Photochemical reactions of trans-[Ru(NH3)4L(NO)] 3+ complexes

Article abstract of DOI:10.1016/j.ica.2003.11.023

The photochemical behavior of a series of trans-[Ru(NH₃) ₄L(NO)]³⁺ complexes, where L=nitrogen bound imidazole, L-histidine, 4-picoline, pyridine, nicotinamide, pyrazine, 4-acetylpyridine, or triethylphosphite is reported. In addition to ligand localized absorption bands (<300 nm), the electronic spectra of these complexes are dominated by relatively low intensity bands assigned as ligand field (LF) and metal to ligand (d_π→NO) charge transfer (MLCT) transitions. Irradiation of aqueous solutions of these complexes with near-UV light (300-370 nm) labilizes NO, i.e.,trans-[Ru(NH₃)₄L(NO)] ³⁺→hν[Ru(NH₃)₄L(H₂O)] ³⁺+NOQuantum yields for [Ru(NH₃)₄L(H ₂O)]³⁺ formation (φ_Ru(III)) are sensitive to the natures of L, λ_irr and pH. The lowest quantum yields (λ_irr=310 nm) were found for L = imidazole (0.03) and L-histidine (0.04), while much higher values were found for L=P(OEt) ₃ (0.30). Irradiation at longer wavelengths does not induce photochemical reactivity. These results are interpreted in terms of the expected reactivities of d_π→NO MLCT state in these systems.

Full text of DOI:10.1016/j.ica.2003.11.023

Inorganica Chimica Acta 357 (2004) 1381–1388
www.elsevier.com/locate/ica
Photochemical reactions of trans-[Ru(NH3)4L(NO)]3þ complexes q
a
a
a
Rose M. Carlos , Alessander A. Ferro c,1, Hildo A.S. Silva , Maria G. Gomes ,
a
b
c,
a,
*
Simone S.S. Borges , Peter C. Ford , Elia Tfouni *, Douglas W. Franco
a
Instituto de Quꢀımica de S~ao Carlos, IQSC–USP, Av. do Trabalhador S~aocarlense, 400 – CP 780, 135666-590, S~ao Carlos, SP, Brazil
Department of Chemistry and Biochemistry, University of California, Santa Barbara, Santa Barbara, CA 93106-9510, USA
b
c
ꢀ
Departamento de Quimica, FFCLRP–USP, Av. dos Bandeirantes, 3900, 14040 – 901, Ribeir~ao Preto, SP, Brazil
Received 31 October 2003; accepted 22 November 2003
Abstract
The photochemical behavior of a series of trans-[Ru(NH3)4L(NO)]3þ complexes, where L ¼ nitrogen bound imidazole,
L-histi-
dine, 4-picoline, pyridine, nicotinamide, pyrazine, 4-acetylpyridine, or triethylphosphite is reported. In addition to ligand localized
absorption bands (<300 nm), the electronic spectra of these complexes are dominated by relatively low intensity bands assigned as
ligand field (LF) and metal to ligand (dp ! NO) charge transfer (MLCT) transitions. Irradiation of aqueous solutions of these
complexes with near-UV light (300–370 nm) labilizes NO, i.e.,
hm
trans-½RuðNH3Þ LðNOÞꢀ3þ !½RuðNH3Þ LðH2OÞꢀ3þ þ NO
4
4
Quantum yields for [Ru(NH3)4L(H2O)]3þ formation (/RuðIIIÞ) are sensitive to the natures of L, kirr and pH. The lowest quantum
yields (kirr ¼ 310 nm) were found for L ¼ imidazole (0.03) and
L-histidine (0.04), while much higher values were found for
L ¼ P(OEt)3 (0.30). Irradiation at longer wavelengths does not induce photochemical reactivity. These results are interpreted in
terms of the expected reactivities of dp ! NO MLCT state in these systems.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Ruthenium nitrosyl photochemistry; Nitric oxide; Nitrosyl; Nitric oxide donors; Ruthenium ammines
1. Introduction
pressure, as a neurotransmitter and as a toxic agent
formed in immune response to pathogens among other
roles [1]. The initial studies have led to a plethora of
studies concerning other possible biological roles for
NO. Also of interest are the interactions of NO and
related species with biologically important targets, the
search for nitric oxide scavengers, and the design of
compounds for therapeutic NO delivery from different
donors, thermally or through activation with light
[2–17].
