Nitric oxide (NO) photo-release in a series of ruthenium-nitrosyl complexes: new experimental insights in the search for a comprehensive mechanism

Article abstract of DOI:10.1039/C8NJ03907K

A series of four ruthenium(ii) complexes built from the [Ru(terpy)(bipy)(NO)]³⁺ core (terpy is 2,2′:6′,2′′-terpyridine and bipy is 2,2′-bipyridine) are investigated. They differ by the presence of zero, one, two, or three 4′-(4-methoxyphenyl) (MP) electron donor substituents introduced at different positions on the pyridine fragments to increase the intramolecular charge transfer capabilities towards the strongly withdrawing nitrosyl (NO) ligand. The UV-visible spectra reflect the presence and position of the MP substituents on the complexes. In the case of species containing the 4′-(MP)-terpy ligand, a low-lying transition is identified as arising from 4′-(MP)-terpy to Ru(NO) intramolecular charge transfer, which is further confirmed by TD-DFT analysis. Irradiation performed at λ = 436 nm on this isolated transition for different complexes leads to quantum yields of NO photo-release equal to 0.002 and 0.011, in a ratio of 6 (instead of 1), which allows concluding non-ambiguously that a single electron transition cannot account for the NO release mechanism.

Full text of DOI:10.1039/C8NJ03907K

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Nitric oxide (NO) photo-release in a series of
ruthenium–nitrosyl complexes: new experimental
insights in the search for a comprehensive
mechanism†
Cite this: New J. Chem., 2019,
43, 755
Max Roose, Marine Tasse, Pascal G. Lacroix * and Isabelle Malfant
*
´
3+
0
0
00
A series of four ruthenium(II) complexes built from the [Ru(terpy)(bipy)(NO)] core (terpy is 2,2 :6 ,2 -
0
terpyridine and bipy is 2,2 -bipyridine) are investigated. They differ by the presence of zero, one, two, or
0
three 4 -(4-methoxyphenyl) (MP) electron donor substituents introduced at different positions on the
pyridine fragments to increase the intramolecular charge transfer capabilities towards the strongly
withdrawing nitrosyl (NO) ligand. The UV-visible spectra reflect the presence and position of the MP
0
substituents on the complexes. In the case of species containing the 4 -(MP)-terpy ligand, a low-lying
0
Received 3rd August 2018,
transition is identified as arising from 4 -(MP)-terpy to Ru(NO) intramolecular charge transfer, which is
Accepted 28th November 2018
further confirmed by TD-DFT analysis. Irradiation performed at l = 436 nm on this isolated transition for
different complexes leads to quantum yields of NO photo-release equal to 0.002 and 0.011, in a ratio of
DOI: 10.1039/c8nj03907k
6
(instead of 1), which allows concluding non-ambiguously that a single electron transition cannot
rsc.li/njc
account for the NO release mechanism.
their coordination environment. In the complexes here under
investigation, the total number of electrons present in the
Introduction
ꢀ
Nitric oxide (NO ) has been recognized as being involved in ruthenium 3d orbitals and the p* orbitals of the nitrosyl ligand
6
many biological functions such as blood pressure regulation, is equal to 6, which is written as {RuNO} in the Enemark and
stimulation of immune response, neurotransmission and cytotoxic Feltham notation which avoids the assignment of a formal
1
,2
5,6
activity in tumor cells by apoptosis. These numerous potential oxidation state of the metal and a charge on the ligand.
6
therapeutic actions have prompted research aimed at developing However, it is reported that almost any {RuNO} electronic
efficient NO donor molecules. Among them, ruthenium–nitrosyl configurations correspond to the formal Ru NO electronic
Ru(NO)] complexes are particularly appealing candidates in structure, which is EPR silent and in which the nitrosyl is
relation to their generally low toxicity, good chemical stability, regarded as a cation. The presence of this resulting diamagnetic
and moreover their capability to release NO under light irradiation singlet ground state has been further supported by several compu-
ꢀ
II
+
[
7
ꢀ
3
,4
8–11
in the l = 300–600 nm domain, exclusively, taking advantage of tational studies.
the noninvasive and highly controllable characteristics of light, the NO radical depicted in eqn (1) occurs after formal electron
Under these assumptions, the photo-release of
ꢀ
according to the following equation:
transfer to the nitrosyl ligand, achieved within an intense low-lying
electron transition having a strong charge transfer character towards
the strongly withdrawing nitrosyl, in all cases.
Â
Ã
Â
Ã
hn
II
þ
III
ꢀ
Ru ðNO Þ þ solvent ƒ! Ru ðsolventÞ þ NO
(1)
ꢀ
Several research teams have targeted NO donors in the class
NO is a classic non-innocent ligand and metal–nitrosyls can
have different electronic structure descriptions depending on
12–14
12a,15–17
of ruthenium–bipyridine
and ruthenium–terpyridine
complexes, built up from ligands of different donating/accepting
capabilities. As a part of this research effort, we have recently
reported on various ruthenium–nitrosyl complexes based on
CNRS, LCC (Laboratoire de Chimie de Coordination), 205 route de Narbonne,
3
1077 Toulouse, France. E-mail: pascal.lacroix@lcc-toulouse.fr,
3
+
18
3+
isabelle.malfant@lcc-toulouse.fr
the [Ru(tpy)(bpy)(NO)] core ([RuT0B0] in Scheme 1) initially
1
19
†
Electronic supplementary information (ESI) available: H NMR for the 6 Ru–X
0
0
00
investigated by Meyer et al. (tpy stands for 2,2 :6 ,2 -terpyridine
and bpy for 2,2 -bipyridine). These various attempts were motivated
by the idea that introducing a donor (either fluorene,
ruthenium complexes (X = Cl, NO
2
, NO) with atom labeling, computational
0
details (optimized structures and UV-visible spectra), and electrochemical and
1
8a
1
photorelease data for [RuT1B0](PF
6
)
3
and [RuT0B2](PF
6
)
3
.
H-NMR-tracking of
and its
18b
18c
ꢀ
carbazole,
or methoxyphenyl ) on the terpyridine ligand
NO release from [RuT1B2](PF
6
)
3
. The IR spectrum for [RuT1B2](PF )
6 3
photoproduct. See DOI: 10.1039/c8nj03907k
would favor intense charge transfer towards the RuNO fragment,
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3 3
CD OD or CD CN as an internal reference. The IR spectra were
recorded with a Perkin-Elmer (FTIR/FIR) 100 spectrometer. The ESI
mass spectra were performed on a UPLC Xevo G2 Q TOF (Waters)
spectrometer. The UV-vis spectra were obtained on a Perkin Elmer
Lambda 35 UV-vis spectrometer. For the deconvolution of the
spectra, the shapes of the transitions were assumed to correspond
to Gaussian functions when drawn against the transition energies.
Elemental analyses were performed at LCC with a Perkin-Elmer
2400 series II instrument. Electron paramagnetic resonance experi-
ments (EPR) were performed on a Bruker ESP 500E spectrometer.
The setup employed for the measurements was the same as that
1
8c
previously reported. [Fe(MGD) ] (MGD = N-methyl-D-glucamine
2
23
dithiocarbamate) was used as a spin trapping reagent. The light
source was a 250 W Oriel Hg lamp (Palaiseau, France).
Synthesis
6 3 6 3
The synthetic route towards [RuT0B2](PF ) , and [RuT0B2](PF )
is summarized in Scheme 2. The atom labeling used for the
1
13
H- and C-NMR assignments is provided as ESI† (Fig. S3).
II
III
[
Ru (terpy)(MP bipy)Cl](Cl). [Ru (terpy)Cl ] (88 mg, 0.2 mmol),
2
3
MP bipy (74 mg, 0.2 mmol), lithium chloride (64 mg, 1.5 mmol),
2
ethylene glycol (27 mL), and triethylamine (0.24 mL, 1.7 mmol)
were successively added into a 50 mL flask. The mixture was
heated at 160 1C for 4 h. After hot filtration, the mixture was
evaporated to dryness under reduced pressure, at 120 1C. The
residue obtained was washed with a large volume of water and
sonicated, and finally filtered and dried under vacuum, providing
2+
Scheme 1 [Ru(terpy)(bipy)(NO)] complexes with 0, 1, 2, and 3 methoxy-
phenyl substituents.
9
9 mg (64%) of the desired compound as a dark purple solid.
ꢀ
1
3
thus increasing the quantum yield of NO release (f = number
NO
H NMR (CD OD, 400 MHz): d 10.15 (1H, d, H , J = 6.0 Hz);
3 6B 6/5
ꢀ
4
4
of NO generated/number of photons absorbed by the complex).
