Mechanism and oxidation state involved in the nitric oxide (NO) photorelease in a terpyridine-bipyridine-based ruthenium nitrosyl complex

Article abstract of DOI:10.1016/j.ica.2018.05.038

A new nitric oxide (NO^[rad]) donor of formula [Ru^II(MeO-Phtpy)(bpy)(NO)](PF₆)₃, where MeO-Phtpy is the 4′-(4-methoxyphenyl)-2,2′:6′,2″-terpyridine and bpy the 2,2′-bipyridine, has been synthesized and fully characterized. The X-ray crystal structure of the complex is provided together with that of its photoproduct of formula [Ru^II(MeO-Phtpy)(bpy)(MeCN)](NO₃)₂. The electronic properties have been investigated theoretically using the time dependent density functional theory (TD-DFT) approach. They reveal that the low-lying electronic transitions exhibit a strong charge transfer character towards the withdrawing nitrosyl unit, which suggest good NO^[rad] donating capabilities under irradiation performed on a large frequency range. The photorelease of NO^[rad] has been investigated in acetonitrile at two different wavelengths, λ = 365 and 436 nm, leading to related quantum yields equal to 0.08 and 0.03, respectively. The evolution of the oxidation states of the species involved within the release process is tracked by EPR and ¹H NMR spectroscopy. It is confirmed that nitrosyl is released as NO^[rad] radicals while the related ruthenium(III) photoproduct is readily reduced to ruthenium(II). The mechanism involved in the release is discussed on the basis of the present experimental data and the TD-DFT analysis.

Full text of DOI:10.1016/j.ica.2018.05.038

Accepted Manuscript
Research paper
Mechanism and oxidation state involved in the nitric oxide (NO) photorelease
in a terpyridine-bipyridine-based ruthenium nitrosyl complex
Valerii Bukhanko, Pascal G. Lacroix, Isabelle Sasaki, Marine Tassé, Sonia
Mallet-Ladeira, Zoia Voitenko, Isabelle Malfant
PII:
S0020-1693(18)30081-1
https://doi.org/10.1016/j.ica.2018.05.038
ICA 18289
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
23 January 2018
3 May 2018
28 May 2018
Please cite this article as: V. Bukhanko, P.G. Lacroix, I. Sasaki, M. Tassé, S. Mallet-Ladeira, Z. Voitenko, I. Malfant,
Mechanism and oxidation state involved in the nitric oxide (NO) photorelease in a terpyridine-bipyridine-based
ruthenium nitrosyl complex, Inorganica Chimica Acta (2018), doi: https://doi.org/10.1016/j.ica.2018.05.038
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____________________________________________________________________________
Mechanism and oxidation state involved in the nitric oxide (NO)
photorelease in a terpyridine-bipyridine-based ruthenium nitrosyl complex
Valerii Bukhankoa,b, Pascal G. Lacroixa*, Isabelle Sasakia, Marine Tasséa, Sonia Mallet-Ladeira,
Zoia Voitenkob, Isabelle Malfanta*
a CNRS, LCC (Laboratoire de Chimie de Coordination), 205 route de Narbonne, BP44099,
Toulouse Cedex 4, F-31077 France.
b Department of Chemistry, Taras Shevchenko National University of Kyiv, Volodymyrska Street,
64/13, Kyiv 01601, Ukraine.
__________________________________________________________________________________
ABSTRACT
A new nitric oxide (NO·) donor of formula [RuII(MeO-Phtpy)(bpy)(NO)](PF6)3, where MeO-Phtpy is
the 4’-(4-methoxyphenyl)-2,2’:6’,2’’-terpyridine and bpy the 2,2’-bipyridine, has been synthesized
and fully characterized. The X-ray crystal structure of the complex is provided together with that of its
photoproduct of formula [RuII(MeO-Phtpy)(bpy)(MeCN)](NO3)2. The electronic properties have been
investigated theoretically using the time dependent density functional theory (TD-DFT) approach.
They reveal that the low-lying electronic transitions exhibit a strong charge transfer character towards
· donating capabilities under irradiation
the withdrawing nitrosyl unit, which suggest good NO
·
performed on a large frequency range. The photorelease of NO has been investigated in acetonitrile at
two different wavelengths, λ = 365 and 436 nm, leading to related quantum yields equal to 0.08 and
0.03, respectively. The evolution of the oxidation states of the species involved within the release
process is tracked by EPR and 1H-NMR spectroscopy. It is confirmed that nitrosyl is released as NO·
radicals while the related ruthenium(III) photoproduct is readily reduced to ruthenium(II). The
mechanism involved in the release is discussed on the basis of the present experimental data and the
TD-DFT analysis.
__________________________________________________________________________________
Keywords: Ruthenium complexes, Nitric oxide, NO release, DFT computations.
* Corresponding authors.
E-mail adresses pascal.lacroix@lcc-toulouse.fr
isabelle.malfant@lcc-toulouse.fr
1

1. Introduction
The last three decades have witnessed an increasing interest for nitric oxide (NO·), which has
gradually been recognized to play a key role in various physiological processes such as blood
regulation, immune response, neurotransmission, respiration and cytotoxic activity in tumor cells by
apoptosis [1,2]. Nevertheless, the toxicological properties of NO· do not have to be unrecognized, and
moreover its therapeutic effect strongly depends on its concentration [3-5]. In this context, exogenous
·
NO donors have become widely investigated, but their relevance has to be evaluated on the basis of
their ability to deliver NO· locally and quantitatively, in order to avoid undesirable effects on
untargeted cells. Among potential candidates, various metal-nitrosyl complexes have been envisioned
[6], Nevertheless, ruthenium nitrosyl complexes have been recognized as the most promising
·
candidates, in relation to their generally low toxicity, good stability and capability of releasing NO
under light irradiation in the λ = 300-600 nm domain, exclusively [7,8]. Therefore, they could be
successfully applied under two-photon irradiation in the 600-1200 nm “therapeutic window” of
relative transparency of biological tissues [9], taking advantage of the noninvasive and highly
controllable characteristics of light, according to the following equation:
ꢒꢓ ꢀ
ꢉ
ꢃꢃꢃ
ꢀ
ꢃꢃꢄ ꢇꢈꢉ
ꢁꢂ ꢅꢆ
+ ꢊꢋꢌꢍꢎꢏꢐꢑ ꢔꢕ ꢁꢂ ꢄꢊꢋꢌꢍꢎꢏꢐꢇ + ꢑꢅꢆ·
(1)
In the search towards alternative NO· donors, we have recently reported on cis(Cl,Cl)-[RuII(R-
Phtpy)Cl2(NO)](PF6) and trans(Cl,Cl)-[RuII(R-Phtpy)Cl2(NO)](PF6) (Scheme 1), where R-Phtpy
stands for 4’-(4-R-phenyl)-2,2’:6’,2’’-terpyridine (R = NO2, H, Br, MeO) [10]. This investigation led
to the conclusion that the cis(Cl,Cl) derivatives always exhibit quantum yields of NO· photorelease
(φNO) at 365 nm larger than those of the related trans(Cl,Cl) derivatives with typical values of 0.3 and
0.1 for cis(Cl,Cl) and trans(Cl,Cl), respectively. More surprisingly, our attempts to correlate the φNO
values with the donating/withdrawing character of R were unsuccessful contrary to our first intuition
that increasing the donating capabilities of the ligand would enhance the charge transfer character of
·
the electronic transitions towards the withdrawing nitrosyl unit and likely encourage the NO release.
In the present contribution, we report on the new [RuII(MeO-Phtpy)(bpy)(NO)](PF6)3
derivative, in which bpy stands for 2,2’-bipyridine (Scheme 1). MeO-Phtpy has been selected within
the R-Phtpy series because it brings an overall donor-acceptor (“push-pull”) electronic charge transfer
character to the complex, in relation to the expectation of additional two-photon capabilities. Indeed,
releasing bioactive molecules under two-photon irradiation has become a powerful tool in life sciences
by virtue of its low damage effects, high spatial selectivity and deep penetration into biological tissues
2