It is now well recognized that nitric oxide (also
known as nitrogen monoxide) has important roles in
mammalian biology. It serves as a bioregulator of blood
q Taken in part from: Hildo A.S. Silva, Sc.D. Thesis, Instituto de
´
~
Quımica de Sao Carlos, USP, Sao Carlos, SP, Brazil, 2001. Simone
S.S. Borges, D.Sc. Thesis, Instituto de Quımica de Sao Carlos, USP,
~
´
~
In the last context, the chemical and photochemical
reactions of various metal nitrosyl complexes and of
related species are being investigated [6–17] with the
goal of developing models for NO delivery agents in
pharmaceutical applications. The physiological func-
tions of NO are complicated and depend on thermo-
dynamic, kinetic, and concentration considerations
[2a,2b], and high or low NO concentrations can be ei-
ther beneficial or harmful; depending on the specific
~
Sao Carlos, SP, Brazil, 1995. Maria G. Gomes, D.Sc. Thesis, Instituto
de Quımica de Sao Carlos, USP, Sao Carlos, SP, Brazil, 1995.
~
~
´
´
Alessander A. Ferro, D.Sc. Thesis, Departamento de Quımica,
~
FFCLRP-USP, Ribeirao Preto, SP, Brazil, 2000.
* Corresponding authors. Tel.: +55166023748; fax: +55166331851.
E-mail addresses: ford@chem.ucsb.edu (P.C. Ford), eltfo-
uni@usp.br (E. Tfouni), douglas@iqsc.sc.usp.br (D.W. Franco).
1
^
Present address: Faculdade de Tecnologia e Ciencias de Feira de
Santana, Rua Artemia Pires Freitas, s/n, SIM, 44.115-000, Feira de
^
Santana, BA, Brazil.
0020-1693/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2003.11.023

1382
R.M. Carlos et al. / Inorganica Chimica Acta 357 (2004) 1381–1388
circumstances. Thus, there is a continuing need for
controlled and site specific NO scavengers and NO de-
livery agents. As a consequence, there is considerable
interest in stable compounds that can, with minimal
toxic effects, release NO in vivo. In this context, water
soluble ruthenium am(m)ine nitrosyls provide the basis
for a systematic study and are shown to have tunable
reactivities toward NO release [8–13]. Recent in vivo
experiments demonstrated that these are able to release
NO after being reduced biologically, resulting in blood
pressure suppression. Furthermore, these ruthenium
complexes were less toxic than sodium nitroprusside,
which is an NO donor used as a vasodilator in hyper-
tensive emergencies [15–17].
Although other ruthenium nitrosyl and related com-
plexes have been studied as potential NO delivery agents,
there has been little systematic study of a series of related
compoundsin thisregard. The trans-[Ru (NH3)4L(NO)]3þ
complexes constitute a system where L can be systemati-
cally varied in order to tune the desired NO properties
[8–17]. Such an investigation can contribute to the fun-
damental understanding necessary to design systems with
tunable thermal and photochemical properties for specific
applications.
acidic medium, or by reacting trans-[Ru(NH3)4L-
(H2O)]2þ, generated from the reduction of the corre-
sponding Ru(III) sulfate, with NOþ generated by
NaNO2 in acidic medium, as described elsewhere [9,10].
The new complexes trans-[Ru(NH3)4L(NO)] (BF4)3
(L ¼ 4-acpy and 4-pic) were prepared by both methods,
with 70% yields; Anal. Calc. (Found) for the 4-acpy
complex: C, 13.79 (13.34); N, 13.78 (13.59); H, 3.14
(3.06)%, and for the 4-pic complex: C, 13.0 (13.0); N,
15.2 (14.9); H, 3.46 (3.4)%. The trans-[Ru(NH3)4(P-
(OEt)3)(NO)] (PF6)3 complex was synthesized by direct
reaction of trans-[Ru(NH3)4 (P(OEt)3)(H2O)]2þ with
nitrous acid [19], with 60% yield; Anal. Calc. (Found):
C, 9.00 (8.98); N, 8.75 (8.85); H, 3.37 (3.47); Ru, 12.63
(12.30)%. The electronic spectra of the anticipated
photoproducts trans-[RuIII(NH3)4L(H O)]3þ were ob-
2
tained by reduction of trans-[Ru(NH3)4L(SO4)]þ with
Zn/Hg in aqueous deaerated ꢂ1:0 ꢃ 10ꢄ2 M CF3COOH
solution followed by addition of a slight excess (10–15%
the stoichiometric amount) of hydrogen peroxide, fol-
lowed by pH adjustment to the pH of photolysis.
