Photo-/Electroinduced Irreversible Isomerization of 2,2’-Azobispyridine Ligands in Arene Ruthenium(II) Complexes

Article abstract of DOI:10.1002/chem.202100142

Novel arene Ru^II complexes containing 2,2’-azobispyridine ligands were synthesized and characterized by using ¹H and ¹³C NMR spectroscopy, UV/vis spectroscopy, electrochemistry, DFT calculations and single-crystal X-ray diffraction. Z-configured complexes featuring unprecedented seven-membered chelate rings involving the nitrogen atom of both pyridines were isolated and were shown to undergo irreversible isomerization to the corresponding E-configured five-membered chelate complexes in response to light or electrochemical stimulus.

Full text of DOI:10.1002/chem.202100142

Accepted Article
Accepted Article
Title: Photo/Electro-Induced Irreversible Isomerization of 2,2’-
Azobispyridine Ligand in Arene Ruthenium(II) Complexes
Authors: Jonathan Long, Divyaratan Kumar, Claire Deo, Pascal
Retailleau, Galina Dubacheva, Guy Royal, Juan Xie, and
Nicolas Bogliotti
This manuscript has been accepted after peer review and appears as an
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To be cited as: Chem. Eur. J. 10.1002/chem.202100142
Link to VoR: https://doi.org/10.1002/chem.202100142
0
1/2020