[11,12]. Along this line, push-pull electronic systems usually possess sizeable two-photon capabilities
and therefore attract an additional interest in research domain where drugs delivery is to be considered.
In addition, a bipyridine ligand has been selected instead of two chloride ligands for the following
reasons:
- The presence of chlorides may lead to undesirable ligands exchanges in biological media,
while a bipyridine provides a more robust molecular structure.
- The presence of cis(Cl,Cl)- and trans(Cl,Cl)- isomers in the reaction mixture leads to a
tedious purification task required to isolate the pure isomers, while the corresponding complex
containing a bipyridine necessarily gives a single product.
- the only product available with the bipyridine in that having the nitrosyl ligand perpendicular
to the terpyridine plane, which corresponds to the cis(Cl,Cl)- derivative. It has invariably be
found to possess a larger φNO value than that of the trans(Cl,Cl)- derivative.
· release capabilities
Ruthenium nitrosyl complexes with NO
Scheme 1
After the report of the synthesis and full characterization of [RuII(MeO-
Phtpy)(bpy)(NO)](PF6)3, its crystal structure will be described with that of the photoproduct
-
-
[RuII(MeO-Phtpy)(bpy)(MeCN)](NO3)2, in which the PF6 counter ions are replaced by NO3 . The
experimental UV-vis spectrum of [RuII(MeO-Phtpy)(bpy)(NO)](PF6)3 will be compared to the
computed data obtained by the time-dependent density functional theory (TD-DFT) in order to
3

· release properties will
identify the nature of the transitions involved in the electronic spectra. The NO
be presented as the quantum yields (φNO) measured at various wavelengths in order to target the
contribution of the different transitions and to provide insights towards the real mechanism involved in
the release process. The issue of the oxidation state of the ruthenium atom (either II or III) in the
photoproduct will be addressed eventually.
2. Experimental
2.1. Starting materials and equipment
4'-(4-methoxyphenyl)-2,2':6',2''-terpyridine was synthesized as previously reported [13]. 2,2'-
bipyridine (TCI), triethylamine (Sigma-Aldrich), sodium nitrite (Alfa-Aesar), and RuCl3.3H2O
(STREM) were used without further purification. The solvents were analytical grade and used without
further purification. Elemental analyses were performed at LCC with a Perkin Elmer 2400 serie II
1
Instrument. H NMR and 13C NMR spectra were obtained at 298K in CD3OD or CD3CN used as
internal references and were recorded on a Bruker Avance 400 spectrometer. The UV-vis spectra were
obtained on a Hewlett Packard 8454A spectrometer. Electron paramagnetic resonance experiments
(EPR) were performed on a Bruker ESP 500E spectrometer. The following setting were employed for
the measurements: microwave power, 20mW, field modulation amplitude, 0.1 mT; field modulation
frequency, 100kHz; microwave frequency, 9.782512GHz. N–methyl-D-glucamine dithiocarbamate
previously synthetized reacted with Mohr salts to get [Fe(MGD)2] [14]. 90 µl of 1mM of [RuII(MeO-
Phtpy)(bpy)(NO)](PF6)3 in acetonitrile were mixed with 10 µl of an aqueous solution of 2mM
[Fe(MGD)2] and injected into quartz capillaries. Samples were irradiated directly in the EPR cavity.
The light source was a 250 W Oriel Hg lamp (Palaiseau, France). The light was passed through an
Oriel WG 400 UV filter (Palaiseau,France, λ > 400 nm) and delivered via an optical fiber to the grid
of the cavity”. Voltammetric measurements were carried out using a potentiostat Autolab
PGSTAT100. The experiments were performed at room temperature in a homemade airtight three-
electrode cell connected to a vacuum/argon line. The reference electrode consisted of a saturated
calomel electrode (SCE) separated from the solution by a bridge compartment. The counter electrode
was a platinum wire of ca. 1 cm2 apparent surface. The working electrode was a Pt microdisk (0.5 mm
diameter). The supporting electrolyte (nBu4N)[PF6] (Fluka, 99% puriss electrochemical grade) was
used as received and simply degassed under argon. Acetonitrile of CHROMASOLVs grade (gradient
grade) was obtained from Aldrich. The solution concentrations used for the electrochemical studies
were typically 10-3 M in complex and 0.1 M in the supporting electrolyte. Before each measurement,
the solutions were degassed by bubbling Ar and the working electrode was polished with a polishing
4

machine (Presi P230). The irradiations were carried out with a 150 W Xenon lamp for at least one
hour. The release of NO· was followed by the disappearance of the reduction waves of the starting
complex at 0.44 V [10]. The scan rate in cyclic voltammetry experiment was 100 mV/s.
2.2. Synthetic procedure
The synthetic route towards the desired complex [RuII(MeO-Phtpy)(bpy)(NO)](PF6) is summarized in
Scheme 2. The assignment for the 1H- and 13C-NMR spectra corresponds to the atom labeling used in
the X-ray crystal structure of [RuII(MeO-Phtpy)(bpy)(NO)](PF6)3.
Scheme 2
Synthetic route for the synthesis of [RuII(MeO-Phtpy)(bpy)(NO)](PF6).
2.2.1. Synthesis of [RuIII(MeO-Phtpy)(Cl)3] (A). 4'-(4-methoxyphenyl)-2,2':6',2''-terpyridine (250 mg,
0.74 mmol) was dissolved in EtOH (96%, 48 mL) upon slight heating. RuCl3·xH2O (40-49% Ru, 192
mg, 0.76 mmol) was then added. The mixture was refluxed for 3 hours, protected from light. The
mixture was cooled down to room temperature and the volume of the solution was reduced to 15-20
mL by evaporation of ethanol under reduced pressure. The mixture was left in a freezer for 2 hours
and then the solid was filtrated. The precipitate was rinsed thoroughly with cold water until the water
becomes colorless, and finally washed with cold ethanol (4-5 mL) and ether. The resulting solid was
dried in vacuum and obtained as a black powder. Yield: 335 mg (83%). Elemental analysis found: C,
48.83; H, 3.05; N 7.60%. C22H17Cl3N3ORu requires C, 48.32; H, 3.13; N, 7.68%.
2.2.2. Synthesis of [RuII(MeO-Phtpy)(bpy)Cl](Cl) (B). [RuIII(MeO-Phtpy)(Cl)3] (335 mg, 0.61 mmol),
2,2'-bipyridine (96 mg, 0.62 mmol), LiCl (146 mg, 3.5 mmol), and triethylamine (0.138 mL, 1 mmol)
were dissolved in 56 mL of 75% EtOH/25% H2O mixture. The solution was refluxed for 3 hours,
5