Potassium trisoxalatoferrate(III) was prepared ac-
cording to Calvert and Pitts [20] for use as a chemical
actinometer.
Preliminary photochemical experiments have shown
that aqueous solutions of certain trans-[Ru(NH3)4L-
(NO)]3þcomplexes are photoactive toward near-UV ex-
citation to give trans-[Ru(NH3)4L(H2O)]3þ and NO as
products [8,9]:
2.2. Instruments
Monochromatic irradiations at 310, 313, 330, 334,
370, 410, 450 and 480 nm were carried out using a 200 W
or a 150 W Xenon lamp in models 68805 or 6253, re-
spectively, Oriel Universal Arc Lamp Source. The irra-
diation wavelengths were selected with Oriel interference
filters for photolysis at the appropriate wavelengths. The
interference filters had an average band pass of 10 nm
and the collimated beam intensities ranged from
4 ꢃ 10ꢄ8 to 1 ꢃ 10ꢄ9 einsteinꢄ1 cmꢄ2 as determined by
ferrioxalate actinometry. The progress of the photore-
actions was monitored spectrophotometrically on a
Hitachi model U-2000 or HP8452A diode array spec-
trophotometers. The ESR frozen aqueous solution
spectra of the reactants and products were recorded on a
Brucker model ESP 300 E EPR spectrometer at 77 K. A
polarographic analyzer/stripping voltammeter Model
264A from Princeton Applied Research (PARC) at-
3þ
trans-½RuðNH3Þ LðNOÞꢀ
4
hm
!trans-½RuðNH3Þ LðH2OÞꢀ3þ þ NO
ð1Þ
4
This article describes the photochemical properties of
the trans-[Ru(NH3)4L(NO)]3þ complexes where L is ni-
trogen bound imidazole (imN), L-Histidine (L-Hist),
4-picoline (4-pic), pyridine (py), 4-acetylpyridine
(4-acpy), pyrazine (pz), nicotinamide (nic), and tri-
ethylphosphite (P(OEt)3).
2. Experimental
2.1. Materials and reagents
ꢀ
tached to a microcomputer employing Microquimica
Ruthenium trichloride (RuCl3 ꢁ nH2O) (Strem or Al-
drich) was the starting material for ruthenium complex
syntheses. HPLC grade solvents were used, except ace-
tone, ethyl ether and ethanol, which were purified ac-
cording to the literature [18]. Doubly distilled water was
used throughout. All other materials were reagent grade.
The Ru(II) complexes trans-[Ru(NH3)4L(NO)]3þ (L ¼
electrochemical software and a model 273 PARC po-
tentiostat/galvanostat were used for the electrochemical
measurements. The electrochemical cells used were of
the conventional three electrode type with an aqueous
saturated calomel electrode (SCE) as a reference elec-
trode and glassy-carbon and Pt wire as working and
auxiliary electrodes, respectively. Emission spectra were
recorded with an Aminco-Bowman spectrofluorimeter
model J4-8960A, with a high-pressure xenon lamp and
an IP28 type photomultiplier. Infrared spectra were
imN, L-Hist, py, nic, and pz) were prepared from trans-
[Ru(NH3)4L(SO4)]þ by the room temperature reaction
of the corresponding sulfate complexes with NaNO2 in

R.M. Carlos et al. / Inorganica Chimica Acta 357 (2004) 1381–1388
1383
recorded in a 1600 FTIR Perkin–Elmer spectropho-
tometer in KBr pellets.
acpy, respectively, falling in the range found for the
other trans-[Ru(NH3)4L(NO)]3þ complexes [8–10].
The electronic absorption spectra of the nitrosyl ru-
thenium ammine complexes, trans-[Ru(NH3)4L(NO)]3þ
,
2.3. Procedures
are quite different from those of analogous ruthenium(II)
complexes [Ru(NH3)5L]2þ and trans-[Ru(NH3)4LL0]2þ
without the nitrosyl ligand [21–23]. The spectra of the
latter complexes show intense metal to ligand charge
transfer (MLCT) absorption bands in the visible when L
and/or L0 are aromatic heterocycles such as pyridines
or pyrazine. However, the spectra of the nitrosyl com-
plexes (e.g., trans-[Ru(NH3)4(4-acpy)(NO)]3þ, Fig. 1) do
not display analogous MLCT bands, even when a pyr-
idine or pyrazine is present in the coordination sphere.
