, 4-pic and 4-
acpy was irradiated continuously in cycles of 20 s at 355
nm light leading to the final photoproduct. The resulting
electronic spectrum shows a band in a visible region that
decreased in time function (Fig. 4).
3. Results
In a previous work we reported a flash-photolysis
study on cis-[RuCl(bpy)2(NO)]2ꢁ in aqueous solution at
room temperature [8]. We concluded therefrom that NO
was dissociated from its ligand site with formation of
cis-[RuCl(bpy)2(OH)]ꢁ. Here we describe complemen-
tary study on the related species cis-[RuL(bpy)2NO]3ꢁ
where L is py, 4-pic and 4-acpy. The photolysis of cis-
[RuL(bpy)2NO]3ꢁ type complex was carried out in
trifluoroacetic buffer solution pH 2.0 to avoid cis-
[RuL(bpy)2NO2]ꢁ formation as a product of nucleo-
philic attack for OHꢃ, as previously described [20]. In
the photochemical experiment, the resulting electronic
spectrum appearing immediately after laser irradiation
at 355 nm as showed in the Fig. 1 for cis-[Ru(4-
pic)(bpy)2NO]3ꢁ is as an example of cis-
4. Discussion
One of the currently investigated problems related to
the electronic structure of nitrosylꢀruthenium com-
/
plexes is the degree of a and p bonding contributions
of co-ligands to NO release in the metal ion coordina-
Fig. 1. Electronic spectra of 1.64ꢄ
/
10ꢃ4
M
cis-[Ru(4-
Fig. 2. Chronoamperogram of NO released by photolysis of 1.64ꢄ
/
pic)(bpy)2(NO)]3ꢁ at pH 2.0 TFA buffer solution during flash-
photolysis with 355 nm light irradiation.
10ꢃ4 M cis-[Ru(4-pic)(bpy)2(NO)]3ꢁ at pH 2.0 TFA buffer solution
using 355 nm light irradiation.

194
M.G. Sauaia et al. / Inorganica Chimica Acta 355 (2003) 191ꢀ196
/
tion reaction is expected to occur considering many of
the reports that present the NO8 specie as a labile ligand
for ruthenium(III) complex [8,19,31ꢀ34]. In that case,
/
we expected that the quantum yield for the NO
production should be higher as stronger is the back-
bonding dp{Ru(II)-p*(NOꢁ)}. Our results indicated a
higher quantum yield for cis-[Ru(4-pic)(bpy)2NO]3ꢁ in
cis-[RuL(bpy)2NO]3ꢁ series for the NO photorealease,
what is consistent with the nature of the p-acceptor
character of ‘L’, which increases as 4-picB
/
pyB4-acpy
/
[20]. These results clearly demonstrate that the electron
density under nitrosyl ligand in cis-[RuL(bpy)2NO]3ꢁ
complex increases in the same order that the p-acceptor
character of ‘L’ decreases. Similar conclusion could be
observed considering electrochemical data previously
described by this class of compounds [20].
Fig. 3. Plot of f(NO) versus time for cis-[Ru(4-pic)(bpy)2(NO)]3ꢁ
during flash-photolysis with 355 nm light irradiation.
In the photodissociation pathway, formation of NO
radical was preceded by an increase and subsequent
decay of absorption at 290 and 430 nm, whose
successive decayings were recorded (Fig. 4). This
may be assigned to the ligand photoisomerization
of
cis-[RuIIL(bpy)2NOꢁ]3ꢁ
producing
cis-
[RuIIL(bpy)2ONꢁ]3ꢁ specie and the absorbance fading
could be interpreted as NOꢁ release with posterior
consumption via various pathways [35]. Although the
NO linkage isomer have not been noticed in aqueous
solutions, some isonitrosyl complexes have been gener-
ated as low temperature photoproducts in the solid state
derived from the corresponding NO complexes [36ꢀ38].
/
The resulting electronic spectrum of the photoproduct
of cis-[RuIIL(bpy)2NOꢁ]3ꢁ after approximately 40 min
shows some similarities with the electronic spectrum of
the cis-[RuIIL(bpy)2H2O]2ꢁ complex (Fig. 5). Although
unequivocal assignment of the substitution was not
Fig. 4. Decay of absorbance in 420 nm region formed after laser flash-
photolysis of cis-[Ru(bpy)2(4-acpy)NO]3ꢁ at pH 2.0 in TFA buffer
solution.
tion sphere [28ꢀ30]. An interesting approach to study
/
this question is based on the synthesis and evaluation of
a series of compounds where the electron density under
nitrosyl ligand was changed by different co-ligand L in
[RuL5NO]nꢁ type complex. In order to understand the
electronic effect under nitrosyl ligand, we have synthe-
sized and performed the photochemistry studies of cis-
[RuL(bpy)2NO]3ꢁ, where L is py, 4-pic and 4-acpy.