protected from light, and led to the formation of an undesirable precipitate. The reaction mixture was
then filtered hot and the solid washed with EtOH. The filtrates were combined and the mixture was
concentrated under vacuum to 10 mL and then left in a freezer for 2 hours. The precipitate was filtered
and washed with chilled 3M HCl, until the filtrate is colorless. It was then washed with 2 ml of
acetone and 5 ml of ether. The compound was dried in vacuum and obtained as a violet powder. Yield:
390 mg (95%). Elemental analysis found: C, 58.05; H, 3.69; N 10.38%. C32H25Cl2N5ORu requires C,
57.58; H, 3.77; N, 10.49. 1H NMR (CD3OD, 400 MHz): δ (ppm) 8.75 (d, J = 5.5 Hz, 1H, H-1), 7.31 (s,
2H, H-19, H-20), 7.26 (d, J = 7,5 Hz, 1H, H-4), 7.09 (d, J = 8.0, 2H, H-15, H-23), 6.97 (d, J = 8.9 Hz,
1H, H-7), 6.81 (td, J = 7.9, 1.6 Hz, 1H, H-3), 6.59 (m, 2H, H-28, H-32), 6.51 (ddd, J = 7.2, 5.6, 1.2
Hz, 1H, H-8), 6.34 (td, J = 7.9, 1.5 Hz, 2H, H-14, H-24), 6.19 (m, 3H, H-12, H-26, H-2), 5.87 (d, J =
6.1 Hz, 1H, H-10), 5.79 (ddd, J = 7.1, 5.5, 1.2 Hz, 2 H, H-13, H-25), 5.62 (m, 2H, H-29, H-31), 5.51
(ddd, J = 7.3, 5.7, 1.3 Hz, 1H, H-9), 2.42 (s, 3H, H-33).
2.2.3. Synthesis of [RuII(MeO-Phtpy)(bpy)(NO2)](Cl) (C). [RuII(MeO-Phtpy)(bpy)Cl](Cl) (390 mg,
0.58 mmol), and NaNO2 (402 mg, 5.83 mmol), were dissolved in 72 mL of 75% EtOH/25% H2O
mixture. The solution was treated like for A, but concentrated to 10 mL before being placed in the
freezer. The precipitate was washed with cold H2O, then 1-2 mL of acetone and 5 ml of ether. D is
obtained as a red powder. Yield: 357 mg (90%). Elemental analysis found: C, 57.12; H, 3.63; N
12.27%. C32H25ClN6O3Ru requires C, 56.68; H, 3.72; N, 12.39. 1H NMR (CD3OD, 400 MHz): δ (ppm)
8.39 (d, J = 5.7 Hz, 1H, H-1), 7.27 (m, 3H, H-19, H-20, H-4), 7.04 (m, 3H, H-15, H-23, H-7), 6.82 (td,
J = 7.9, 1.5 Hz, 1H, H-3), 6.54 (m, 3H, H-28, H-32, H-8), 6.42 (td, J = 7.9, 1.6 Hz, 2H, H-14, H-24),
6.34 (td, J = 7.9, 1.5 Hz, 1H, H-2), 6.27 (dd, J = 5.5, 1.5 Hz, 2H, H-12, H-26), 5.96 (m, 1H, H-10),
5.85 (ddd, J = 7.2, 5.4, 1.3 Hz, 2H, H-13, H-25), 5.63 (m, 3H, H-29, H-31, H-9), 2.42 (s, 3H, H-33).
13C NMR (CD3OD, 400 MHz): δ (ppm) 163.24, 160.23, 159.21, 158.25, 157.41, 154.09, 153.66,
151.74, 149.77, 139.13, 138.59, 138.43, 130.14, 129.70, 128.64, 128.26, 127.59, 125.08, 125.03,
124.45, 120.88, 115.97, 56.05.
[RuII(MeO-Phtpy)(bpy)(NO)](PF6)3 (D). [RuII(MeO-Phtpy)(bpy)(NO2)](Cl) (50 mg, 0.074 mmol) was
dissolved in 8 mL of ethanol, and the solution was then added to a mixture of 12M HCl (8 mL, 0.096
mol) with ethanol (16 mL). The mixture was stirred for 1 hour at 60oC, protected from light. The
mixture was then concentrated under vacuum to 6 ml and NH4PF6 (200 mg, 1.2 mmol in 1 mL of
water) was added. The solution was left in a freezer for 2 hours. The precipitate was collected by
filtration and was washed with cold EtOH (0.5 mL) and ether. The compound was dried in vacuum
and obtained as an orange powder. Yield: 42 mg (54%). IR: νmax (cm−1) 1940 (N–O). Elemental
analysis found: C, 36.66; H, 2.30; N 7.84%. C32H25F18N6O2P3Ru requires: C, 36.21; H, 2.37; N 7.92%.
1H NMR (CD3CN, 400 MHz): δ (ppm) 9.32 (d, J = 5.5 Hz, 1H, H-1), 9.04 (s, 2H, H-19, H-20), 8.86
(d, J = 8.0 Hz, 2H, H-15, H-23), 8.81 (d, J = 8.3 Hz, 1H, H-4), 8.69 (m, 1H, H-3), 8.62 (d, J = 8.0 Hz,
6

1H, H-7), 8.47 (t, J = 7.8 Hz, 2H, H-14, H-24), 8.28 (m, 4H, H-28, H-32, H-2, H-8), 8.02 (d, J = 5.4
Hz, 2H, H-12, H-26), 7.71 (t, J = 6.4 Hz, 2H, H-13, H-25), 7.50 (t, J = 6.7 Hz, 1H, H-9), 7.33 (m, 3H,
H-29, H-31), 3.99 (s, 3H, H-33). 13C NMR (CD3CN, 400 MHz): δ (ppm) 164.84 (C-30), 159.08 (C-
19), 157.73 (C-16, C-22), 155.75 (C-12, C-26), 155.29 (C-6), 154.56 (C-17, C-21), 154.36 (C-5),
153.94 (C-1), 148.97 (C-10), 145.61 (C-8), 145.24 (C-14, C-24), 144.88 (C-3), 131.58 (C-2), 131.53
(C-28, C-32), 131.12 (C-9), 131.06 (C-13, C-25), 128.49 (C-15, C-23), 127.91 (C-4), 127.52 (C-27),
127.32 (C-7), 124.51 (C-18, C-20), 116.47 (C-29, C-31), 56.67 (C-33).
2.3. X-ray analysis
Single crystals of [RuII(MeO-Phtpy)(bpy)(NO)](PF6)3 were grown as follows: 5 mg of
powdered [RuII(MeO-Phtpy)(bpy)(NO)](PF6)3 were dissolved in 2 mL of acetonitrile. The filtered
solution transferred into a vial was placed in a large beaker in the presence of a flask, filled with a
large amount of diethyl ether, and let aside in the dark. Slow diffusion of diethyl ether into the solution
of acetonitrile led to the appearance of several single crystals suitable for X-ray analysis, within a few
days. Single crystals of the photoproduct were obtained as follows: [RuII(MeO-
Phtpy)(bpy)(NO)](PF6)3 (11 mg) in solution in deuterated acetonitrile (1 mL) was irradiated with an
home-made set of LED-lamp in the λ = 350-450 nm range. The completeness of the irradiation was
controlled by NMR tracing of the evolution in solution. The resulting solution was transferred into a
vial and single crystals were grown following the procedure previously described for [RuII(MeO-
Phtpy)(bpy)(NO)](PF6)3.
Data were collected at low temperature (100(2) K) on a Bruker Kappa Apex II diffractometer
equipped with a 30 W air-cooled microfocus, using MoKα radiation (λ = 0.71073 Å), and an Oxford
Cryosystems Cryostream cooler device. Phi- and omega- scans were used for data collection. An
empirical absorption correction with SADABS was applied [15]. The structure was solved by intrinsic
phasing method (SHELXT) [16]. All non-hydrogen atoms were refined anisotropically by means of
least-squares procedures on F² with SHELXL [17]. All the hydrogen atoms were refined isotropically
at calculated positions using a riding model with their isotropic displacement parameters constrained
to be equal to 1.5 times the equivalent isotropic displacement parameters of their pivot atoms for
terminal sp3 carbon and 1.2 times for all other carbon atoms. In both crystal structures, the counter-
anions were disordered. For [RuII(MeO-Phtpy)(bpy)(NO)](PF6)3, one unique hexafluorophosphate
anion was disordered over two positions in a 74:26 ratio. But for the [RuII(MeO-
Phtpy)(bpy)(MeCN)](NO3)2 photoproduct, the nitrate anions were highly disordered over three
positions in a ratio 48:22:30 and 40:29:3, and were difficult to model reliably. Furthermore, the
anisotropic displacement parameters were restrained by DELU, SIMU and ISOR commands. CCDC
1818137-1818138 contains the supplementary crystallographic data for this paper.
7