Table 1 summarizes the relevant spectral features of the
complexes studied here. The absorption spectra of all
the trans-[Ru(NH3)4L(NO)]3þ complexes are similar
and consist of several sets of bands, strong ppꢆ ligand
localized transitions below 300 nm, broad bands cen-
tered around 330 nm and ꢂ400–500 nm and, in some
cases, a band at >500 nm. (The spectrum of aqueous
trans-[Ru(NH3)4(pz)(NO)]3þ was previously reported to
have kmax at 302, 468 and 584 nm [10], but reexamina-
tion of the spectrum with a higher concentration showed
the lowest energy band to be centered at 508 nm.)
Theoretical computational studies [11,24] of several
trans-[Ru(NH3)4L(NO)]3þ ions have addressed the
electronic absorption spectra. The seven to ten calcu-
lated transitions involve ligand field (LF) (d–d), ligand-
to-ligand charge transfer (LLCT) (L ! NOþ), internal
ligand (IL) (L ! L), MLCT (Ru dp ! NOþ) and MLCT
(Ru ! L) transitions. The transitions that should be
involved in the photochemistry reported here are cen-
tered around 330 nm and 450 nm. According to the
calculations, absorptions at 300–350 nm involve LF and
Ru dp ! NO MLCT transitions; and that at ꢂ400–500
nm involves a Ru dp ! NO MLCT transition.
Photolysis experiments were carried out at 25.0 ꢅ
0.3 °C in 1.0 cm path length quartz cells capped with a
rubber septum. The solutions (10ꢄ4 mol lꢄ1 initial
complex concentration) were deaerated by bubbling
with argon prior to photolysis and stirred during irra-
diation. The solutions were photolyzed to approxi-
mately 5% reaction to minimize inner filter effects.
Simultaneous dark reactions were carried out with
identical solutions. As analyzed from their UV–Vis
spectra, these samples were stable in the dark on a time
scale longer than that of the photochemical experiments.
Photoproducts were identified by UV–Vis voltam-
metric measurements by comparison with respective
data of separately prepared ruthenium complexes.
Quantitative analyses were made monitoring electronic
spectral changes, and it was assumed that the quantum
yield for appearance of product is equal to that of dis-
appearance of the starting material, consistent with the
stoichiometry indicated in Eq. (1). Quantum yields were
determined from initial spectral changes and were
plotted versus reaction percentage and extrapolated
back to 0% reaction to minimize inner filter effects. The
reported quantum yields are the average of, at least,
three independent experiments.
3. Results and discussion
3.1. Syntheses and spectroscopic properties
The syntheses of the previously unreported complexes
trans-[Ru(NH3)4(4-acpy)(NO)]3þ
(I)
and
trans-
[Ru(NH3)4(4-pic)(NO)]3þ (II) followed the general
procedures described for other trans-[Ru(NH3)4L
(NO)]3þ (L ¼ py, pz, nic,
L-Hist, imN) [9,10]. The
physical properties, such as electrochemical and spec-
tral, are similar to those with L ¼ py, pz, nic,
L-Hist,
imN [8]. Crystallographic structural studies [8] have
confirmed the trans configuration for such complexes
with the ammines in the equatorial plane and the linear
nitrosyl and L in the axial positions and suggest a
nitrosonium character for NO [8]. All are ESR silent,
consistent with a diamagnetic [RuII(NOþ)] core. The
infrared spectra (KBr pellets) of trans-[Ru(NH3)4L
(NO)](BF4)3 each displayed a single mNO band at 1930
and 1934 cmꢄ1 for I and II, respectively, consistent with
the nitrosyl configuration with a NO triple bond. The
reduction potentials of the coordinated nitrosyl in the I
and II complexes, in aqueous solution, were determined
to be –0.108 and +0.002 V versus NHE for 4-pic and 4-
Fig. 1. Absorption spectra of trans-[Ru(NH3)4(4-acpy)(NO)]3þ com-
plex in CF3COOH/CF3COONa solution pH 2.0, l ¼ 0:1 mol lꢄ1. (A)
2:9 ꢃ 10ꢄ4 mol lꢄ1; (B) 1:2 ꢃ 10ꢄ3 mol lꢄ1; (C) 8:7 ꢃ 10ꢄ3 mol lꢄ1
.