0
0
0
9.17 (1H, d, H3B, J3/5 = 2.0 Hz); 8.88 (1H, d, H3 B, J3 /5 = 1.9 Hz);
3
3
In the present contribution, we wish to target the role of an
electron donating substituent in the optical and photo-chemical
properties of the ruthenium nitrosyl species through the investiga-
tion of a set of substitutions achieved either on the terpyridine
8
.66 (2H, d, H30T + H50T, J3 /4 = J5 /4 = 8.1 Hz); 8.54 (2H, d,
0
0
0
0
3
3
3
H
3T + H3 T, J3/4 = J3 /4 = 7.9 Hz); 8.28 (1H, dd, H5B, J5/6 = 6.1 Hz,
00
00 00
4
J
5/3 = 1.9 Hz); 8.17–8.20 (2H, m, H2P + H6P); 8.14–8.17 (1H, t, H4 T
0
,
=
+
3
3
3
3
3
J
4 /3 = J4 /5 = 8.1 Hz); 7.94 (2H, td, H4T + H4 T, J4/3 = J4/5
0
0
0
0
00
3
+
3+
3
4
4
(
[RuT1B0] ), on the bipyridine ([RuT0B2] ), or on both ligands
J400 00 = J 00 00 = 7.9 Hz, J = J 00 00 = 1.6 Hz); 7.82 (2H, m, H6T
/3 4 /5 4/6 4 /6
3+
(
[RuT1B2] ), as shown in Scheme 1. After the report and complete H600T); 7.77 (2H, m, H
0
+ H
0
); 7.36 (2H, m, H5T + H 00 ); 7.28 (2H,
2 P
6 P 5 T
characterization of the ruthenium complexes, their optical spectra
will be presented experimentally and approached computationally
within the framework of density functional theory (DFT) to
determine the origin of the observed transitions. fNO will be
investigated by irradiation of the low-lying ‘‘push–pull’’ transitions,
and tentatively analyzed in an attempt to provide new features
towards a comprehensive description of the release process.
m, H 0 + H60B); 7.23 (2H, m, H3P + H ); 7.02 (2H, m, H + H 0 );
0
5
B
5P
3 P
5 P
1
3
3 3 3
3.95 (3H, s, OCH ); 3.82 (3H, s, OCH ). C NMR (CD OD,
1
00 MHz): d 163.18 (C4P or C40P); 163.05 (C4P or C40P); 160.61
(
(
C20B); 160.33 (C2T + C2 T); 159.65 (C20T + C60T); 158.10 (C2B); 153.47
00
C
6B); 153.36 (C6T + C6 T); 152.48 (C60B); 149.94 (C4B); 149.01 (C40B);
00
1
38.30 (C4T + C4 T); 135.22 (C40T); 130.04 (C2P + C6P); 129.86 (C1P);
00
129.72 (C2 P + C ); 129.13 (C
0
0
0
); 128.56 (C5T + C 00 ); 124.76
6 P
1 P
5 T
(
(
(
C3T + C 00 ); 124.69 (C ); 124.08 (C
0
); 123.65 (C
0
+ C
0
); 121.48
); 55.34
3 T
5B
5 B
3 T
5 T
C ); 121.35 (C
0
); 115.97 (C3P + C ); 115.76 (C
0
+ C
0
3
B
3 B
5P
3 P
5 P
Experimental
Starting materials and equipment
+
3 3
OCH ); 55.24 (OCH ). ESI-MS: m/z: 738.5 [M] .
II
II
[
Ru (terpy)(MP
2 2 6
bipy)(NO )](PF ). [Ru (terpy)(MP
2
bipy)(Cl)](Cl)
0
0
20
2
The 4,4 -bis(4-methoxyphenyl)-2,2 -bipyridine (MP bipy) and (95 mg, 0.12 mmol) was dissolved in a mixture of ethanol
0
0
0
00
21
4
-(4-methoxyphenyl)-2,2 :6 ,2 -terpyridine (MPterpy) ligands (8.6 mL) and water (2.9 mL). After complete dissolution, NaNO
2
were synthesized as previously described in the literature. (83 mg, 1.2 mmol) was added and the violet resulting solution
3
0
Likewise, [Ru(terpy)(Cl) ] and [Ru(MPterpy)(Cl) ] was synthetized was heated under reflux for 3 h. The solution turned dark red.
3
3
22
following Adcock’s procedure. Triethylamine (Sigma-Aldrich), After cooling down to room temperature, an excess of NH PF6
4
LiCl (Alfa Aesar), ethylene glycol (Fluka), NH PF (Alfa Aesar), and (250 mg, 1.53 mmol) dissolved in 1 mL of water was added. The
4
6
NaNO
received. The H NMR and C NMR spectra were recorded at the resulting solution was cooled in a fridge overnight. After
98 K with a Bruker Avance 400 spectrometer, using CDCl filtration and washing with cold water, a dark red solid was
2
(Fluka) were the highest purity grade and were used as solution was concentrated to three-quarters of its volume and
1
13
2
3
,
7
56 | New J. Chem., 2019, 43, 755--767
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6 3 6 3
Scheme 2 Synthetic route towards [RuT0B2](PF ) and [RuT1B2](PF ) .
dried under vacuum, affording 73 mg (68%) of the desired Filtrating, washing with cold water, and eventually drying
1
compound. H NMR (CD
3
CN, 400 MHz): d 9.82 (1H, d, H6B
,
,
under vacuum afforded 75 mg (77%) of a dark yellow solid.
3
4
1
3
J
J
8
6/5 = 6.1 Hz); 8.93 (1H, d, H3B, J3/5 = 2.0 Hz); 8.69 (1H, d, H
3
0
B
H NMR (CD
3
CN, 400 MHz): d 9.23 (1H, d, H6B, J6/5 = 6.2 Hz);
4
3
3
4
0
0
= 1.8 Hz); 8.49 (2H, d, H
0
+ H
00/400 = 8.0 Hz); 8.21–8.16 (2H, m,
); 8.10 (2H, m, H + H ); 7.94 (2H, td, H + H 00
0
, J
0
0
= J
0
0
= 8.0 Hz); 9.16 (1H, d, H3B, J3/5 = 2.0 Hz); 9.00 (1H, m, H
0
); 8.95–8.92 (3H, m,
3
/5
3
3
T
5
T
3
/4
5
/4
4
T
3
3
3
.37 (2H, d, H3T + H
3
00
T
, J3/4 = J
3
H
3
0
T
+ H
5
0
T
+ H
3
0
B
); 8.76 (2H, d, H3T + H
3
00
T
, J3/4 = J
3
00 /400 = 7.9 Hz);
H
+ H
0
,
4 T
8.51–8.45 (3H, m, H + H 00 + H ); 8.24 (2H, m, H + H ); 8.17
5
B
4 T
2P
6P
4T
4T
4 T
5B
2P 6P
3
3
3
3
4
4
3
3
4
4
J4/3 = J = J 00 00 = J 00 00 = 7.8 Hz, J = J 00 00 = 1.5 Hz); 7.83 (2H, dd, H6T + H 00 , J = J 00 00 = 5.7 Hz, J = J 00 00 = 1.4 Hz); 7.91
4/5
4 /3
4 /5
4/6
4 /6
6 T 6/5
6 /5
6/4
6 /4
3
3
4
4
(
7
H
3
2H, dd, H6T + H
.74 (2H, m, H
+ H3P + H5P); 7.04 (2H, m, H
.83 (3H, s, OCH
ou C ); 162.62 (C4P ou C
); 158.14 (C
C600T); 151.81 (C
6
00
T
, J6/5 = J
+ H ); 7.34–7.22 (6H, m, H5T + H
+ H ); 3.94 (3H, s, OCH
CN, 100 MHz): d 162.69 (C4P (3H, s, OCH
6
00/500 = 5.5 Hz, J6/4 = J
6
00/400 = 1.4 Hz); (2H, m, H
2
0
P
+ H
6
0
P
); 7.76 (2H, m, H5T + H
= 5.7 Hz, J
0 0
5 /3
5
00
T
); 7.62 (1H, ddd, H
=1.3 Hz); 7.31 (2H, m, H3P + H5P);
); 7.11 (1H, m, H ); 4.00 (3H, s, OCH ); 3.97
). C NMR (CD CN, 100 MHz): d 164.36 (C ); 164.20
(C4P); 157.6 (C2T + C ); 156.00 (C ); 155.66 (C6T + C ); 155.57
(C2B + C ); 155.16 (C + C ); 154.69 (C4B); 153.44 (C6B); 148.31
(C ); 147.90 (C
5 B
0
,
3
3
4
0
0
00
+ H
0
+ = 7.8 Hz, J
); 7.13 (2H, m, H
J
5
0
0
0
0
2
P
6
P
5
T
5
B
/4
5
/6
0
0
0
0
+ H
0
0
6
B
3
P
5
P
3
3
P
5
P
6
B
3
1
3
13
). C NMR (CD
); 159.56 (C2T + C
); 157.19 (C2B); 153.70 (C6B); 153.50 (C6T
); 149.50 (C ); 149.20 (C4B); 138.74 (C4T
3
3
3
0
4 P
3
0
0
00
T
); 159.09 (C
0
+
+
+
00
0
00
6 T
4
P
4
P
2
2
T
2
T
2
B
C
6
0
T
2
0
0
B
4
0
B
2
0
T
0
6 T
0
0
0
); 145.36 (C4T + C 00 ); 131.27 (C5T + C 00 ); 131.01
6 B
4 B
6 B
4 T
4 T
5 T
C400T); 136.40 (C
0
); 129.91 (C2P + C ); 129.84 (C
0
+ C
0
); (C2P + C6P ou C
0
+ C
0
); 130.95 (C2P + C6P ou C
0
+ C ); 128.66
6 P
0
4 T
6P
2 P
6 P
2 P
6 P
2 P
1
C
29.34 (C1P); 128.72 (C
1
0
P
); 128.44 (C5T + C
+ C + C ); 121.57 (C3B); 121.10 126.74 (C
); 115.92 (C3P + C5P); 115.63 (C + C ); 56.34 (OCH
). ESI-MS: m/z: 749.1 [M] ; 372 [M–NO
5
00
T
); 124.57 (C3T
+
(C3T + C
3
00
T
); 128.44 (C
); 126.24 (C
3
5
P
0
T
B
+ C
); 124.34 (C3B); 123.47 (C
); 56.54 (OCH ); 56.52 (OCH
3
5
0
T
); 127.28 (C1P); 127.18 (C5B);
); 116.26
).