2.4. Computational studies
[RuII(MeO-Phtpy)(bpy)(NO)]3+ was optimized using the Gaussian-09 program package [18]
within the framework of the Density Functional Theory (DFT). The double-ζ basis set 6-31G* was
used for all atoms except the heavy ruthenium atom, for which the LANL2DZ basis set was applied to
account for relativistic effects [19]. To be consistent with our previous investigations on
Ru(terpyridine)(NO) complexes [20], and in agreement with a previous investigation of ruthenium-
nitrosyl by Mascharak [21], we have selected the hybrid functional B3PW91 for the optimization.
B3PW91 has been shown to outperform other hybrid functionals (e.g. B3LYP) and pure functionals
(e.g. PW91) in numerous cases of ruthenium complexes, especially when back bonding ligands (like
NO) are present [21, 22]. The vibrational analyses were performed at the same level to verify that the
stationary point corresponds to a minimum on the potential energy surfaces. The UV-vis electronic
spectrum was computed using various standard time-depending TD-DFT methods (B3LYP, CAM-
B3LYP, B3PW91, and PBE0). Solvent effects were included by using the polarizable continuum
model (PCM) implemented in Gaussian09 for acetonitrile (ε = 35.688) both at the optimization and
UV-vis spectrum level. Finally, the CAM-B3LYP functional was selected for its good efficiency to
reproduce experimental transition energies. Molecular orbitals were plotted with GABEDIT 2.4.8.
[23]. The optimized geometries are provided in supporting information.
2.5. Photochemical investigations
Photochemistry: Kinetic studies on the photolysis reactions were carried out with a diode array
Hewlett Packart 8454A spectrophotometer. Solutions of 2 mL of [RuII(MeO-Phtpy)(bpy)(NO)](PF6)3
(3.64.10-5 mol.L-1 for irradiation at 365 nm and 4.42.10-5 mol.L-1 for irradiation at 436 nm) in non-
deoxygenated acetonitrile were used. The optical fiber was fixed laterally from the cuvette. Absorption
spectra were taken after each minute, in fast scan mode, during a period of 80 minutes and 7 hours for
irradiations at 365 nm and 436 nm, respectively, which allows reaching apparent stable absorption
conditions. The UV-vis spectra were recorded under irradiation realized with a Muller reactor device
equipped with a cooling water filter and a mercury arc lamp equipped with appropriate interference
filter to isolate the desired irradiation wavelength (λmax = 365, 436 nm). The temperature was
maintained at 25°C during the whole experiment.
Quantum yield measurements: Light intensities were determined before each photolysis experiments
by chemical actinometry procedure. The actinometers used were potassium ferrioxalate to λirr=365 nm
(I0 = 8.2.10-7 mol.L-1.s-1) and λirr = 436 nm (I0 = 7.81.10-7 mol.L-1.s-1). The quantum yield (φΑ) for
8

[RuII(MeO-Phtpy)(bpy)(NO)](PF6)3 was determined by the program Sa3.3 written by D. Lavabre and
V. Pimienta [24]. It allows the resolution of the differential equation (2):
ꢀ
ꢉ = −ΦꢙꢑIꢚꢗ ꢑ= −ΦꢙꢑAbsꢗꢛ IꢜF
(2)
ꢖ ꢗ
ꢖꢘ
where ꢑIꢚꢗ is the intensity of the light absorbed by the precursor; Absꢗꢛ , the absorbance before
irradiation; Absꢝꢛꢞꢘ, the total absorbance; I0, the incident intensity measured at 365 or 436 nm; and F,
the photokinetic factor given by equation (3):
ꢩ
ꢢꢣꢤꢥ
ꢦꢧꢨ
ꢪ
F = ꢟ
(3)
ꢠꢡꢠꢜ
ꢩ
ꢗꢫꢬ
ꢦꢧꢨ
The equation was fitted with the experimental data Absꢝꢛꢞꢘ = ꢭꢄꢐꢇ and 2 parameters φΑ and εΒ (εΒ is
the molar extinction coefficient measured at the end of the reaction) at two wavelengths (λirr=365 nm,
λobs = 470 nm; λirr = 436 nm, λobs = 370 nm). λobs was chosen because it corresponds to a large
difference between molar extinction coefficient at the initial and final time of the photochemical
reaction. Simulation and optimization procedures were performed by using numerical integration and
a non-linear minimization algorithm for the fitting of the model to the experimental data [24,25].
When λirr = 365 nm:
= 3.64ꢑ10ꢡꢮmol. Lꢡꢠ,εꢙꢯꢰꢮ = 18874ꢑmolꢡꢠ. L. cmꢡꢠꢑ,εꢙꢱꢲꢜ = 7335ꢑmolꢡꢠ. L. cmꢡꢠꢑ,
ꢀ ꢉ
A
ꢜ
ꢑεꢯꢳꢰꢮ = 5244ꢑmolꢡꢠ. L. cmꢡꢠꢑ, εꢳꢱꢲꢜ = 17672ꢑmolꢡꢠ. L. cmꢡꢠ
ꢑ
When λirr = 436 nm:
= 4.42ꢑ10ꢡꢮmol. Lꢡꢠ,εꢙꢱꢯꢰ = 11292ꢑmolꢡꢠ. L. cmꢡꢠꢑ,εꢙꢯꢲꢜ = 18735ꢑmolꢡꢠ. L. cmꢡꢠꢑ,
ꢀ ꢉ
A
ꢜ
εꢱꢳꢯꢰ = 13412ꢑmolꢡꢠ. L. cmꢡꢠꢑ, εꢳꢯꢲꢜ = 5554ꢑmolꢡꢠ. L. cmꢡꢠ
ꢑ
3. Results and discussion
3.1. Synthesis and characterization
In our first report on ruthenium-nitrosyl complexes with substituted terpyridine, the Ru(NO)
fragment was introduced in one step by means of the [RuIICl5(NO)]2- reagent [20a]. By contrast, in the
present synthetic route, the ruthenium atom is introduced first from the reaction of RuIIICl3, followed
9