1384
R.M. Carlos et al. / Inorganica Chimica Acta 357 (2004) 1381–1388
Table 1
UV–Vis spectral data for trans-[Ru(NH3)4L(NO)]3þ in aqueous solu-
3þ
trans-½RuðNH3Þ LðH2OÞꢀ
4
tiona
2þ
ꢀ trans-½RuðNH3Þ LðOHÞꢀ þ Hþ
ð2Þ
4
L
kmax (nm) (e/L molꢄ1 cm-1)
The EPR spectrum of the photolysis solutions at pH
2 indicates the presence of a paramagnetic photoproduct
(Fig. 2). Although free NO is paramagnetic, its aqueous
solution spectrum does not exhibit an EPR signal [13].
Furthermore, excited states generated in 25 °C solution
would not live long enough to be detected by EPR.
Thus, the paramagnetic species observed in the photo-
lyzed solutions of trans-[Ru(NH3)4L(NO)]3þ must be
trans-[Ru(NH3)4L(H2O)]3þ and this is confirmed by
spectral simulation and comparison with an authentic
sample of this species.
The formation of NO as a photoproduct was con-
firmed using a nitric oxide selective electrode and by
differential pulse polarography (DPP). Fig. 3 shows the
differential pulse polarogram obtained after 330 nm ir-
radiation of trans-[Ru(NH3)4(4-pic)(NO)]3þ. The anodic
peak observed at 0.80 V was assigned to the oxidation of
the released NO. This was not observed when the so-
imNb;c
233 (9300); 324 (1000); 460 (40)
L
-Histb;d
242 (2200); 294sh (3700); 336sh (600); 461 (40)
239 (11 200)g; 262 (3550)g; 325 (220); 460 (20)
231 (9100); 267 (2300); 324 (160); 47 (23)
224 (9400)g; 242 (3500)g; 272 (2500)g; 320 (160); 485
(23)
230 (10 000); 276 (4400); 302 (660)g;468 (40)g; 508
(40); 584 (43)g
4-pice;f
pyb;h
nicb;i;j
pzk;l
4-acpyb;m
P(OEt)3
238 (19 000); 286 (3700); 330 (340); 464 (40)
260 (1400); 316 (216)
a Values previously reported [9,10] were either from experimental
absorption peaks or result of Gaussian deconvolution. Values reported
here are from experimental absorption peaks, except where noted.
b In 0.1 M CF3COOH.
c [Ru] ¼ 4.5 ꢃ 10ꢄ4, 1.7 ꢃ 10ꢄ4 and 4.3 ꢃ 10ꢄ3 mol lꢄ1
.
d [Ru] ¼ 5.4 ꢃ 10ꢄ3 and 1.2 ꢃ 10ꢄ2mol lꢄ1
.
e In CF3COOH/CF3COONa solution pH 3.0, l ¼ 0:1 mol lꢄ1
.
f [Ru] ¼ 9.4 ꢃ 10ꢄ5 and 5.1 ꢃ 10ꢄ3 mol lꢄ1
.
g From Gaussian deconvolution.
h [Ru] ¼ 1.0 ꢃ 10ꢄ4 and 1.0 ꢃ 10ꢄ3 mol lꢄ1
.
i The 224 nm value was reported as doubtful for possible interfer-
ence from electrolyte absorption [9].
j [Ru] ¼ 8.6 ꢃ 10ꢄ5 mol lꢄ1
.
k In pH 3.96 CF3COOH solution.
l [Ru] ¼ 1.0 ꢃ 10ꢄ4, 1 ꢃ 10ꢄ4, 7.3 ꢃ 10ꢄ4, and 8.0 ꢃ 10ꢄ3 mol lꢄ1
.
m [Ru] ¼ 2.9 ꢃ 10ꢄ4, 1.2 ꢃ 10ꢄ3 and 8.7 ꢃ 10ꢄ3 mol lꢄ1
.