00
+ C5B); 123.73 (C
0
0
0
0
0
0
3 B
3
T
3
T
5
T
5
B
1
P
(C
3
0
B
3
0
P
5
0
P
3
); (C3P + C5P); 116.14 (C
3
0
+ C
5
0
P
3
+
2+
ꢁ1
5
2
5.24 (OCH
52. IR: n (cm ): 1387 (nNO2), 1346 (nNO2), 839 (nPF6), 558 (nPF6). [Ru(terpy)(MP
RuT0B2](PF [Ru(terpy)(MP bipy)(NO )](PF (75 mg, [Ru(terpy)(MP
.083 mmol) was dissolved in a mixture of ethanol (30 mL) (55 000 L mol cm ), 292 nm (43 000 L mol cm ), 322 nm
3
2
+ CH
3
CN] ; IR: n (cm ): 1937 (nNO), 816 (nPF6), 552 (nPF6). ESI-MS: m/z: 360
ꢁ1
2+
2+
2
bpy)(H bpy)(CH
2 3
bpy)(NO )] . UV-vis (CH CN): lmax (e): 229 nm
2
O)] ; 372 [Ru(terpy)(MP
2
3
CN)] ; 749
+
[
6
)
3
.
2
2
6
)
2
ꢁ
1
ꢁ1
ꢁ1
ꢁ1
0
ꢁ
1
ꢁ1
and HCl 37% (7 mL). The resulting dark red-brown solution (sh), 360 nm (33 000 L mol cm ). Anal. calcd for C H F N O -
39 31 18 6 3
was heated at 60 1C for 1 hour. The mixture was left at room
temperature, and an excess of NH PF (250 mg, 1.53 mmol) dis-
solved in 3 mL of water was added. The beginning of precipitation 0.25 mmol), MP
P
3
Ru: C, 40.12; H, 2.68; N, 7.2. Found: C, 40.43; H, 2.34; N, 7.2.
[Ru(MPterpy)(MP bipy)Cl](Cl). [Ru(MPterpy)Cl ] (137 mg,
bipy (92 mg, 0.25 mmol), lithium chloride
(80 mg, 1.875 mmol [7.5 eq.]), ethylene glycol (29 mL), and
4
6
2
3
2
3
0
was noticed. The solution was put in a fridge for 2 h.
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triethylamine (0.29 mL, 2.125 mmol) were successively intro- 161.61 (C4Pb or C
duced into a 50 mL flask. The mixture was heated at 170 1C for (C2T + C ); 157.91 (C
h. After hot filtration, the solution was evaporated to dryness (C ); 152.58 (C + C 00 ); 150.74 (C
4
0
Pb or C4Pt); 161.60 (C4Pb or C
+ C ); 157.23 (C
); 148.47 (C
0
4 B
4
0
Pb or C4Pt); 158.79
00
0
0
0
); 156.28 (C2B); 152.70
); 148.11 (C4B);
2
T
2
T
6
T
2
B
8
0
6B
6T
6 T
6 B
under reduced pressure at 120 1C. The residue obtained was 147.43 (C4
0
T); 137.63 (C4T + C 00 ); 129.10 (C2Pt + C6Pt); 129.00 (C1Pt);
4 T
washed with a huge amount of water and sonicated. The 128.91 (C2Pb + C6Pb or C
0
+ C
0
); 128.86 (C2Pb + C6Pb or C
0
+
2 Pb
6 Pb
2 Pb
resulting dark purple solid was filtered, washed with water
C
6
0
Pb) 128.38 (C1Pb); 127.76 (C
); 123.59 (C5B); 122.73 (C
); 120.01 (C + C ); 114.93 (C3Pb + C5Pb or C3Pt + C5Pt);
OD, 400 MHz): d 10.20 (1H, d, H6B, J6/5 = 6.0 Hz); 9.19 (1H, 114.86 (C3Pb + C5Pb or C3Pt + C5Pt); 114.63 (C Pb + C Pb); 55.35
). IR: n (cm ): 1334 (nNO2), 843 (nPF6), 826
1
0
Pb); 127.35 (C5T + C
5 T
00
); 123.63
and a minimum of ethanol and ether, and dried under vacuum, (C3T + C
3
00
T
0
5 B
); 120.57 (C3B); 120.11
1
affording 180 mg (82%) of the desired compound. H NMR (C
3
0
B
3
0
T
0
5 T
3
(
CD
d, H3B, J3/5 = 1.9 Hz); 8.90 (3H, m, H
3
3
0
0
5
4
ꢁ1
0
+ H
0
+ H
0
); 8.69 (OCH
ꢂ 2); 55.24 (OCH
3
T
5
T
3
B
3
3
3
3
+
(
2H, d, H3T + H 00 , J = J 00 00 = 7.9 Hz); 8.31 (1H, dd, H5B, (nPF6), 557 (n ). ESI-MS: m/z: 855.2 [M] .
3 T
3/4
3 /4
PF
6
3
4
3
J5/6 = 6.2 Hz, J = 1.9 Hz); 8.21 (2H, d, H
+ H6Pb, J
=
[RuT1B2](PF ) . [Ru(MPterpy)(MP bipy)(NO )](PF ) (60 mg,
6 3 2 2 6
5/3
2Pb
2/3
3
3
3
J6/5 = 8.4 Hz); 8.15 (2H, d, H2Pt + H6Pt, J = J = 8.3 Hz); 7.93 0.060 mmol) was dissolved in a mixture of ethanol (22.5 mL)
2/3
6/5
3
3
(
5
2H, m, H4T + H
.4 Hz); 7.79 (2H, d, H
3H, m, H + H5T + H
3 5 Pb
4H, m, H3Pb + H5Pb + H3Pt + H5Pt); 7.04 (2H, m, H Pb + H ,
0
4
00
T
); 7.84 (2H, d, H6T + H
0 0 0 0 0 0
Pb + H Pb, J = J
2 6 2 /3 6 /5
6
00
T
, J6/5 = J
= 8.5 Hz); 7.38–7.36 60 1C for 1 h. The solution was left to cool to room temperature,
); 7.31–7.29 (1H, m, H ); 7.26–7.21 and an excess of NH PF (350 mg, 2.13 mmol) dissolved in 3 mL
of water was added. The resulting solution was evaporated under
); vacuum at a temperature maintained below 30 1C, until the
.84 (3H, s, OCH3). C NMR (CD OD, 100 MHz): d 161.79 beginning of precipitation occurred. The mixture was put in a
6
00/500
=
and HCl 37% (5 mL). The resulting brown solution was heated at
3
3
(
(
6
0
B
5
00
T
5
0
B
4
6
0
3
3
J
3
0
/2
0
= J
5 /6 3 3
= 8.5 Hz); 3.97 (3H, s, OCH ); 3.96 (3H, s, OCH
0 0
1
3
3
3
(
C4Pb); 161.65 (C
0
); 161.59 (C4Pt); 159.31 (C
+ C
0
); 159.18 (C2T
+
fridge overnight. After filtration and washing with cold water,
0
); 156.82 (C2B); 152.12 (C6B); 152.00 the resulting dark orange solid was dried under vaccum (72 mg,
6 T
4 Pb
2 B
C200T); 158.13 (C
0
2 T
1
3
(C
6T + C
); 136.80 (C4T + C
6
00
T
); 151.09 (C
6
0
B
); 148.43 (C4B); 147.52 (C
); 128.95 (C1Pt); 128.63 (C2Pt + C6Pt
Pb + C Pb); 127.78 (C Pb); 8.92–8.89 (3H, m, H
); 123.26 (C5B); 122.67
); 119.29 (C + C
C3Pb + C5Pb or C3Pt + C5Pt); 114.55 (C3Pb + C5Pb or C3Pt + C5Pt);
4
0
B
); 146.55 94%). H NMR (CD
6.2 Hz); 9.13 (1H, d, H3B, J3/5 = 2.1 Hz); 9.08 (2H, s, H
+ H3T + H ); 8.51 (2H, td, H4T + H
3
CN, 400 MHz): d 9.23 (1H, d, H6B, J6/5
=
);
4
(
C
1
C
4
0
T
4
00
T
+
3
0
T
+ H
5
0
T
2Pb + C6Pb); 128.50 (C1Pb); 128.30 (C
27.04 (C5T + C ); 123.54 (C3T + C
); 120.07 (C3B); 119.92 (C
2
0
6
0
1
0
3
0
3
B
3
00
T
4
00
T
,
3
3
3
4
4
00
00
J
4/3 = J4/5 = J
00/300 = J
00/500 = 7.9 Hz, J4/6 = J
00/600 = 1.5 Hz); 8.47
5
T
3
T
4
4
4
3
4
(
(
C
5
0
B
3
0
B
3
0
T
5
0
T
); 114.57 (1H, dd, H5B, J5/6 = 6.2 Hz, J5/3 = 2.1 Hz); 8.32 (2H, m, H2Pt
+
=
);
3
H
); 8.23 (2H, m, H
+ H6Pb); 8.17 (1H, dd, H6T + H 00 , J
6Pt
2Pb 6 T 6/5
3
4
4
1
14.34 (C3
0
+ C
0
); 54.63 (OCH ꢂ 2); 54.51 (OCH ). ESI-MS:
J
00 00 = 5.6 Hz, J = J 00 00 = 1.4 Hz); 7.90 (2H, m, H
0
+ H
0
Pb
5 Pb
3
3
6 /5
6/4
6 /4
2 Pb
6 Pb
+
3
m/z: 844.4 [M] .