Chemistry - A European Journal
10.1002/chem.202100142
FULL PAPER
Photo/Electro-Induced Irreversible Isomerization of 2,2’-
Azobispyridine Ligand in Arene Ruthenium(II) Complexes
[
a]
[a]
[a]
[b]
[a,c]
Jonathan Long, Divyaratan Kumar, Claire Deo, Pascal Retailleau, Galina V. Dubacheva,
Guy
[
c]
[a]
[a]
Royal, Juan Xie, and Nicolas Bogliotti*
[
a]
J. Long, D. Kumar, C. Deo, Dr. G. Dubacheva, Prof. J. Xie, Dr. N. Bogliotti
Université Paris-Saclay, ENS Paris-Saclay, CNRS, Photophysique et Photochimie Supramoléculaires et Macromoléculaires
9
1190, Gif-sur-Yvette, France
E-mail: nicolas.bogliotti@ens-paris-saclay.fr
[
[
b]
c]
Dr. P. Retailleau
Université Paris-Saclay, CNRS, Institut de Chimie des Substances Naturelles, UPR 2301
9
1198, Gif-sur-Yvette, France
Dr. G. V. Dubacheva, Prof. G. Royal
Université Grenoble Alpes, CNRS, Département de Chimie Moléculaire (UMR5250)
F38400, Grenoble, France
Supporting information for this article is given via a link at the end of the document.
Abstract: Novel arene Ru(II) complexes containing 2,2’-
coordination of the two nitrogen atoms of the pyridine rings of Z-
1
azobispyridine ligands were synthesized and characterized using H-
2,2’-azobispyridine, suggested by Baldwin as early as 1969,
1
3
[33]
and
C-NMR, UV/Vis spectroscopy, electrochemistry, DFT
have never been reported so far.
calculations and single crystal X-ray diffraction. Z-configured
complexes featuring unprecedented 7-membered chelate ring
involving the nitrogen atom of both pyridines were isolated and were
shown to undergo irreversible isomerization to the corresponding E-
configured 5-membered chelate complexes in response to light or
electrochemical stimulus.
4
,4'-azobispyridine
3,3'-azobispyridine
E-Z photo-isomerization
E-Z photo-isomerization
M
N
N
N
N
N
N
N
M
N
II
I
E-2,2'-azobispyridine
no photo-isomerization
Z-2,2'-azobispyridine
photo/electro-isomerization
(this work)
(M)
Introduction
N
N
M
N
Azopyridine is
a
photochromic organic compound that
N
N
IIIa
N
N
M
undergoes reversible E–Z isomerization upon UV irradiation. It is
[
1]
N
N
also a well-known ligand for metal center , with resulting
N
[
2]
N
N
IV
novel coordination pattern
complexes used as magnetic resonance imaging probes ,
[
3,4,5]
cytotoxic agents against cancer cell,
components in switchable materials.
or as building
M
IIIb
[6,7,8,9,10]
The ligand
properties of azobispyridine have been investigated towards a
Figure 1. Common coordination pattern exhibited by azobispyridine (I, II, III)
and a novel 7-membered chelate ring described in the present study (IV).
[
11,12,13,14,15,16]
[13,15,17,18]
number of metals, such as Cd(II)
, Cu(II)
,
,
,
[
19,20,21]
[13,18,22]
[13,15,23]
[15,23]
[24,25]
Re(I)
, Cu(I)
, Ni(II)
, Zn(II)
, Ru(II)
[
26,27]
26]
[11,13]
[21]
[28]
[15]
Ag(I)
,
Co(II)
,
Re(II) and (V)
,
Pt(II)
, Hg(II)
[
[13]
[29]
[30]
Au(I) , Mn(II) , W(0) and Os(0) , and Ti(II) , giving rise to
In this work, we report the synthesis of novel arene ruthenium(II)
complexes derived from 2,2’-azobispyridine, notably exhibiting
an unprecedented Z-configured seven-membered chelate ring
with the metal center (Scheme 1). The study of their behavior in
response to light or redox stimuli revealed a complete and
irreversible Z→E isomerization with concomitant rearrangement
of the coordination pattern of the ligand.
new types of polymer structures involving π-π and π-p stacking
[
13,14,15,16,30]
of the ligand
. The coordination properties to the metal
center are obviously impacted by the position of the nitrogen
atom in the pyridine ring of the ligand. Linear patterns such as I
[
11-
and II are most common for 4,4’- and 3-3’-azobispyridine
1
5,19,20,22,24,27,29]
(
Figure 1), while bidentate coordination mode with
[
16-18,21,23,26,28]
five-membered chelate rings such as IIIa
rarely tridentate coordination such as IIIb, are found with E-2,2’-
, or more
[
31,32]
azobispyridine.
A peculiar property of the latter ligand in
these systems is that it usually does not undergo E-Z photo-
isomerization as a consequence of the geometrical constraints
imposed by the strong bonding to the metal center, with one
[
26]
notable exception found in the case of Ag(I) complex. To the
best of our knowledge, metal complexes of type IV involving
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FULL PAPER
R
R
1
2
ligands, the H-NMR spectra in dichloromethane-d of all E
photon
complexes revealed a similar pattern with a set of 8 signals
(each integrating for 1H) between 7.5 and 9.5 ppm (see SI),
corresponding to the magnetically non-equivalent H atoms of the
+
+
Ru
Ru
Cl
N
Cl
N
or
N
N
N
redox input
N
N
[
35]
N