3.2. Photochemical studies
All the complexes described are robust in solution
and in the solid state and only undergo NO release
upon photolysis. In a preliminary investigation with
trans-[Ru(NH3)4L(NO)]3þ (L ¼ py and nic), it was
observed that photolyses at higher energy (310–370
nm) light leads to the formation of NO and
[Ru(NH3)4L(H2O)]3þ as products Eq. (1) [9]. Similar
photochemical behavior is found for the other com-
plexes reported here, but irradiation with kirr P 380
nm does not lead to observable photochemistry under
the conditions used (25 °C).
Fig. 2. EPR spectrum of trans-[Ru(NH3)4(py)(NO)]3þ complex in KCl
solution, pH 4.34, l ¼ 0:1 mol lꢄ1 after 4% complex irradiation at
330 nm.
3þ
trans-½RuðNH3Þ LðNOÞꢀ
4
hm
!trans-½RuðNH3Þ LðH2OÞꢀ3þ þ NO
ð1Þ
4
At pH ꢂ6.0, the Ru(III) products are largely in the
hydroxo form Eq. (2). The aqua and hydroxo products
were identified by comparing the UV–Vis spectra, elec-
trochemical and EPR data with those of authentic
samples of previously characterized trans-[Ru(NH3)4
L(OH2)]3þ and trans-[Ru(NH3)4L(OH)]2þ [25]. Forma-
tion of Ru(II) complexes, such as trans-Ru(NH3)4
L(OH2)]2þ, can easily be ruled out, since these would
have strong MLCT bands in the visible range
(e >ꢂ 8 ꢃ 103).
Fig. 3. Differential pulse voltammogram of trans-[Ru(NH3)4(py)-
(NO)]3þ complex in KCl solution, pH 4.34; l ¼ 0:1 mol lꢄ1 at glassy-
carbon electrode with platinum wire as an auxiliary electrode and SCE
as the reference electrode. The voltammogram was taken after 4%
complex irradiation at 330 nm.

R.M. Carlos et al. / Inorganica Chimica Acta 357 (2004) 1381–1388
1385
Table 2
Quantum yield values (/RuIII
lution of trans-[Ru(NH3)4(4-pic)(NO)](PF6)3 was kept
in the dark.
)
for the photolysis of trans-
[Ru(NH3)4L(NO)L]3þ in aqueous solution
Fig. 4 illustrates typical electronic spectral changes
accompanying photolysis of aqueous trans-[Ru(NH3)4(4-
acpy)(NO)]3þ at pH 2. These reactions occur quite
smoothly and photolyses were allowed to proceed up to
the limit of 5% conversion. There is an increase in ab-
sorbance around 330 nm and the spectral changes are
consistent with the formation of trans-[Ru(NH3)4(4-
L
kirr (nm)
pH
URuðIIIÞ
4-pic
310
330
370
410
480
2.0
2.0
2.0
2.0
2.0
0.06 ꢅ 0.01
0.08 ꢅ 0.01
0.04 ꢅ 0.02
<0.001
<0.001
imN
313
330
370
2.13
2.13
2.13
0.042 ꢅ 0.002
0.033 ꢅ 0.004
0.017 ꢅ 0.003
acpy)(H2O)]3þ
.
Thus, extensive data from three independent tech-
niques show that irradiation of aqueous trans-
[Ru(NH3)4L(NO)]3þ gives trans-[Ru(NH3)4L(H2O)]3þ
and NO as the only observable products at pH 2.0, and
25 °C Eq. (1). Increasing the solution pH results in in-
L
-Hist
313
334
370
2.13
2.13
2.13
0.045 ꢅ 0.003
0.039 ꢅ 0.005
0.022 ꢅ 0.002
py
nic
310
330
370
3.0
3.0
4.5
0.17 ꢅ 0.01
0.10 ꢅ 0.01
0.07 ꢅ 0.02
creasing concentrations of trans-[Ru(NH3)4L(OH)]3þ
,
which become dominant about pH 6 owing to the equi-
librium indicated by Eq. (2).