The quantum yield of NO release (FNO) for cis-
[RuL(bpy)2NO]3ꢁ decreases as the p-acceptor character
of L increases, which is consistent with the decrease of
electron density under nitrosyl ligand. Assuming that
the metal ligand charge transfer between Ru(II) and
NOꢁ occurs at 330 nm for cis-[RuIIL(bpy)2NOꢁ]3ꢁ
complex, the irradiation in this region could easily
generate cis-[RuIIIL(bpy)2NO+]3ꢁ at excited state.
Fig. 5. Electronic spectra in UVꢀ
Vis region of cis-[RuII(4-
/
pic)(bpy)2(H2O)]2ꢁ (dashed line), cis-[Ru(4-pic)(bpy)2(NO)]3ꢁ after
flash-photolysis at 355 nm (dot-dashed line) and cis-[Ru(4-
pic)(bpy)2(NO)]3ꢁ after approximately 40 min of flash-photolysis at
355 nm (line).
Since the transient is formally RuIIIꢀNO+, the substitu-
/

M.G. Sauaia et al. / Inorganica Chimica Acta 355 (2003) 191ꢀ
/
196
195
possible, the electronic spectrum of the photoproduct
suggesting the decay occurs by a chemical reaction.
The overall light-induced process can be represented
by Scheme 1.
The assumption of cis-[RuIII(OH)L(bpy)2]2ꢁ forma-
tion from the photolytic product of cis-
[RuL(bpy)2(NO)]3ꢁ is based mainly on the electroche-
mical behavior of cis-[RuIIL(bpy)2(H2O)]2ꢁ. The results
of differential pulse voltammetry on aqueous solution
containing cis-[RuIIL(bpy)2(H2O)]2ꢁ for the expected
Ru(III)/(II) couple are pH-dependent. The dependence
of the potentials as a function of pH are shown
in the Epaꢀ
/
pH diagram in Fig.
6
for cis-
[RuII(py)(bpy)2(H2O)]2ꢁ as an example of cis-
[RuL(bpy)2(NO)]3ꢁ compounds, where L was 4-pic,
py and 4-acpy.
Fig. 6. pH potential regions of stability for different oxidation states
of cis-[Ru(H2O)(bpy)2(py)]ꢁ complex. The vertical lines in the various
E versus pH regions show the pKa values.
In the diagram (Fig. 6) the linear relation shows how
the potential of Ru(III)/(II) varies with pH. The line is
drawn according to the Nernst equation with slope of 0
or 59 mV/pH unit corresponding to proton to electron
ratios 0 and 1.0. This pH dependence allows one to
obtain the pKa values for the cis-[RuIIL(bpy)2(H2O)]2ꢁ
specie. The observed pKa for the cis-[RuL(bpy)2H2O]2ꢁ
complex where L is py, 4-pic and 4-acpy is 10.6, 11.2,
11.5 and for cis-[RuL(bpy)2H2O]3ꢁ it is 1.6, 1.2 and 1.4,
respectively. Controlled potential oxidation of cis-
[RuIIL(bpy)2(H2O)]2ꢁ in a trifluoroacetic buffer solu-
tion pH 2.0 yields a specie with no absorption in visible
region which is consistent with its formulation as cis-
suggested based on the monitoring NO with NO sensor
during light irradiation. The quantum yield observed for
the NO formation involved in the photolysis of cis-
[RuL(bpy)2(NO)]3ꢁ decreases with the p acceptor
character of L increase. It is consistent with the increase
of electron density under NOꢁ.
Acknowledgements
[RuIIIL(bpy)2(OH)]2ꢁ
.
The authors thank the Brazilian agencies CAPES and
FAPESP for their financial support.
5. Conclusion
We have shown that the photolysis of cis-
[RuL(bpy)2(NO)]3ꢁ complex where L is py, 4-pic and
4-acpy produces NO when it is irradiated with laser at
355 nm. The major product of the irradiation of cis-
[RuL(bpy)2(NO)]3ꢁ is an intermediate which was
suggested to be cis-[RuIIIL(bpy)2(NO+)]3ꢁ that in
aqueous solution releases NO to produce cis-
[RuL(bpy)2(OH)]2ꢁ. The photochemical pathway was
References
[1] T.M. Dawson, V.L. Dawson, S.H. Synder, Ann. Neurol. 32
(1992) 297.
[2] P.L. Feldman, O.W. Griffith, D.J. Stuehr, Chem. Eng. News 20
(1993) 26.
[3] L.J. Ignarro, Pharm. Res. 6 (1989) 651.
[4] S. Moncada, R.M.J. Palmer, E.A. Higgs, Pharmacol. Rev. 43
(1991) 109.
[5] L.J. Ignarro, G.M. Buga, K.S. Wood, R.E. Byrns, G. Chaudhuri,
Proc. Natl. Acad. Sci. USA 84 (1987) 9265.
[6] D.W. Godwin, D. Che, D.M. O’Malley, Q. Zhou, J. Neurosci.
Methods 73 (1997) 91.
[7] D.J. Sexton, A. Muruganandam, D.J. McKenney, B. Mutus,
Photochem. Photobiol. 59 (1994) 463.