-
by that of the nitrosyl from the reaction of a nitrite (NO2 ) switched to NO+ in acidic medium. This
alternative route avoids the production of a large amount of the undesirable homoleptic [RuII(terpy)2]2+
species observed with the [RuIICl5(NO)]2- reagent.
The use of TEA in the second step of the synthesis (Scheme 2) is justified to facilitate the
reduction of RuIII to RuII.
1
The reactions towards C and D are easily monitored by H-NMR spectroscopy, from the
observation of the shift of one proton located in position 6 on the bipyridine (H(1) linked to C(1) in
Figure 1), spatially close to the monodentate ligands (Cl, NO2 or NO). Indeed, the distances between
H(1) and atoms N(1) and O(1) in complex D measured from the present crystallographic data are
equal to 2.595(1) Å and 2.719(1) Å, respectively. It is therefore submitted to the deshielding influence
of the substituent. Thereby, switching from -Cl to -NO2 substituent and subsequently to -NO leads to
the peak-shifting of the proton in position 6 from 10.26 ppm (in MeOD) to 9.90 ppm (in MeOD) and
finally to 9.32 ppm (in CD3CN).
Applying two-dimensional NMR-techniques HMBC, HMQC and COSY allows the complete
1
assignment of the peaks observed in H- and 13C- NMR spectra for the final complex D. The
magnitude of the stretching vibration frequency of NO is equal to 1940 cm-1, very close to the ν(NO)
value of 1942 cm-1 previously reported in a Ru(NO) complex having similar donor-substituted-
terpyridine bipyridine coordination sphere [20b], but significantly higher than the values reported in
Ru(NO) complexes having the terpyridine-Cl2 coordination sphere (1894 cm-1 and 1901 cm-1 [20a];
1891 cm-1 and 1892 cm-1 [10]). This tendency for higher ν(NO) frequencies when a bipyridine is
present in the coordination sphere could be related to the π-backbonding character of the Ru-bpy
interaction, leading to a decrease of the electron density on the antibonding molecular orbital of the
NO-ligand. No evolution of the ν(NO) band (frequency and intensity) is observed in solid state which
indicates that the compound is sensitive to light only in solution.
3.2. Crystal structures
[RuII(MeO-Phtpy)(bpy)(NO)](PF6)3 crystallizes in the P-1 triclinic space group. Crystal data
are gathered in Table 1. The asymmetric unit cell is shown in Figure 1, with the atom-labeling scheme
-
employed. It is built up from one [RuII(MeO-Phtpy)(bpy)(NO)]3+ cations, three PF6 anions and an
additional molecule of acetonitrile. There is no packing observed between the ligands (either
terpyridine or bipyridine). The coordination spheres (main interatomic distances and angles) of the
ruthenium atoms are found nearly identical in both crystal structures (Table S1 in supplementary
10

-
materials). The nitrosyl ligand is slightly interacting with two PF6 anions through short contacts O(1)
– F(9) = 2.767(6) Å, and O(1) – F(9) = 2.873(6) Å.
Table 1
Crystal data and structure refinement parameters for [RuII(MeO-Phtpy)(bpy)(NO)](PF6)3, and
[RuII(MeO-Phtpy)(bpy)(MeCN)](NO3)2
______________________________________________________________________________
[RuII(MeO-Phtpy)(bpy)(NO)](PF6)3
[RuII(MeO-Phtpy)(bpy)(MeCN)](NO3)2
______________________________________________________________________________
Formula
Formula weight
Crystal size/mm3
Crystal system
Space group
T/K
C32H25N6O2Ru, 3(F6P), C2H3N
1102.61
C34H28N6ORu, 2(NO3), C2H3N
802.77
0.20×0.20×0.02
triclinic
0.08×0.06×0.06
monoclinic
P-1
P21/c
100(2)
100(2)
Radiation
a/Å
MoKα (λ = 0.71073 Å)
10.149(2)
MoKα (λ = 0.71073 Å)
11.6612(10)
17.7203(13)
17.7839(13)
90
b/Å
11.626(2)
c/Å
17.718(3)
α/°
77.537(6)
β/°
83.961(7)
86.138(8)
2027.9(6)
2
95.375(3)
γ/°
V/Å3
90
3658.7(5)
Z
4
Absorpt. coef./mm-1
0.632
0.489
F(000)
1096
1640
θ range
2.02 to 33.14°
1.754 to 26.416
hkl
-15 ≤ h ≤ 15
-17 ≤ k ≤ 17
-27 ≤ l ≤ 27
105491
-14 ≤ h ≤ 13
-22 ≤ k ≤ 22
-20 ≤ l ≤ 22
62466
Collected reflections
unique reflections
Rint
15447
7523
0.0219
0.0643
a
R1 [I > 2sigma(I)]
0.0202
0.0842
wR2b [I > 2sigma(I)]
GOF(F2)
0.0553
0.2499
1.053
1.069
______________________________________________________________________________
a
ꢴ ꢶ
ꢶ
ꢶ
ꢶ ꢹꢴꢶ ꢶ
ꢵ
ꢵ
ꢁꢠ =
ꢷꢜ − ꢷꢸ
ꢷꢜ ꢑ
b
ꢻ
ꢻ
ꢀꢴꢀ
ꢻꢉꢹꢴꢀꢄ ꢜꢻꢇꢻꢉꢉ
ꢺꢷ
ꢠ/ꢻꢑ
ꢺꢁꢻ =ꢑ
ꢺꢄꢷꢜ − ꢷꢸ ꢇ
11

Fig. 1 Asymmetric unit for [RuII(MeO-Phtpy)(bpy)(NO)](PF6)3 and atom-labeling scheme. Displacement
ellipsoids are drawn at the 50 % probability level, and hydrogen atoms are omitted for clarity.
The terpyridine fragment appears roughly planar with longest deviation of 0.154 Å from the
mean plane (18 atoms) observed at C(14). The bipyridine ligand is more planar with longest deviation
of 0.043 Å from the mean plane (12 atoms) observed at C(9). Both terpyridine and bipyridine are
roughly orthogonal with tortion angle of 87.88°. Interestingly, the torsion angle between the
terpyridine and the methoxyphenyl ring is equal to 6.59°, thus allowing an almost perfect transmission
of the donating electronic effect towards the Ru(NO) drawing moieties.
The electronic structure of the M(NO) core is a key parameter in the biological activities of
metal-nitrosyl complexes [26]. In the case of our complexes, the issue is to determine if their
electronic structure is either RuII(NO+) or RuIII(NO·), which correspond to the general Enemark-
Feltham notation {Ru(NO)}6 [27]. In the present case, the linear Ru-NO linkage (Ru-N-O angle =
172.94(8)°) corresponds to the RuII(NO+) electronic state. Furthermore, this electronic structure is
consistent with the observation of a ν(NO) stretching frequency equal to 1940 cm-1 [20b, 28].
Alternatively, the Ru-Npy bond lengths might tentatively be used to determine the oxidation state of
the metal center. Nevertheless, it has previously been reported that the ruthenium-nitrogen distances
are similar in [RuII(bpy)3]2+ and [RuIII(bpy)3]3+, with values equal to 2.053(2) Å and 2.057(3) Å,
respectively [29]. It is therefore impossible to conclude from the simple observation of this
geometrical parameter.
After irradiation of [RuII(MeO-Phtpy)(bpy)(NO)](PF6)3 performed in acetonitrile, the
subsequent photo product crystallizes in the in the P21c monoclinic space group. Crystal data are
gathered in Table 1. The asymmetric unit cell is shown in Figure 2, with the atom-labeling scheme
12