310
334
370
480
2.0
2.0
2.0
2.0
0.07 ꢅ 0.01
0.08 ꢅ 0.01
<0.001
3.3. Quantum yields
<0.001
Table 2 summarizes the quantum yields for the pho-
toreaction depicted in Eq. (1) determined from changes
in the optical spectra of aqueous solutions of, initially,
trans-[Ru(NH3)4L(NO)]3þ, when irradiated with 300–
370 nm light. URuðIIIÞ (/RuðIIIÞ and /NO are assumed to
be the same) is somewhat sensitive to the nature of L,
pz
313
334
370
450
2.13
2.13
2.13
2.0
0.13 ꢅ 0.03
0.10 ꢅ 0.01
0.07 ꢅ 0.01
<0.01
isn
310
330
370
3.0
3.0
3.0
0.04 ꢅ 0.01
0.05 ꢅ 0.01
<0.01
and over the series 4-pic,
L-hist, py, nic, 4-acpy, pz,
P(OEt)3 at pH 2.0 the quantum yields span an order of
magnitude. The lowest quantum yields are found for the
complexes, where L is a strong r-donor such as imN or
4-acpy
P(OEt)3
313
334
370
2.13
2.13
2.13
0.10 ꢅ 0.02
0.07 ꢅ 0.01
0.04 ꢅ 0.02
L
-hist, while the highest URuðIIIÞ observed for
310
2.0
0.30 ꢅ 0.05
L ¼ P(OEt)3, is a good p-acceptor. The URuðIIIÞ values
correlate roughly with the formal Ru(III)/Ru(II) re-
duction potentials for the trans-[Ru(NH3)4L(H2O)]3þ
,
which range from –0.30 V (versus Ag/AgCl) for L ¼
L
-
The photochemical behavior of trans-[Ru(NH3)4
L(NO)]3þ is completely different from that of other
Ru(II) ammines [8,21], for which MLCT and LF
states, the order of which can be tuned, are the lowest
energy excited states (LEES). For the latter complexes,
ligand photosubstitution is the major photochemical
pathway, and this is usually attributed to the ligand
field excited states [21]. Photooxidation of Ru(II) to
Ru(III) has been observed for UV irradiation into
charge transfer to solvent (CTTS) states. However,
with the nitrosyl ruthenium complexes, visualizing
these as RuII(NOþ) complexes as described above
leads to the view that the photoreaction depicted in
Eq. (1) represents photooxidation of the metal center
simultaneous with ligand (NO) labilization.
Hist to +0.46 V for L ¼ P(OEt)3. The quantum yields
are also pH dependent (Table 3).
Among the UV–Vis transitions predicted by earlier
DFT calculations [24], the (dp ! NOþ) MLCT transi-
tions around 330 and 450 nm would appear to be logical
predecessors of the trans-[RuIII(NH3)4L(OH )]3þ and
NO products. While there is also an LF transition around
330 nm which would be expected to be substitution labile,
2
Fig. 4. Spectral changes of the absorption spectrum accompanying
photolysis of trans-[Ru(NH3)4(4-acpy)(NO)]3þ in CF3COOH/CF3-
COONa solution pH 3.0, l ¼ 0:1 mol lꢄ1, kirr ¼ 330 nm.

1386
R.M. Carlos et al. / Inorganica Chimica Acta 357 (2004) 1381–1388
g1-N-bonded complex upon warming [26]. While this
Table 3
pH Dependence of
/
values for the photolysis of trans-
RuðIIIÞ
might have occurred under longer wavelength excitation
of the trans-[Ru(NH3)4L(NO)]3þ ions, such a process
would have likely gone undetected under the present
experimental conditions (25 °C) if it did not lead to li-
gand labilization products.
[Ru(NH3)4L(NO)L]3þ in aqueous solution
L
k
irr/nm
pH
/
RuðIIIÞ
4-pic
330
330
330
330
2.0
3.45
5.0
6.0
0.08 ꢅ 0.01
0.12 ꢅ 0.01
0.26 ꢅ 0.02
0.40 ꢅ 0.01
A particularly interesting observation is the pH de-
pendence of the URuðIIIÞ values recorded for the trans-
[Ru(NH3)4L(NO)]3þ photolyses, where higher solution
pH results in larger quantum yields. We attribute this to
the dissociation of Hþ from the product aqua ion ac-
cording to Eq. (2). Except for trans-[RuIII(NH3)4
py
330
330
330
3.0
4.4
6.4
0.11 ꢅ 0.01
0.13 ꢅ 0.01
0.18 ꢅ 0.01
pz
313
313
313
2.13
3.45
4.32
0.13 ꢅ 0.03
0.17 ꢅ 0.03
0.23 ꢅ 0.03
(P(OEt)3)(OH2)]3þ
,
the pKaÕs of various trans-
[RuIII(NH3)4L(OH2)]3þ complexes fall in the range 4–5,
and are reasonably independent of L [11,27]. For exam-
ple, the pKas of trans-[RuIII(NH3)5(OH2)]3þ (4.1) and
trans-[RuIII(NH3)4(4-pic)(OH2)]3þ (ꢂ4.5) are quite simi-
lar. As the pH increases, a larger fraction of the product is
in the hydroxo form and this is likely to be much less re-
active toward the back reaction with NO than the corre-
sponding aqua product. Thus, the net quantum yield for
NO should increase with pH, as was observed.