7.76 (2H, m, H + H 00 ); 7.62 (1H, dd, H
0
, J
0
0
= 6.4 Hz,
5
T
5 T
5 B
5 /6
4
[
Ru(MPterpy)(MP
2
bipy)(NO
2
)](PF
6
).
J
5
0
/3
0
= 2.1 Hz); 7.36 (2H, m, H3Pt + H5Pt); 7.31 (2H, m, H3Pb
+
+
3
[Ru(MPterpy)(MP
2
bpy)(Cl)]Cl (130 mg, 0.15 mmol) was dis-
H
H
5Pb); 7.24 (1H, d, H
Pb); 4.02 (3H, s, OCH
6
0
B
, J
); 4.00 (3H, s, OCH
CN, 100 MHz): d 163.90 (C4Pt); 163.37
Pb); 163.22 (C4Pb); 158.07 (C ); 156.90 (C2T + C ); 154.99
); 154.63 (C6T + C 00 + C
); 154.54 (C4B); 153.68 (C
6 /5 3 Pb
= 6.4 Hz); 7.13 (2H, m, H
0 0 0
solved in a mixture of ethanol (12 mL) and water (4 mL). NaNO
96 mg, 1.40 mmol) was added and the reaction mixture was OCH ). C NMR (CD
3 3
heated under reflux for 4 h. The dark brown solution was (C
cooled down to room temperature, and an excess of NH PF (C
2
5
0
3
3
); 3.89 (3H, s,
1
3
(
2
0
0
0
00
2 T
4
4
T
0
0
0
+
2 T
4
6
4 B
6 T
2 B
(350 mg, 2.1 mmol) dissolved in 1 mL of water was added. The C6
0
T); 153.61 (C2B); 152.33 (C6B); 147.34 (C
0
); 144.25 (C4T + C400 T);
6 B
solution was concentrated to three-quarters of its volume, and 130.58 (C2Pt + C6Pt); 130.10 (C5T + C 00 ); 130.00 (C2Pb + C6Pb or
5 T
the resulting solution was cooled in a fridge for 1 h. After
filtration and washing with cold water, the resulting dark
C
C
2
0
Pb + C
); 126.63 (C1Pt); 126.30 (C1Pb); 126.21 (C5B); 125.76 (C
); 123.53 (C + C ); 123.28 (C3B); 122.42 (C
compound (138 mg) was purified as follows: the solid was 115.54 (C3Pt + C5Pt); 115.28 (C3Pb + C5Pb); 115.14 (C Pb + C
dissolved in 135 mL of dichloromethane and the solution was 55.73 (OCH ); 55.58 (OCH ); 55.52 (OCH ). IR: n (cm ): 1933
filtrated. Petroleum ether (550 mL) was slowly added to the (nNO), 829 (nPF6), 556 (n ). ESI-MS: m/z: 855.2 [Ru (MPterpy)-
6
0
Pb); 129.93 (C2Pb + C6Pb or C
2
0
Pb + C
6
0
Pb); 127.48 (C3T
+
00
0
1
Pb);
3
T
brown solid was dried under vacuum in a desiccator. The crude 125.25 (C
5
0
B
3
0
T
5
0
T
0
);
3 B
0
0
Pb);
3
5
ꢁ
1
3
3
3
II
PF6
+
II
2+
filtrate under stirring. The resulting mixture was put in a freezer (MP bipy)(NO )] ; 425.1 [Ru (MPterpy)(MP bipy)(CH CN)] ;
for 3 h. Filtrating, washing with cold petroleum ether and drying 419.6 [Ru (MPterpy)(MP bipy)(NO )] ; 413.6 [Ru (MPterpy)-
under vacuum provided 93 mg (62%) of the desired compound, (MP
[Ru (MPterpy)(MP
bpy)] . UV-Vis (CH CN): lmax (e): 227 (52 000 L mol cm ),
3 /5 3 T 5 T 3 T
= 2.0 Hz); 8.71 (2H, s, H + H ); 8.50 (2H, m, H3T + H ); 275 (sh), 291 (51 000 L mol cm ), 315 (sh), 365 (39 000 L
0 0 0 0 00
2
2
2
3
II
ꢀ
2+
II
2
2
+
II
2+
2
III
bipy)(H
2
O)] ; 404.6 [Ru (MPterpy)(MP
2
bpy)] ; 283.4
1
3+
III
as a brown solid. H NMR (CD
3
CN, 400 MHz): d 9.87 (1H, d, H6B
,
2
bpy)(CH
3 2
CN)] ; 269.4 [Ru (MPterpy)(MP -
3
4
3+
ꢁ1
ꢁ1
J
J
6/5 = 6.0 Hz); 8.97 (1H, d, H3B, J3/5 = 2.0 Hz); 8.73 (1H, d, H
3
0
B
,
3
4
ꢁ1
ꢁ1
3
4
ꢁ1
ꢁ1
8
.23 (1H, dd, H5B, J5/6 = 6.1 Hz, J5/3 = 2.0 Hz); 8.14 (2H, m, H2Pb
+
=
mol cm ), 440 (sh). Anal. calcd for C46
43.37; H, 2.93; N, 6.6. Found: C, 43.25; H, 2.69; N, 6.47.
37 18 6 4 3
H F N O P Ru: C,
3
H
); 8.10 (2H, m, H + H ); 7.96 (2H, td, H + H 00 , J
6
Pb
2Pt 6Pt 4T 4 T 4/3
3
3
3
4
4
1
J4/5 = J 00 00 = J 00 00 = 7.9 Hz, J = J 00 00 = 1.6 Hz); 7.88 (2H, m,
[Ru(terpy)(MP bipy)(CH CN)](PF ) .
H
NMR (CD CN,
3
4 /3
4 /5
4/6
4 /6
2
3
6 2
3
H6T + H 00 ); 7.77 (2H, m, H
0
+ H
0
); 7.39–7.33 (3H, m, H
0
+
400 MHz): d 9.55 (1H, d, H , J = 6.0 Hz); 8.97 (1H, d, H3B
3/5 = 2.0 Hz); 8.70 (1H, d, H
,
6 T
2 Pb
6 Pb
6 B
6B
6/5
4
4
H
5T + H
5
00
T
); 7.30–7.23 (5H, m, H
Pb); 3.97 (6H, s, OCH
CN, 100 MHz): d 161.71 (C4Pb or C
5
0
B
+ H3Pb + H5Pb + H3Pt + H5Pt); 7.06
J
3
0
B
, J
3
0
/5
0
= 2.1 Hz); 8.58 (2H, d,
3
3
(
2H, m, H
C NMR (CD
3
0
Pb + H
5
0
3
ꢂ 2); 3.85 (3H, s, OCH
3
).
H
3
0
T
+ H
5
0
T
, J
3
0
/4
0
= J
5
0
/4
0
= 8.1 Hz); 8.44 (2H, m, H3T + H );
= 8.1 Hz); 8.19 (1H, dd, H5B
3
00
T
1
3
3
3
0
Pb or C4Pt); 8.32 (1H, t, H
0
,
J
0
0
=
J
0
0
,
3
4
4
T
4
/3
4
/5
7
58 | New J. Chem., 2019, 43, 755--767
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NJC
Paper
3
4
J
5/6 = 6.0 Hz, J5/3 = 2.0 Hz); 8.13 (2H, m, H2Pbpy + H6Pbpy); 8.01 (lmax = 365 nm or 436 nm). The temperature was maintained at
3
3
3
3
4
(2H, td, H4T + H
4
00
T
, J4/3 = J4/5 = J
4
00/300 = J
4
00/500 = 7.9 Hz, J4/6
=
25 1C during the whole experiment.