[8] V. Togniolo, R.S. Silva, A.C. Tedesco, Inorg. Chim. Acta 316
(2001) 7.
[9] P.G. Wang, M. Xian, X. Tang, X. Wu, Z. Wen, T. Cai, A.J.
Janczuk, Chem. Rev. 102 (2002) 1091.
[10] A.R. Butler, C. Glidwele, Chem. Soc. Rev. 16 (1987) 361.
[11] A. Wieraszko, M.J. Clarke, D.R. Lang, L.G.F. Lopes, D.W.
Franco, Life Sci. 68 (2001) 1535.
[12] P.C. Ford, I.M. Lorkovic, Chem. Rev. 102 (2002) 993.
[13] F.W. Flitney, I.L. Megson, J.L.M. Thomson, G.D. Kennovin,
A.R. Butler, Br. J. Pharmacol. 117 (1996) 1549.
Scheme 1.

196
M.G. Sauaia et al. / Inorganica Chimica Acta 355 (2003) 191ꢀ196
/
[14] T.D. Carter, N. Bettache, D. Ogden, Br. J. Pharmacol. 122 (1997)
971.
[27] D.A. Chaisson, R.E. Hintze, D.H. Stuemer, J.D. Petersen, D.P.
McDonald, P.C. Ford, J. Am. Chem. Soc. 94 (1976) 6665.
[28] J.N. Abelson, M.I. Simon, Methods in Enzimology, Academic
Press, California, 1996, p. 96.
[15] M. Hoshino, M. Kogure, J. Phys. Chem. 93 (1989) 5478.
[16] M. Hoshino, S. Arai, M. Yamaji, Y. Hama, J. Phys. Chem. 90
(1986) 2109.
[29] S.S.S. Borges, C.V. Davanzo, E.E. Castellano, J.Z. Schepector,
S.L. Silva, D.W. Franco, Inorg. Chem. 37 (1998) 2670.
[30] M.G. Gomes, C.U. Davanzo, S.C. Silva, L.G.F. Lopes, P.S.
Santos, D.W. Franco, J. Chem. Soc., Dalton Trans. 94 (1998) 601.
[31] C.W.B. Bezerra, S.C. Silva, M.T.P. Gambardella, R.H.A. Santos,
L.M.A. Plicas, E. Tfouni, D.W. Franco, Inorg. Chem 38 (1999)
5660.
[17] I.M. Lorkovic, K.M. Miranda, B. Lee, S. Bernhard, J.R. Schoon-
over, P.C. Ford, J. Am. Chem. Soc. 120 (1998) 11674.
[18] P.C. Ford, J. Bourassa, K. Miranda, B. Lee, I. Lorkovic, S.
Boggs, S. Kudo, L. Laverman, Coord. Chem. Rev. 171 (1998)
185.
[19] M.G. Gomes, C.U. Davanzo, S.C. Silva, L.G.F. Lopes, P.S.
Santos, D.W. Franco, J. Chem. Soc., Dalton Trans. 4 (1998)
601.
[32] C.F. Works, C.J. Jocher, G.D. Bart, X.H. Bu, P.C. Ford, Inorg.
Chem. 41 (2002) 3728.
[33] E. Tfouni, Coord. Chem. Rev. 196 (2000) 281.
[20] M.G. Sauaia, R.S. Silva, Trans. Metal. Chem. 28 (2003) 254.
[21] B.A. Moyer, T.J. Meyer, Inorg. Chem. 20 (1981) 436.
[22] D.A. Wink, J.F. Darbyshire, R.W. Nims, J.E. Saavedra, P.C.
Ford, Chem. Res. Toxicol. 6 (1993) 23.
[34] C.F. Works, P.C. Ford, J. Am. Chem. Soc. 122 (2000) 7592.
[35] W.H. Koppenol, in: J.N. Abelson, M.I. Simon (Eds.), Thermo-
dynamics of Reactions Involving Nitrogen-Oxigen Compounds,
[23] S. Kudo, J.L. Bourassa, S.E. Boggs, Y. Sato, P.C. Ford, Anal.
Biochem. 247 (1997) 193.
vol. 268, Academic Press, London, 1996, pp. 7ꢀ12.
/
[36] K. Szacilowski, J. Oszajca, A. Barbieri, A. Karocki, Z. Sojka, S.
Sostero, R. Boaretto, Z. Stasicka, J. Photochem. Photobiol. A:
Chem. 143 (2001) 99.
[24] V. Mori, M. Bertotti, Analyst 125 (2000) 1629.
[25] R.P. Waine, Principles and Applications of Photochemistry,
Oxford Science Publication, New York, 1991, p. 8.
[26] A.W. Adamson, W.L. Waltz, E. Zinato, D.W. Watts, P.D.
Fleischauer, R.D. Lindholm, Chem. Rev. 68 (1968) 541.
[37] T. Wondimagegn, A. Ghosh, J. Am. Chem. Soc. 123 (2001) 5680.
[38] P. Coppens, I. Novozhilova, A. Kovalevsky, Chem. Rev. 102
(2002) 861.