-
employed. It is built up from one [RuII(MeO-Phtpy)(bpy)(MeCN)]2+ cation, two NO3 anions, and an
additional molecule of acetonitrile.
Fig. 2 Asymmetric unit for [RuII(MeO-Phtpy)(bpy)(MeCN)](NO3)2 and atom-labeling scheme.
Displacement ellipsoids are drawn at the 50 % probability level, and hydrogen atoms are omitted for
clarity.
The first thing which strikes from the examination of the structure is the replacement of the
-
PF6- anions by NO3 . The presence of nitrate can be related to the possible oxidation of the NO·
radicals generated under irradiation in the non-deoxygenated solution. We have already reported on
-
-
such a PF6 /NO3 anion exchange in a previous investigation devoted to [RuII(FT)Cl2(NO)](PF6) (FT =
fluorenylterpyridine) [30]. Nevertheless, it must be pointed out that the crystal growth experiment led
to the appearance of very few crystals, however exhibiting different shapes (blocks or plates). The
present structure corresponds to the blocks. Our attempts to solve the crystal structure of the plates
-
were unsuccessful, but they may likely include the PF6 anions present in solution. Surprisingly, the
-
NO3 anions appear distorted from the ideal planar geometry with the nitrogen atoms N(8) and N(9)
lying respectively at 0.217 Å and 0.405 Å above the mean plane of the three oxygen atoms. The fact
that the nitrate anions are highly disordered may tentatively account to some extent for this effect.
Additionally, it is worth pointing out that only two anions are present instead of three, as could have
been anticipated from equation (1) which suggests that the charge of the resulting complex should be
3+. This allows us to ascribe unambiguously the oxidation state of ruthenium in the isolated
photoproduct as being RuII. This issue will be further discussed in the last section.
13

3.3. Spectroscopic data
The UV-vis spectra of [RuII(MeO-Phtpy)(bpy)(NO)](PF6)3 and that of the related MeO-Phtpy
ligand recorded in acetonitrile are compared in Figure 3. The ligand exhibits a single and very intense
band with an absorption maximum (λmax at 284 nm (
= 37 700 mol-1 L cm-1). By contrast,
[RuII(MeO-Phtpy)(bpy)(NO)](PF6)3 exhibits a more complex spectrum. While the high energy band is
present (band 1, λmax = 292 nm,
= 38 900 mol-1 L cm-1) and slightly red-shifted, two additional
)
ε
ε
features appear at 375 nm (band 2) and 420 nm (band 3) with intensities of 18 900 mol-1 L cm-1 and
12 400 mol-1 L cm-1, respectively. The differences observed between ligand and metal complex
strongly suggest that the contributions of the ruthenium nitrosyl withdrawing fragment are important
in the low energy bands 2 and 3. Therefore, it encourages a computational analysis of the related
electronic transitions.
Fig. 3 UV/vis spectra of MeO-Phtpy (blue) and [RuII(MeO-Phtpy)(bpy)(NO)](PF6)3 (red) in acetonitrile
showing the three (1-3) bands of interest present in the ruthenium(II) complex.
The comparison between the experimental (UV-vis) and computed (TD-DFT) data is provided
in Table 2. Although the computed wavelengths are slightly blue-shifted, the agreement is found
satisfactory, thus making the interpretation at the orbital level fully reliable. The transitions related to
the three UV-vis bands are further described in Table 3. Band 1 exhibits a
π → π* character, and
→ 21 and 1 → 24 transitions centered on the bipyridine and the
involves the contribution of two 1
terpyridine, respectively. As anticipated, both low-energy bands 2 and 3 possess a strong charge
transfer character towards the Ru(NO) fragment. This is further illustrated in Figure 4, where the main
orbitals of interest contributing to bands 2 and 3 are drawn, with their relative energy levels.
14

Table 2
Low-lying UV-vis bands for [RuII(MeO-Phtpy)(bpy)(NO)](PF6)3 and TD-DFT computed
bands obtained at the CAM-B3LYP/6-31G* level.
________________________________________________________________________
Experimental (UV-vis) Computed (TD-DFT)
λmax (nm) λmax (nm)
ε (mol-1 L cm-1)
________________________________________________________________________
f
Band 1
Band 2
Band 3
292
375
420
38 900
18 900
12 400
265
335
417
0.628
0.632
0.227
________________________________________________________________________
15

Table 3
TD-DFT-computed absorption maxima (λmax), oscillator strengths (f), composition of the configuration interaction expansion and character of
the relevant transitions of [RuII(MeO-Phtpy)(bpy)(NO)]3+.
_________________________________________________________________________________________________________
Transition λmax character
dominant contribution1
f
_________________________________________________________________________________________________________
Band 1
→ 21
→ 24
→
1
1
270
258
0.266
0.362
87 % χ141 → χ148
bpy bpy
→ t
53 % χ142 → χ149 + 20 % χ140 → χ147
tpy
py
Band 2
1 → 7
355
335
327
0.265
0.176
0.191
30 % χ142 → χ146 + 19 % χ144 → χ147 + 15 % χ137 → χ146
42 % χ142 → χ146 + 40 % χ144 → χ147
MeO-phtpy → terpyRu(NO)
MeO-phtpy → terpyRu(NO)
MeO-phtpy + bpy → terpyRu(NO)
1 → 10
1 → 11
22 % χ144 → χ147 + 18 % χ142 → χ146 + 17 % χ141 → χ146
Band 3
→ 3
→ Ru(NO)
MeO-phenyl
1
417
0.227
76 % χ144 → χ145
_________________________________________________________________________________________________________
1 Orbital 144 is the HOMO and orbital 145 is the LUMO in [RuII(MeO-Phtpy)(bpy)(NO)]3+.
16

Fig. 4 Relevant orbitals involved in band 2 and 3, for [RuII(MeO-Phtpy)(bpy)(NO)]3+.
Band 3 is readily described as a HOMO → LUMO based charge transfer with the excited state
electron density being totally located on the Ru(NO) fragment. Therefore, this transition should induce
· release
an efficient NO
→ 7, 1 →
process. By contrast, band 2 involves the contribution of three (1
→ 11) transitions in which the
10, and 1
dominant charge transfer occurs towards the LUMO+1 level.
Interestingly, the electronic features of the LUMO+1 are closely related to those of the LUMO, with a
rotation of 90° around the Ru-NO axis, and hence with the same expected effect in term of NO release
capabilities. Nevertheless, the transitions involved in band 2 also involve a charge transfer towards
orbitals #147 which is strongly located on the terpyridine fragment (Figure 4). Based on the
assumption that the NO· release process implies a single charge transfer transition towards the nitrosyl
fragment, it should readily be concluded that an irradiation will be less efficient on band 2 than on
band 3. This hypothesis is tested experimentally in the next section.
17