4
3
3
J400 00 = 1.5 Hz); 7.81 (2H, d, H6T + H 00 , J = J 00 00 = 5.5 Hz);
Quantum yield measurements. Light intensities were deter-
/6
6 T
6/5
6 /5
7
7
H
.75 (2H, m, H2
0
+ H
+ H
0
); 7.38 (2H, m, H5T + H 00 ); mined before each photolysis experiment by a chemical actino-
+ H3Pbpy + H5Pbpy); 7.05 (2H, m, metry procedure. The actinometers used were potassium ferri-
Pbpy
6 Pbpy
5 T
.30–7.21 (4H, m, H6
0
0
B
5 B
ꢁ
7
ꢁ1 ꢁ1
0
Pbpy + H
0
Pbpy); 3.95 (3H, s, OCH
Ru(MPterpy)(MP bipy)(CH CN)](PF
00 MHz): d 9.60 (1H, d, H6B
3B, J3/5 = 2.1 Hz); 8.83 (2H, s, H
.61 (2H, m, H3T + H ); 8.23 (1H, dd, H5B, J5/6 = 6.0 Hz, J5/3
); 3.83 (3H, s, OCH
).
oxalate at lirr = 436 nm (I
= 2.59 ꢂ 10 mol L
s
), and at
3
5
3
3
0
ꢁ6
1
ꢁ1 ꢁ1
[
2
3
6
)
2
.
H NMR (CD
3
CN,
l
irr = 365 nm (I
0
= 5.30 ꢂ 10 mol L
s
). The quantum yield (f )
A
3
4
H
8
2
,
J
0
6/5 = 6.0 Hz); 9.00 (1H, m, was determined by the program Sa3.3 written by D. Lavabre and V.
4
30
0
); 8.74 (1H, m, H
0
); Pimienta. It allows one to solve the following differential equation:
3
T
+ H
5
T
3
B
3
4
00
=
3
T
d½Aꢃ
A
l
A
.0 Hz); 8.16 (4H, m, H + H6Pt + H2Pb + H6Pb); 8.07 (2H, td,
¼ ꢁFAIa ¼ ꢁFA Abs I0F
(2)
2
Pt
3
dt
3
3
3
4
4
H4T + H 00 , J = J = J 00 00 = J 00 00 = 7.9 Hz, J = J 00 00
=
4 T 4/3 4/5 4 /3 4 /5 4/6 4 /6
3
3
A
l
1
.5 Hz); 7.85 (2H, d, H6T + H 00 , J = J 00 00 = 5.5 Hz); 7.78 (2H, where I
is the intensity of the light absorbed by the precursor, Abs
a A
6 T 6/5 6 /5
l
m, H
H
3
2
0
Pb + H
+ H3Pt + H5Pt + H3Pb + H5Pb); 7.08 (2H, m, H
3
.99 (3H, s, OCH ); 3.98 (3H, s, OCH ); 3.86 (3H, s, OCH ).
6
0
Pb); 7.40 (2H, m, H
6
0
B
+ H
5
0
B
); 7.34–7.27 (6H, m, the absorbance before irradiation, AbsTot the total absorbance, I
Pb); incident intensity measured at 436 nm, and F the photo-kinetic
factor given defined as:
0
the
5T + H
5
00
T
3 5
Pb + H
0 0
3
3
ꢀ
ꢁ
l
Abs
ꢁ 10ꢁ Tot
Abs
1
Computational details
The four ruthenium–nitrosyl complexes were fully optimized
F ¼
(3)
l
Tot
2
4
using the Gaussian-09 program package within the frame-
work of Density Functional Theory (DFT). In all cases, the
calculations were performed in the presence of acetonitrile,
which was modelled using the Polarizable Continuum Model
l
Eqn (2) was fitted with the experimental data Abs
= f (t)
Tot
and 2 parameters f and e (e is the molar extinction coeffi-
A
B
B
cient measured at the end of the reaction) at two wavelengths
irr = 436 nm, lobs = 470 nm for [RuT1B2](PF ; lirr = 365 nm,
obs = 420 nm for [RuT0B0](PF and lirr = 365 nm, lobs
70 nm for [RuT0B2](PF ). lobs was chosen because it corre-
(
l
4
l
6 3
)
2
5
(
SCRF = PCM method). The double-z basis set 6-31G* was
6
)
3
=
used for all atoms except the heavy ruthenium atom, for which
the LANL2DZ basis set was applied to account for relativistic
6 3
)
sponds to a large difference between the 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 fitting of the model to the experimental data.
2
6
effects. We have selected the hybrid functional B3PW91 for
the optimization, for better coherency with previous investigations.
Indeed, B3PW91 has been shown to outperform other hybrid
functionals (e.g. B3LYP) and pure functionals (e.g. PW91) in
30,31
It
2
7
numerous cases of ruthenium complexes, especially when
has to be pointed out that fNO was previously published elsewhere
2
8
back bonding ligands (like NO) are present. In our previous
18c
for [RuT1B0](PF
6
)
3
.
Nevertheless, we have found that this previous
. Contrary to
3
+ 18c
investigation of [RuT1B0] ,
several functionals were tested
value was overestimated due to a wrong evaluation of I
0
for the calculations of the UV-visible spectra (B3PW91, B3LYP,
the previous study, the pH of the solution was carefully lowered by
addition of sulfuric acid in the actinometry procedure in the present
case, thus leading to a better evaluation of I and a concomitant
29
PBE0, and CAM-B3LYP) by time-dependent (TD)-DFT. CAM-B3LY
was finally selected, for its better accuracy to reproduce the experi-
mental transition energies (o0.5 eV in all cases). Using the same
conditions was found to be satisfactory here as well. The computed
0
reduction of fNO. The experimental data reported here are all based
on this new procedure.
II
+
geometries of FT, and cis- and trans-[Ru (FT)Cl (NO)] are provided
2
Data for [RuT0B0](PF
6
)
3
as ESI† with the details of the related UV-visible computed spectra.
ꢁ5
ꢁ1
365
B
365
A
ꢁ1
ꢁ1
420
B
[
A]
0
= 8.66 ꢂ 10 mol L , e
= 7500 mol L cm ,
= 2246 mol L cm , e
4
20
ꢁ1
ꢁ1
ꢁ1
ꢁ1
e
6
A
= 577 mol L cm , e
=
Photochemical investigations
ꢁ1
ꢁ1
813 mol L cm
Data for [RuT0B2](PF )
6 3
.
Photochemistry. Kinetic studies on the photolysis reactions
were carried out with a diode array Hewlett Packard 8452A
ꢁ
5
ꢁ1
365
A
ꢁ1
ꢁ1
[A]
= 2.33 ꢂ 10 mol L , e
= 31 759 mol L cm
= 14 116 mol L cm , e
,
=
0
4
70
ꢁ1
ꢁ1 365
ꢁ1
ꢁ1 470
spectrophotometer. Solutions of 2 mL of [RuT0B2](PF
6
)
3
(2.33 ꢂ
e
A
= 3527 mol L cm , e
B
B
ꢁ5
ꢁ1
ꢁ1
ꢁ1
1
0
mol L for irradiation at 365 nm) and [RuT1B2](PF
6
)
3
180 58 mol L cm
.
ꢁ
5
ꢁ1
(3.75 ꢂ 10
mol L
for irradiation at 436 nm) in non-
Data for [RuT1B2](PF )
6 3
ꢁ
5
ꢁ1
436
A
ꢁ1
ꢁ1
deoxygenated acetonitrile were used. The optical fiber was fixed
laterally to the cuvette. Absorption spectra were taken after each
[A]
0
= 3.76 ꢂ 10 mol L , e
= 17 872 mol L cm
= 15 131 mol L cm , e
,
=
4
70
ꢁ1
ꢁ1 436
ꢁ1
ꢁ1 470
e
A
= 8318 mol L cm , e
B
B
ꢁ
1
ꢁ1
3
minutes, in fast scan mode, during a period of 21 hours, 20 126 mol L cm .
which allows reaching apparently stable absorption conditions.
The UV-vis spectra were recorded under irradiation realized
Electrochemical studies
with a Muller reactor device equipped with a cooling water Electrochemical experiments were performed with a potentiostat
filter and a mercury arc lamp equipped with an appropriate Autolab PGSTAT100 (EcoChemie, The Netherlands) controlled by
interference filter to isolate the desired irradiation wavelength GPES 4.09 software. Experiments were carried out at room
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Paper
NJC
temperature in a homemade airtight three-electrode cell connected 10.18 ppm (in CD
3
CN) to 9.82 ppm (in CD
3
CN) and finally to
3 6 3
to a vacuum/argon line. The reference electrode was a saturated 9.23 ppm (in CD CN) in the case of [RuT0B2](PF ) and from
calomel electrode (SCE) separated from the solution by a bridge 10.20 ppm (in CD OD) to 9.87 ppm (in CD CN) and finally to
3
3
compartment. The counter electrode was a platinum wire of ca. 9.23 ppm (in CD CN) in the case of [RuT1B2](PF ) .