isn
330
330
330
3.02
4.63
4.95
0.05 ꢅ 0.01
0.05 ꢅ 0.01
0.07 ꢅ 0.01
4-acpy
313
313
313
2.13
3.45
4.32
0.10 ꢅ 0.02
0.13 ꢅ 0.03
0.19 ꢅ 0.03
L
-Hist
313
313
313
2.13
3.45
4.32
0.045 ꢅ 0.003
0.067 ꢅ 0.004
0.086 ꢅ 0.004
imN
313
313
313
2.13
3.45
4.32
0.042 ꢅ 0.002
0.063 ꢅ 0.003
0.079 ꢅ 0.003
4. Summary
The present study has shown that near-UV irradiation
of trans-[Ru(NH3)4L(NO)]3þ solutions at kirr, corre-
sponding to a Ru dp-NO MLCT transition of these
complexes, leads to NO photoaquation and formation of
the Ru(III) ion trans-[Ru(NH3)4L(H2O)]3þ. Interest-
ingly, this photochemistry is not observed when the
lowest energy MLCT band(s) are excited. The quantum
yields for this process is somewhat dependent on the
nature of L and on the solution pH. Higher NO phot-
olabilization quantum yields were seen for the p-acceptor
ligand P(OEt)3 than for more strongly donating ligands
but the trend, if any, among the other L was not obvious.
The pH dependence of /NO is consistent with a mecha-
nism where the initially formed photoproducts trans-
[Ru(NH3)4L(H2O)]3þ and NO undergo back reactions to
the initial nitrosyl complex in competition terms with
dissociation apart. The effect of higher pH may be to trap
the aqua complex to give a less labile hydroxo complex,
hence facilitating the separation of the products.
the formation of a Ru(III) might be expected to result
from a charge transfer of the type given in Eq. (3). How-
ever, the
hm
L-RuIIðNOþÞ!½L-RuIIIðNO0Þꢀꢆ
ð3Þ
photochemistry depicted in Eq. (1) is only observed for
photoexcitation at kirr < 370 nm, so, if such (dp ! NOþ)
is responsible, then it is only that at higher energy which
serves this purpose. In this context, the calculations [24]
show that the higher energy (dp ! NOþ) MLCT ES is
the result of a transition from the Ru–NO p-bonding Ru
dxz;yz orbitals to the pꢆ NO orbitals. The lower energy
(dp ! NOþ) MLCT ES involves transition from the
2
nonbonding dx2–y2 to the pꢆ NO orbitals. Thus, the
former might be expected to be more dissociative than
the latter as observed. The higher energy excitation
might also provide more impetus to the trajectory for
NO dissociation.
It has been reported that lower energy irradiation
(kirr > 400 nm) of certain Ru nitrosyls leads to the ob-
servation (at T < 0 °C) of metastable linkage isomers
with O- or g2-bonded NO, which decayed back to the
Acknowledgements
The authors thank grants and fellowships from
~
2
Reported calculations of reference 24 for the py and pz complexes
ꢁ
~
Fundaßcao de Amparo a Pesquisa do Estado de Sao
Paulo (FAPESP) (Grant No. 99/07109-9), Conselho
used the following axes orientation: the L–Ru–NO axes is the z axis
and the two NH3-Ru-NH3 bond axes in the equatorial plane are
bisected by the x and y axes, with the L plane in the yz plane. As a
result, in this orientation, the dxy and dxz orbitals are bonding and the
ꢀ
Nacional de Desenvolvimento Cientifico e Tecnologico
ꢀ
~
(CNPq), and Fundaßcao Coordenadoria de Aperfeißcoa-
mento do Pessoal de Ensino Superior (CAPES). Rose
dx
2–y2
orbital is non-bonding.

R.M. Carlos et al. / Inorganica Chimica Acta 357 (2004) 1381–1388
1387
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