· release from [RuII
3.4. NO (MeO-Phtpy)(bpy)(NO)](PF6)3
As previously reported in several Ruthenium nitrosyl complexes with fluorenylterpyridine
ligand, [20] the resulting release of nitric oxide is followed by the formation of a solvent bound
ruthenium(III) photoproduct, according to equation (1). At this stage, it should be pointed out that
equation (1) does not preclude any further chemical evolution of [(L)RuIII(solvent)], with the outcome
of an observed photoproduct being possibly different than the expected species. Owing to the presence
of two bands in the UV-vis spectrum having absorption maxima located at 375 nm (band 2) and 420
nm (band 3), the irradiations were performed at 365 nm and 436 nm using a set of available filters, in
order to determine the NO· release efficiencies for bands 2 and 3, respectively.
The changes in the electronic absorption spectrum of [RuII(MeO-Phtpy)(bpy)(NO)](PF6)3
exposed to 365 nm light in acetonitrile are shown in Figure 5a. The presence of isosbestic points at
268, 344 and 434 nm indicates a clean conversion of the RuII(NO+) complexes to related photolysed
species. No back-reaction is observed when the light is turned off. In the photolysed species, new
bands located at 286, 308, 434 and 470 nm arise. The fact that the photoproduct does not absorb
significantly at 365 nm leads to a fast photoreaction process, completed after 80 minutes without
further evolution. The changes in the electronic absorption spectrum of [RuII(MeO-
Phtpy)(bpy)(NO)](PF6)3 exposed to 436 nm light in acetonitrile are shown in Figure 5b. The presence
of isosbestic points at 266, 346 and 436 nm indicates a clean conversion of the RuII(NO+) complexes
to related photolysed species. No back-reaction is observed when the light is turned off. In the
photolysed species, new bands located at 286, 308,434 and 470 nm arise. Due to a significant
absorption of the photoproduct at 436 nm, the release takes 7 hours to reach completeness.
Nevertheless, both experiments leading to the same photoproduct, the two Figures 5a and 5b appear
nearly identical.
II
· release (
The quantum yield of NO
φNO) observed for [Ru (MeO-Phtpy)(bpy)(NO)](PF6)3 is
equal to 0.08 and 0.03 under light irradiation at 365 nm (band 2) and 436 nm (band 3), respectively.
These values are in complete disagreement with the expectation of a more efficient NO· release when
the charge transfer towards the Ru(NO) fragment appears more significant (Table 3 and Figure 4).
They confirm a previous observation that, along the series of [RuII(R-phenylterpyridine)Cl2(NO)]+
→
(Scheme 1) in which the dominant electronic transition exibits a significant R-phenylterpyridine
Ru(NO) charge transfer character, the use of a better donating (R) substituent favors the push-pull
character towards Ru(NO) but does not necessarily imply the observation of a larger quantum yield
·
(φNO) [10]. Within these experimental features, the issue of the real mechanism involved in the NO
release process is naturally addressed. Clearly, an explanation based on the analysis of a single charge
transfer transition appears to be an oversimplified picture of a mechanism, which seems to be more
complex. A recent theoretical investigation has suggested that the NO· release could require more than
one photon, with the contribution of an intermediate triplet state [31]. Moreover, the investigation of
18

· release
the ultrafast dynamics of the photoinduced NO
in a ruthenium-nitrosyl complex with
terpyridine has also provided experimental evidences for a secondary photochemical process within a
few ps, although it was not possible to determine the nature of the intermediate state [32].
Figure 5
Evolution in the absorption spectra of [RuII(MeO-Phtpy)(bpy)(NO)](PF6)3 in acetonitrile under
irradiation at = 365nm (a) and at = 436nm (b). Blue line: before irradiation; red line: after
λ
λ
completeness of the photo-chemical process.
Additionally, it is interesting to observe that the shape of the final product (red line in Figure
5) corresponds to a RuII complex (lack of band at 600 nm characteristic of RuIII species) [23a]. This
suggests that the photoproduct in equation (1) may be reduced immediately after the photoreaction,
leading to a RuII complex as the final observable derivative. Nevertheless, the isosbestic points
evidenced in Figure 5 indicate that this second step process is probably extremely fast. Similar features
have been reported in other ruthenium-nitrosyl species [20c].
3.5. Electrochemical studies
19

The voltammograms of [RuII(MeO-Phtpy)(bpy)(NO)](PF6)3 (D) were compared to that of its
related photoproduct. The results are gathered in Table 4 and displayed in Figure 6 (square wave
technics), and Figure S1 (cyclic voltammetry, supplementary material). Complex D exhibits first
reduction waves at 0.44 V and -0.27 V (vs SCE). The first one is find reversible (∆E = 100 mV) and is
ascribable to a RuIINO+ → RuIINO· reduction process, which disappears in the photoproduct, free of
nitrosyle ligand. The second one is irreversible (∆E = 180 mV) and tentatively ascribable to a second
II
· → RuII
NO¯ process, according to previous reports pointing out the presence of a second
Ru NO
Ru(NO) reduction with E1/2 redox value lowered by 30-50 mV in polypyridine RuNO systems
[23a,33]. Additional waves are observed at much lower potentials, which likely involve the organic
ligands.
Complex D is oxidized irreversibly at 1.99 V, a value similar to the 1.93 V measured on the
photoproduct. Both waves are therefore readily ascribed to an oxidation process involving the organic
ligands, most likely the electron rich substituted terpyridine. The same behavior has previously been
observed in [Methoxyphenylterpyridine-RuCl2(NO)]+ with oxidation potential found equal to 1.90 V
[10]. Additionally, D exhibits a new oxidation wave at 1.27 V after irradiation, ascribable to a
reversible RuII → RuIII process (∆E = 100 mV). The observation of this RuII/RuIII couple in the
oxidation regime indicates that the observable photoproduct corresponds to a RuII species (vide infra).
Table 4.
Electrochemical data (E1/2 vs SCE) for [RuII(MeO-Phtpy)(bpy)(NO)](PF6) (D), before and after
irradiation at 350-450 nm. Data for [RuII(MeO-Phtpy)Cl2(NO)](PF6) are given as references
[10].
Before irradiation
After irradiation
Oxidation Eeq Reduction Oxidation
compounds
Eeq
Reduction
MeOPh-
NO+/NO• tpy and
bpy
MeOPh-
terpy
MeO-
terpy
MeOPh
-tpy
RuIII/II
0.48
0.44
-1.49
-1.62
-1.71
1.99
0.69
-1.29
-1.56
-1.63
-1.71
1.27
1.93
D
cis-[RuII(MeO-Phtpy)Cl2(NO)]
-0.13
1.90
0.4
(PF6)
20