3
6 3
2
1
cm apparent surface area. The working electrode was a Pt micro-
Applying two-dimensional NMR-techniques HMBC, HMQC
disk (radius = 0.25 mm). The supporting electrolyte (nBu4N)(PF6) and COSY allows the complete assignment of the peaks observed in
1
13
(Fluka, 99% electrochemical grade) was used as received and simply
2
H- and C-NMR spectra for the four diamagnetic Ru(Cl), Ru(NO ),
degassed under argon. Acetonitrile was freshly purified prior to use. and Ru(NO)-based species. The stretching vibrations (nNO) are
ꢁ
1
ꢁ1
The solutions used during the electrochemical studies were observed at 1933 cm and 1937 cm for [RuT1B2](PF
6
)
3
, and
ꢁ3
ꢁ1
ꢁ1
ꢁ1
10
mol L and 10 mol L respectively for the ruthenium com- [RuT0B2](PF ) , respectively. These values fall close to those of
6 3
ꢁ
1
ꢁ1
18c
plexes and for the supporting electrolyte. Before each measurement, 1940 cm and 1944 cm previously recorded in [RuT1B0](PF ) ,
6
3
3
3
the solutions were degassed by bubbling argon through them, and and [RuT0B0](PF ) . Nevertheless, a tendency for decreasing
6
3
the working electrode was polished with a polishing machine (Presi frequencies is apparent when the number of methoxyphenyl
P230). Typical instrumental parameters for recorded square wave donating substituent increases. This observation is in complete
voltammograms were: SW frequency f = 20 Hz, SW amplitude Esw
=
agreement with the idea that increasing the donating capabilities of
20 mV, and scan increment dE = 5 mV. The irradiation performed the complexes gradually increases the charge transfer towards the
prior to recording the voltammograms of the photoproducts was antibonding p* level of the withdrawing nitrosyl, and reduces its
performed with a mercury lamp. The voltammograms for the new bond order and eventually its stretching frequency.
[
RuT0B2](PF ) and [RuT1B2](PF ) compounds and the complete
6 3 6 3
redox potentials tables along the [RuT0B0](PF ) , [RuT1B0](PF ) , Optical properties
6
3
6 3
[
RuT0B2](PF ) , [RuT1B2](PF ) series are provided as ESI.†
6 3 6 3
The UV-visible spectra recorded in acetonitrile are compared in
Fig. 1 for the four compounds under investigation. At first glance,
a rough tendency for higher intensity is observed as the size of the
p-skeleton of the ligands increases, from [RuT0B0](PF6)3 to
Results and discussion
Synthesis and characterization
[
RuT1B2](PF ) . More precisely, and apart from high energy transi-
6 3
The synthesis of [RuT0B2](PF
here follows the general route previously reported for [RuT1B0](PF )
which implies the introduction of the ruthenium atom as Ru Cl
followed by that of the nitrosyl ligand through a first reaction of
nitrite (NO
procedure avoids the production of a large amount of undesirable
homoleptic [Ru (terpy)
Ru(NO) is introduced with the [Ru Cl
Contrary to the synthesis of related complexes built up from
6
)
3
and [RuT1B2](PF
6
)
3
reported
tions in the 250–300 nm range having likely p - p* intra-ligand
character, two main features are observed at about 375 nm and at
about 425 nm. The transition at 375 nm is present in the four
complexes, with a tendency for higher intensity in the species
6
3
III
3
ꢁ
+
18c
2
) switched to NO in acidic medium. This synthetic
2
containing the MP bipy ligand. The transition at 425 nm of weaker
intensity is present in species containing the MPterpy ligand, only.
Interestingly, the UV-visible spectrum of [RuT1B2](PF ) seems to
II
2+
2
]
species which was observed when
6
3
II
2ꢁ
32
5
(NO)] reagent.
result grossly from the sum of that of [RuT1B0](PF ) , and that of
6
3
[
RuT0B2](PF
the origin of the electronic transition at 425 nm is roughly the
same for both [RuT1B2](PF , and [RuT0B2](PF . This idea is
6 3
) . This observation suggests that, to a large extent,
1
8
unsubstituted bipyridines, the first step of the present synthesis
Scheme 2) requires the use of the rather insoluble MP bipy,
(
2
6
)
3
6 3
)
which led to unexpected difficulties. Eventually, the reactions
were successfully achieved in ethylene glycol, following a pro-
cedure previously used in our group in the case of ruthenium
further developed by DFT analysis.
DFT analyses
3
3
nitrosyl complexes with insoluble ligands. The use of an
excess of triethylamine (TEA) is justified at this step to facilitate
Before discussing the computed optical properties, it is interesting
to comment on the evolution of a few relevant data from the ground
III
II
the reduction of Ru to Ru according to a mechanism
3
4,35
previously suggested,
which implies electron transfer from
TEA to the hexacoordinated ruthenium center without direct
binding. Additionally, we have observed in most cases that a
III
III
[
Ru (terpy)(bipy)X] complex undergoes spontaneous Ru
-
II
Ru reduction, even in the absence of a clear reducing agent
(
vide infra). In the case of [RuT1B2](PF
6
)
3
, the substitution of
ꢁ
2
for Cl required a purification step as previously reported
ꢁ
NO
2
2
by Snow et al. The reaction steps 2 and 3 in Scheme 2 are easily
monitored by H-NMR spectroscopy, from the observation of
1
the shift of one proton located in position 6 on the bipyridine,
spatially close to the monodentate ligands (Cl, NO2 or NO),
and therefore subject to their deshielding influence. Thereby,
2
switching from –Cl to –NO substituents and subsequently to
Fig. 1 Comparison of the UV-visible spectra of the ruthenium complexes,
–
NO leads to the peak-shifting of the proton in position 6 from in acetonitrile.
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3+
3+
Table 1 Relevant data for the DFT-computed ruthenium nitrosyl complexes in 394 nm for [RuT1B0] and [RuT0B2] , respectively. The detailed
the ground state: NO bond length (dNO in Å), NO stretching frequency
analysis of these transitions is provided in Table 2, to check this
ꢁ1
(n
NO in cm ), and torsion angle between the methoxyphenyl and the
hypothesis.
The data gathered in Table 2 lead to the suggestion that the
related pyridine rings (a in 1)
3
+
a
two dominant transitions of [RuT1B2] (1 - 5 and 1 - 7) are
3
+
3+
Compound MP
d
NO
n
NO
2
MP-terpy MP -bipy
reminiscent of those of [RuT1B0] (1 - 3) and [RuT0B2]
3
+
+
+
+
(1 - 5), respectively. Indeed, they are found to possess roughly
the same energies, relative intensities and charge transfer
properties. The comparison of these transitions is further
[
[
[
[
RuT0B0]3
0
1
2
3
1.132 2054
1.134 2043 28.81
1.133 2045
RuT1B0]3
a
b
b
RuT0B2]
RuT1B2]
26.70(t) and 28.28(c)
3
a
1.135 2033 29.13
27.09(t) and 28.57(c)
depicted at the molecular level in Fig. 3. It allows concluding
a
3+
Methoxyphenyl on the pyridine in the trans position with respect to unambiguously that the two transitions present in [RuT1B2]
b
the Ru(NO). Methoxyphenyl on the pyridine in the cis position with
respect to the Ru(NO).
3+
3+
are those of the parent [RuT1B0] and [RuT0B2] species
without any significant mixing. This absence of mixing can
be tentatively related to the symmetries of the orbitals involved
state optimizations in order to evaluate the charge transfer features in the transitions.
along the [Ru(NO)] series. They are gathered in Table 1.
Within the assumption of an ideal octahedral RuN
As previously discussed, the charge transfer effects on NO nation sphere, schematized in Scheme 3, the terpyridine ligand
increasing dNO, and decreasing nNO values) are more pronounced appears to lie in the XZ plane and the bipyridine ligand to lie in the
6
coordi-
(
3+
when the number of MP donor units increases. Nevertheless, the XY plane. In [RuT1B0] , the dominant 1 - 3 transition possesses
opposite is observed for the donating counterparts. Indeed, the MP
* character, while the dominant 1 - 5 transition of
to pyridine charge transfer is reduced (increasing torsion angle) [RuT0B2] possesses pZ(bipy) to dYZ * character. Both charge
when the number of donors increases from 0 to 3. This evolution is transfer phenomena being strictly orthogonal, it is therefore not
p
Y(terpy) to dXYp
X
3+
p
Z
3+
readily understood from the fact that the withdrawing capability of a surprising that they found their counterpart in [RuT1B2] , without
single NO is distributed on several donors, thus leading to a any mixing.
reduction of the effect on each individual donor. Additionally, the
data gathered in Table 2 suggest that the effect of one substitution
achieved on the terpyridine ligand is as efficient as that of two
ꢀ
NO photo-release from [RuT1B0](PF
[
6
)
3
, [RuT0B2](PF
6
)
3
and
6 3
RuT1B2](PF )
substitutions achieved on the bipyridine. A similar behavior has Numerous ruthenium nitrosyl complexes have been observed
ꢀ
previously been observed in our group for a fluorenyl-based Ru(NO) to release NO under irradiation, with very different quantum
3
3
complex.
yields, depending on the nature of the ligands. For example, in
2ꢁ3+
The TD-DFT computed spectra are compared in Fig. 2 for the the case of [Ru(bipy) L(NO)]
four [RuT0B0] , [RuT1B0] , [RuT0B2] , and [RuT1B2] complexes. a fNO value of 0.98 when L = Cl . This value drops to 0.17 with
Although some discrepancies are apparent in general between the L = 4-picoline and furthermore to 0.07 with L = 4-acetylpyridine.
theoretical and experimental spectra, it is possible to point out a few In [Ru(terpy)L(NO)] complexes, fNO = 0.14 when L = bipy, but
computational features supporting the experimental data: (i) the rises to 0.46 when L is an orthophenylenediamine. The authors
tendency for higher overall intensity as the size of the p-skeleton of relate these evolutions to the withdrawing character of the L
the ligands increases is evidenced in full agreement with the ligand, which hampered efficient Ru - NO charge transfer,
experiment; (ii) the presence of a sizeable low-lying transition at assuming that this charge transfer is the dominant parameter to
complexes, Da Silva et al. reported
2
3+
3+
3+
3+
ꢁ 12
12
3+
17
ꢀ
l 4 400 nm is observed only for the complexes in which a donor is account for the NO release.
present on the terpyridine (MPterpy); and (iii) the spectrum of
Clearly ruthenium–nitrosyl complexes with polypyridyl ligands
possess a very broad range of possible fNO values. However,
3+
3+
[
RuT1B2] seems to result from the sum of those of [RuT1B0]
3+
and [RuT0B2] , in the low-lying 375–425 nm wavelength domain. the mechanism responsible for the release process was never
More precisely, the transitions at 413 nm and 382 nm computed for unambiguously established. According to theoretical analyses
3+
[RuT1B2] seem to correspond to those computed at 417 nm and by Mascharak, the release is ascribed in all cases to the
Table 2 Details of the low energy transitions involved in the TD-DFT computed spectra of the ruthenium–nitrosyl complexes: absorption maxima (lmax),
oscillator strengths (f), composition of the configuration interactions (CI), and characters
a
Compound
Transition
lmax (nm)
f
CI expansion
Main character
3
+
[
RuT0B0]
1 - 7
345
324
417
434
393
413
382
0.058
0.042
0.227
0.029
0.276
0.269
0.359
59% w116-117 + 14% w112-117
88% w115-118
76% w144-145
33% w172-174 + 32% w166-174
47% w171-174 + 31% w172-174
65% w198-201
terpy - Ru(NO)
bipy - Ru(NO)
MP(terpy) - Ru(NO)
MP(bipy) + bipy - Ru(NO)
MP(bipy) - Ru(NO)
MP(terpy) - Ru(NO)
MP(bipy) - Ru(NO)
1
- 10
3
+
+
[
[
RuT1B0]3
RuT0B2]
1 - 3
1 - 3
1
- 5
1 - 5
- 7
3+
[
RuT1B2]
1
37% w199-202 + 36% w200-202
a
3+
3+
3+
3+
Orbitals 116 (117), 144 (145), 172 (173), and 200 (201) are the HOMO (LUMO) in [RuT0B0] , [RuT1B0] , [RuT0B2] , and [RuT1B2] , respectively.
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supporting a mechanism in one step. Apart from this simple
picture, different dissociation mechanisms have been suggested
with a first excitation towards an additional ligand, however still
followed by conversion to the antibonding dRu–pNO* excited
3
6–38
state.
Recently, molecular dynamics beyond the Born–
Oppenheimer approximation has been used to point out the
possibility of intersystem crossing towards a triplet state susceptible
to undergoing additional excitation towards higher triplet states or
ꢀ
isomerization to a metastable bent conformation prior to the NO
releasing.
9,39
The use of multiconfigurational wave functions has
also been proposed to provide a better description of the
40
bonding properties of Ru(NO) metal complexes. These proposed
mechanisms should further be tested by experimental data to
support the computational suggestions, although it is difficult to
compare experiments conducted on different species at different
wavelength irradiation, owing to the number of potential para-
meters to be taken into account.
A recent investigation in our group has come to the conclusion
that the quantum yield of photo-release (fNO) is hardly correlated
with the donating capabilities of the ligands, along a series of
+
41
substituted [Ru(R-terpy)Cl (NO)] complexes. However, the
2
irradiation was performed at a constant wavelength on these
complexes in which the absorption maxima, and the nature
and the number of the involved transitions were variable, thus
hampering reliable conclusions to be established. By contrast,
we present here an evaluation of fNO at l = 436 nm in
[
RuT1B0](PF ) and [RuT1B2](PF ) , which undergo the same
6 3 6 3
transition at this wavelength, selected among our set of filters
available as the most suitable for this investigation. First, the
deconvolution of the UV-visible spectra in the low energy
domain (Fig. 4) is examined to verify the effect of the irradiation
on the different species. The wavelengths of the de-convoluted
transitions are compared to the TD-DFT data in Table 3, to
verify the reliability of the analysis. In the case of [RuT1B0](PF
transition at 425 nm is the only one active under irradiation at
36 nm. By contrast, the deconvolution reveals the presence of three
transitions (l = 340 nm, 369 nm, and 421 nm) in closer proximity in
RuT1B2](PF , so the resulting shape shows a shoulder at the
6 3
) , the
4
[
6 3
)
irradiation wavelength. Nevertheless, it appears that about 90% of
the light provided at 436 nm is used to promote the transition at
421 nm (Fig. 4, bottom). It can therefore be concluded that, to a
large extent, the behaviors of [RuT1B2](PF and [RuT1B0](PF )
6
)
3
6 3
should be identical under this irradiation. By contrast, the decon-
volution clearly reveals that the irradiation has to be conducted
around l = 350 nm in [RuT0B2](PF ) , for the only intense transition
6
3
observed at low-energy. More precisely, l = 365 nm was selected as
corresponding to the most suitable wavelength in our set of filters
available.
Fig. 2 Comparison of the UV-visible spectra computed at the CAM-
B3LYP/6-31G* level for the ruthenium nitrosyl complexes.
The change in the electronic absorption spectrum of
[
RuT1B2](PF
is shown in Fig. 5. It reveals the presence of isosbestic points at
population of an excited state having strong dRu–pNO* anti- l = 222, 266, 348 and 442 nm, which indicate a clean conversion
6 3
) in acetonitrile, under irradiation at l = 436 nm,
3
a,36
II
+
bonding character.
Interestingly, the associated transitions of the Ru (NO ) complexes to the related photolysed species.
tend towards the overall process depicted in eqn (1), which No back-reaction is observed when the light is turned off. In the
+
ꢀ
leads to: (i) electron transfer to NO which is switched to NO , resulting photo-product, new bands arise at 282, 312, and 476 nm.
ꢀ
(
ii) weakening of the Ru–NO bond and finally, (iii) NO release Similar experiments were conducted on [RuT1B0](PF
6
)
3
at 436 nm
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3+
3+
3+
Fig. 3 Main orbitals involved in the dominant low-energy transitions of [RuT1B0] , [RuT0B2] , and [RuT1B2]
.
II
complex is invariably reduced to Ru , in the case of complexes
in which 5 pyridines are present in the coordination sphere, so
II
[
Ru (solvent)] is the only observable species arising after the
ꢀ
18
NO release. A previous investigation has pointed out the
unexpected appearance of nitrates in the crystal structure of the
II
Ru photoproduct of RuNO complexes containing 5 pyridines.
ꢀ
This leads to the suggestion of NO as the potential reducing
agent in the fast reduction process. Indeed, one third of the
released NO available could be used to reduce the photo-
1
8c
Scheme 3 Partial coordination sphere in the XY plane for a [Ru(terpy)-
ꢀ
3+
(
6
bipy)(NO)] species in the case of ideal RuN octahedral symmetry.
products, as described below:
þ1e
III
II
Ru ƒƒ ! Ru
(4)
(5)
6 3
and on [RuT0B2](PF ) at 365 nm (see ESI†). Moreover, the three
photoproducts exhibit rather similar UV-visible spectroscopic
features with a transition in the 450–500 nm domain and a
multi-transition band located between 250 and 350 nm.
It is worth pointing out that the shape of the UV-visible
ꢁ
1e
ꢀ
ꢁ
1
=3NO ƒƒ ! 1=3NO3
No other investigations were carried out to elucidate the
spectra of the photoproducts (red curve in Fig. 5 and Fig. S7 in nature of the reducing agent accounting for the fast reduction
ESI†) does not exhibit the large and broad band centered at process.
III
II
III
6
00 nm characteristic of Ru , which is predicted by eqn (1).
The presence of Ru instead of Ru after irradiation is
+
ꢀ
This might suggest that NO is released as NO rather than NO . further confirmed by electrochemistry, with the appearance of
However, this hypothesis has to be abandoned after the exam- an oxidation wave after irradiation along the RuNO series, the
ination of the EPR spectra recorded under irradiation in the values of which are gathered in Table 4. Interestingly, the
II
presence of [Fe (MGD) ] which behaves as a powerful spin oxidation becomes more favorable (shifted to lower potentials)
2
trapping reagent and leads to the observation of a stable spin as the number of donating substituents increases, in agree-
ꢀ
adduct exhibiting the characteristic triplet signal of NO (Fig. 6). ment with chemical intuition. A similar effect is evidenced
+
ꢀ
These behaviors are exemplified here for [RuT0B2](PF
Fig. 6, with a hyperfine splitting constant value of a
= 1.21 ꢂ potential indicating that it becomes more difficult to reduce the
cm and a g-factor equal to 2.040, values which are fully species having more donating substituents. Altogether the
6 3
) , in before irradiation with the NO - NO wave shifted to lower
N
ꢁ
3
ꢁ1
10
II
consistent with reports found in the literature for [Fe (MGD)
2
-
spectroscopic and electrochemical data lead to the conclusion
2
3
NO] adducts. Note that the residual signal observed without that, although the nature of the reducing agent is not clearly
irradiation (upper part of Fig. 6) is related to the fact that established, eqn (1) has to be re-written as follows:
experiments are never strictly conducted in the dark.