Figure 6
Square wave voltamograms in reduction (top) and in oxidation (bottom) for complex D before
irradiation (blue) and after irradiation at = 350-450 nm (red).
λ
3.6. Oxidation state of ruthenium within the release process
The present investigation provides a set of experimental data (UV-vis spectroscopy, X-ray
crystal structure) which allow concluding that, after irradiation, the observed photoproduct
corresponds to a ruthenium (II) complex instead of a ruthenium (III) species. This unexpected result
raises the issue of the real nature of the species produced under irradiation, which might not
correspond to that depicted in equation (1), but to the alternative, and undesirable following reaction:
ꢒꢓ ꢀ
+ ꢊꢋꢌꢍꢎꢏꢐꢑ ꢔꢕ ꢁꢂ ꢄꢊꢋꢌꢍꢎꢏꢐꢇ +ꢑꢅꢆ
ꢉ
ꢃꢃ
ꢈ
ꢀ
ꢃꢃꢄ ꢇꢈꢉ
ꢁꢂ ꢅꢆ
(4)
Nevertheless, the photo-generation of NO· (equation (1)) instead of NO+ (equation (4)) can be
demonstrated experimentally, by EPR spectroscopy under standard excitation, since spin trapping
21

combined with EPR spectroscopy is considered as one of the best methods for the direct detection of
NO· radicals [34]. Namely, we used Iron(II)-N-methyl-D-glucamine dithiocarbamato [FeII(MGD)2] to
trap NO· due to its high probability of adduct formation and to the high stability of its spin adduct. The
· is shown in Figure 7
appearance of the characteristic triplet signal of NO
, with a hyperfine splitting
constant value of aN = 1.24 × 10-3 cm-1 and a g-factor equal to 2.040. These values are fully consistent
with the literature report for [FeII(MGD)2-NO] adduct [14]. The weak signal observed in the control
spectrum (top of Figure 7) is due to a trace of NO· and related to the fact that the manipulation is never
strictly conducted in the dark.
25000
15000
5000
-5000
-15000
-25000
25000
15000
5000
-5000
-15000
-25000
3375
3395
3415
3435
3455
3475
Magnetic field (G
)
II
· trapped by [Fe(MGD)
2] for [Ru (MeO-Phtpy)(bpy)(NO)](PF6)3
Figure 7
Triplet EPR signals from NO
upon one photon excitation at room temperature and at λ > 400 nm (Hg lamp) (bottom), and
control signal before irradiation (top).
· confirms the
The observation of NO
validity of the process depicted in equation (1) with the
outcome of a fast reduction of the concomitant [RuIII(solvent)] species into [RuII(solvent)], in such a
way that no transient RuIII complex is detectable in the reaction medium in the UV-vis spectra (Figure
5). The absence of significant amount of paramagnetic complex can be further monitored by 1H-NMR
1
spectroscopy at any time of the photochemical process. The evolution of the H-NMR spectrum of
[RuII(MeO-Phtpy)(bpy)(NO)](PF6)3 under irradiation in the 350-450 nm range is presented in Figure
8. The gradual transformation of the complex into a diamagnetic photoproduct is apparent in the figure
(e.g. the singlet of methyl group protons at 4.00 ppm decreases while the corresponding signal of
photoproduct at 3.95 ppm rises), without any production of paramagnetic RuIII complex.
22

Figure 8
1H-NMR-tracking of NO· release from [RuII(MeO-Phtpy)(bpy)(NO)](PF6)3 irradiated with
continuous light in the 350-450 nm range, in CD3CN.
The most noticeable evolution in Figure 8 is the shift of a peak tentatively attributed to traces
of water from 2.13 ppm in the initial solution to 6.85 ppm after 155 minutes of irradiation. A possible
explanation for this observation is the augmentation of acidity of the solution during irradiation, which
could likely be related to the oxidation of NO· according to the following equation:
ꢡ
4ꢅꢆ· + 3ꢆꢻ +ꢑ2ꢼꢻꢆ → 4ꢅꢆꢯ + 4ꢼꢈ
(5)
The electrochemical data can be used to target the relative stability of RuII/RuIII species
depending on the coordination spheres of the metal center. Indeed, the RuIII complexes should be more
reducible in the case of a coordination sphere in which five pyridine units are present, due to the fact
that the electron density transferred to the metal center can be delocalized by d-π* overlap towards the
pyridine ligands, thus leading to an increase of the RuII → RuIII oxidation potential . This tendency is
observed here, with a shift of the RuIII/RuII redox potential from 0.40 V in trans-Cl,Cl[Ru(MeO-
Phtpy)Cl2(MeCN)]+/0 [10] to 1.27 V in [Ru(MeO-Phtpy)(bpy)(MeCN)]3+/2+ (Table 4).
The true nature of the reducing agent involved in the reduction process has not been
thoroughly investigated. At first, it might be inferred from the observation of nitrate anions in the
· species could be involved in
crystal structure of the photo-product (Figure 2), that some released NO
-
the fast reduction process. On the other hand, the appearance of NO3 is evidenced in the investigation
23

of the photo-release from [RuII(FT)Cl2(NO)](PF6) as well [30], where no RuIII → RuII reduction is
observed in the photo-product. Therefore, the presence of nitrate may result from further oxidation of
NO· in the non-deoxygenated reaction mixture (equation (5)), and does not prove any interaction
between NO· and the ruthenium complex.
Conclusion
[RuII(MeO-Phtpy)(bpy)(NO)]3+ releases NO· under irradiation in the UV-vis domain, with a
quantum yield which appears significantly reduced when the involved transition exhibits a stronger
push-pull charge transfer towards NO. This behavior indicates that the real mechanism of the NO
release cannot be restricted to the effect of an individual transition. Although NO is undoubtedly
generated as NO· radical, the concomitant RuIII photoproduct is readily reduced to a RuII complex of
formula [RuII(MeO-Phtpy)(bpy)(MeCN)]2+, probably by the action of a fraction of the nitric oxide
photo-released during the irradiation process.
Supplementary materials
DFT optimized [RuII(MeO-Phtpy)(bpy)(NO)]3+ and [RuII(MeO-Phtpy)(bpy)(MeCN)]2+
coordinates,
X-ray
data
for
[RuII(MeO-Phtpy)(bpy)(NO)](PF6)3
and
[RuII(MeO-
Phtpy)(bpy)(MeCN)](NO3)2 including details on the coordination spheres (Table S1), and cyclic
voltamogramms for D and for [Ru(MeO-Phtpy)(bpy)(MeCN)]3+/2+ (Fig. S1).
Acknowledgments
The authors wish to thank Prof. Norberto Farfan and Dr. Margarita Romero (Faculty of
Chemistry of the Universidad Nacional Autonoma de México – UNAM) for fruitful discussions, Dr
Alix Sounia-Saquet (LCC – CNRS) for performing the electrochemical experiments, Lionel Rechignat
(LCC – CNRS) for his assistance in EPR experiment. V.B. Thanks the French/Ukrainian institution
for a grant and Campus France (Europe, 835930K) for financial support.
24

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27

HIGHLIGHTS
A ruthenium-nitrosyl complex undergoes NO release under irradiation around 400 nm
The generation of NO is monitored by EPR spectroscopy
The quantum yield of photo release is investigated at different wavelengths
The Ru(III) photoproduct is readily reduced to Ru(II)
The data suggests a NO release involving more than one photon
28