Â
Ã
Â
Ã
hn
II
þ
III
ꢀ
Ru ðNO Þ þ solvent ƒ! Ru ðsolventÞ þ NO
Eventually, the observation of a ruthenium(II) complex as the
stable final photoproduct is not so surprising. Indeed, we have
observed that, after NO release, the expected [Ru (solvent)]
(6)
Â
Ã
fast
ƒ! Ru ðsolventÞ þ NO
II
ꢀ
ꢀ
III
3+
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6 3
Fig. 5 Changes in the absorption spectrum of [RuT1B2](PF ) , in aceto-
nitrile, under irradiation at l = 436 nm. The blue curve corresponds to the
starting complex and the red curve corresponds to the photo-product.
Fig. 4 UV-visible spectra of [RuT1B0](PF
black solid lines) with deconvolutions: the red dashed lines are the sum of the
contribution of dotted lines (see text).
6 3 6 3 6 3
) , [RuT0B2](PF ) , and [RuT1B2](PF )
(
Table 3 Comparison of the de-convoluted (left) and computed (right)
transitions in the UV-visible spectrum of [RuT1B0](PF , [RuT0B2](PF
and [RuT1B2](PF : data in blue are ascribed to T1 - RuNO transitions and
ꢀ
II
Fig. 6 Representative triplet signal from NO trapped by [Fe (MGD)
acetonitrile, after irradiation of [RuT0B2](PF with a Hg lamp (bottom).
The signal recorded before irradiation is given as a reference (top).
2
] in
6
)
3
6 3
) ,
6 3
)
6 3
)
data red to B2 - RuNO transitions
Experimental data
TD-DFT computation
6 3 6 3 6 3
of [RuT1B0](PF ) , [RuT0B2](PF ) , and [RuT1B2](PF ) are shown
Compound
Deconvoluted transition Compound Transition
in Table 5. Surprisingly, and while the transitions arise from the
same general MP - RuNO charge transfer behavior in all cases,
their associated fNO values scale from 0.002 to 0.024, in a ratio of
1 to 12. More precisely, the experiments conducted under the
same irradiation conditions (l = 436 nm) on [RuT1B2](PF ) and
3+
3+
3+
[RuT1B0](PF
[RuT0B2](PF
[RuT1B2](PF
6
6
6
)
)
)
3
3
3
425 nm
69 nm
458 nm (weak)
56 nm
421 nm
[RuT1B0]
[RuT0B2]
[RuT1B2]
417 nm
355 nm
434 nm (weak)
393 nm
413 nm
3
3
6
3
3
3
69 nm
40 nm
382 nm
349 nm
[
RuT1B0](PF ) lead to f values of 0.002 and 0.011, respectively,
6 3 NO
in a ratio 1 to 6, contrary to the expectation of unvarying properties
arising from identical (MP-terpy - RuNO) charge transfer
Finally, the fNO values obtained by treatment of the time transitions. This striking difference forces us to recognize that
dependence of the absorbance of the intense low-energy transitions the simple one step (transition - release) mechanism cannot
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Table 4 Selected redox potentials (vs. SCE electrode) observed in the such as bipyridine, has been observed under irradiation in
square-wave voltammograms of [RuT0B0](PF , [RuT1B0](PF , [RuT0B2](PF
and [RuT1B2](PF
6
)
3
6
)
3
6
)
3
,
43–46
some cases.
Nevertheless, a set of experimental features
6 3
)
allow us to exclude this possibility as a significant behavior:
Before irradiation
NO /NO
After irradiation (i) isosbestic points in the UV-vis spectra revealing the presence
III II
+
ꢀ
Compound
Ru /Ru
of a single photoproduct; (ii) photochemical analysis applied
satisfactorily on the basis of a single photoproduct, (iii) a lack
of NO in the photoproduct confirmed by EPR and IR spectro-
scopy, and (iv) finally the presence of a single photoproduct
[RuT0B0](PF
[RuT1B0](PF
[RuT0B2](PF
[RuT1B2](PF
6
6
6
6
)
)
)
)
3
3
3
3
0.47a
0.44
0.42
0.39
1.29a
1.27
1.23
1.20
1
monitored by H NMR spectroscopy.
a
Data from ref. 12c.
account for the observation of such different properties. It has
Conclusion
therefore to be inferred that the singlet excited state promoted
ꢀ
after one-photon absorption cannot release NO spontaneously. The report of the new [RuT0B2](PF
)
3
, and [RuT1B2](PF
)
3
6
6
II
Alary et al. have recently proposed that, in [Ru (terpy)- compounds allowed us to obtain a series of four ruthenium
+
3+
Cl
2
(NO)] complexes, the Ru–NO bond cannot be broken in nitrosyl complexes built up from the [Ru(terpy)(bipy)(NO)] core
the singlet excited state, but is rather switched to a triplet state with 0, 1, 2, and 3 methoxyphenyl donating units. Computational
9
by an intersystem crossing reaction. Furthermore, this triplet DFT analysis conducted in support of the experimental data leads to
could be subject to evolution towards a bent Ru–N–O inter- a comprehensive report of the role of the charge transfer effects in
mediate, the so called MS2 state, which is subject to additional both ground and excited states with low-energy transitions arising
photon absorption, and finally, to the release of nitric oxide. A from methoxyphenyl to ruthenium nitrosyl charge transfer, in all
two-photon mechanism was recently evidenced in the RuNO/ cases. Despite these similarities, irradiation performed on these
ꢀ
RuON photo-isomerization of [Ru(py)
4
Cl(NO)](PF
6
)
2
, a reaction transitions leads to NO photorelease with significantly different
which frequently competes with the photorelease and implies quantum yields, in the broad 0.002–0.02 range. More precisely, and
4
2
first the breaking of the ruthenium–nitrosyl bond. Owing to due to orbital orthogonality, the low energy transitions are found to
this observation, the issue of a possible sequential two-photon be identical in [RuT1B0](PF and [RuT1B2](PF , nevertheless,
6
)
3
6 3
)
mechanism along the present RuNO series cannot be avoided. with different quantum yields in a ratio of 1 to 6. This unexpected
To the best of our knowledge, the requirement for a second result proves that the intuitive mechanism based on a simple
photon and the need for a triplet state mechanism were never ‘‘absorption + release’’ scheme is unable to account for the
proven experimentally in the release process. In the present experimental data. More sophisticated models are required
case, the requirement for an intermediate (e.g. MS2) species is either by introducing very reactive intermediates or intersystem
not verified experimentally, due to the presence of isosbestic conversion to hypothetical triplet states, as suggested by theory.
points in Fig. 5 which favors a simple two-species (A - B) Additional experimental data are necessary to clarify if more
ꢀ
mechanism. If it is involved here, MS2 must necessarily be than one photon is required to release NO in these species.
extremely photo-reactive, so its concentration stays very weak
throughout the entire release process. Although this possibility
cannot be strictly dismissed, we clearly have no experimental Conflicts of interest
data in hand suggesting that this two-photon mechanism takes
There are no conflicts to declare.
place in our compounds.
As a final comment, it is worth pointing out that the photo-
6 3
product generated under irradiation of [RuT1B2](PF ) under-
Acknowledgements
goes significant absorption at 436 nm (Fig. 5). As a consequence
the completion of the photo-release process required a long
irradiation time of about 40 hours. Under these conditions, the
issue of possible additional photoreactions of the photoproduct
with dissociation of the chelating ligands could have been
considered. Indeed, release of bidentate p-acceptors ligands,
The authors wish to thank Dr Alix Sounia-Saquet (LCC-CNRS)
for performing the electrochemical experiments, Lionel Rechignat
(LCC-CNRS) for his assistance with EPR experiments, and
Dr Isabelle Sasaki (ISM-Bordeaux) for helpful discussions.
M. R. thanks the Minist `e re de l’Enseignement Sup ´e rieur et de
la Recherche (MESR) for a PhD fellowship.
ꢀ
Table 5 Quantum yield of NO photo release (fNO) after irradiation (lirr
)
of the intense low-energy transition (lmax
)
6 3
of [RuT0B0](PF ) ,
[
RuT1B0](PF
6
)
3
, [RuT0B2](PF
6
)
3
, and [RuT1B2](PF
6
)
3
References
Compound
lmax (nm)
lirr (nm)
f
NO
1
L. J. Ignarro, Nitric oxide biology and pathobiology, Academic
Press, San Diego, 1st edn, 2000.
[
[
[
[
RuT0B0](PF
RuT1B0](PF
RuT0B2](PF
RuT1B2](PF
6
6
6
6
)
)
)
)
3
3
3
3
352
425
356
421
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