azobispyridine ligand, in agreement with previous reports. The
non-symmetric nature of the azobispyridine ligand in these
Z
E
(
7-membered chelate)
(5-membered chelate)
1
3
complexes was also confirmed by
C-NMR spectroscopy
showing eight signals accounting for pyridine CH groups in the
range 116–154 ppm. The two quaternary carbons next to
Scheme 1. Photon or redox input-induced isomerization of Z-configured 7-
membered chelate Ru complexes into E-configured 5-membered chelate
complexes.
+
nitrogen atoms appeared at 163.0 and 162.4 ppm in E-3 (only
+
one visible at 164.4 ppm in E-4 ), while none were observed in
+
the case of E-2 , presumably due to long relaxation times.
Results and Discussion
+
+
Single crystals of E-3 and E-4 suitable for X-ray analysis were
obtained by chloroform vapor diffusion in an acetone solution
(Figure 2). The molecular structure of both complexes exhibits a
classical "three-legged piano stool" geometry around Ru center,
with Ru–N bond lengths ranging between 1.997 and 2.056 Å,
Ru-centroid distances of 1.719 and 1.740 Å, azo N–N bond
lengths around 1.27 Å, and N1-Ru-N3 and N1-Ru-Cl angles
being around 75° and 84°, respectively (Table 1). These values
+
+
+
+
Synthesis and molecular structure of E-2 , E-3 , E-4 , Z-5
+
and Z-6
irr.
15 nm
N
N
N
N
3
N
N
N
CH Cl2
2
N
E-1
Z-1 (41%)
[
35]
[
Ru(Ar)Cl2]2
NH PF
[Ru(Ar)Cl2]2
NH PF6
MeOH
compare well with those previously reported. The most striking
difference between the two complexes is the relative orientation
of both pyridine rings, which are almost coplanar in the case of
4
6
4
MeOH
+
+
PF6–
PF6–
E-3 and significantly twisted in the case of E-4 , as revealed by
dihedral angle N1-C2-C1-N4 values of -30.4° and -128.6°,
respectively, thereby evidencing the influence of arene ligand
substitution on the geometry of the complex.
Cl
Ru+
Ru+
N
N
N
Cl
N
N
N
N
E-2+ (92%)
N
+
Z-5 (89%)
–
PF6–
PF
6
Cl
Ru+
Ru+
N
N
Cl
N
N
N
N
N
N
E-3+ (71%)
Z-6 (82%)
+
PF6–
bz
p-cym
Ru+
Figure 2. ORTEP representation (thermal ellipsoids drawn at 50% probability
N
Cl
N
+
+
level) of E-3 (left) and E-4 (right) with atom numbering. The hydrogen atoms
Ar =
hmbz
N
N
-
6
and PF anion have been removed for clarity.
E-4+
(
(
86% from E-1)
63% from Z-1)
+
Table 1. Selected bond lengths (Å) and angles (deg.) for complexes E-3 and
E-4 .
+
+
+
+
+
+
+
+
E-3
E-4
Scheme 2. Synthesis of Ru complexes E-2 , E-3 , E-4 , Z-5 and Z-6 .
Ru-N1
Ru-N3
2.035(5)
2.046(5)
1.267(7)
1.719
2.056(3)
1.997(2)
1.275(3)
1.740
Irradiation of
a
dichloromethane solution of the known
[
33,34]
–2
azobispyridine E-1
at 315 nm (P = 45 mW.cm ) for 4 h
N2-N3
resulted in a ca. 6:4 mixture of E and Z isomers, which were
separated by column chromatography to yield 41% of Z-1, stable
for several months when stored in the dark below 4°C (Scheme
[
a]
Ru-cent.
N1-Ru-N3
N1-Ru-Cl
75.2(2)
83.70(2)
-30.4(8)
0.8(5)
74.8(1)
84.13(7)
-128.6(3)
1.3(2)
2
). Reaction of E-1 in methanol with ruthenium dimers
Ru(Ar)Cl , where Ar = benzene (bz), para-cymene (p-cym) or
hexamethylbenzene (hmbz), followed by counter-ion exchange
[
2 2
]
+
+[35]
N1-C2-C1-N4
N1-Ru-N3-N2
with NH
4
PF
6
yielded the expected complexes E-2 , E-3
and
+
E-4 as dark red powders in good yield (71 to 92%, Scheme 2).
1
13
The identity of these complexes was assessed by H- and C-
NMR spectroscopies, HRMS analysis and X-ray diffraction
studies. Besides the expected signals corresponding to arene
[a] cent.=centroid of arene moiety
2
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Photo-induced isomerization studies
The absorption spectra of CH Cl solutions of Z-5 and Z-6
2 2 2 2
Reaction of Z-1 with [Ru(bz)Cl ] and [Ru(p-cym)Cl ] in the dark
+
+
under otherwise the same conditions yielded the dark green and
2
2
+
+
showed a similar profile, with a sharp band at 270 nm and 276
nm, respectively and a broader one in the visible region at 424
nm and 419 nm, respectively (Figure 4).
deep yellow complexes Z-5 (89%) and Z-6 (82%), respectively.
1
2
The H-NMR spectra recorded in dichloromethane-d revealed a
significant simplification of the signals as compared to the
+
Upon irradiation of Z-5 at 438 nm, the intensity of the band at
corresponding E isomers, with four peaks between 7.41 and
2
70 nm decreases significantly while two main bands at 363 nm
9
.04 ppm each integrating for 2H, ascribed to magnetically
and 501 nm, as well as a broader signal centered at 638 nm
appear. Two isosbestic points are observed at 250 and 285 nm
equivalent hydrogen atoms of the pyridine rings. The symmetric
nature of the azobispyridine ligand in those complexes was also
+
1
3
and the resulting spectrum perfectly matches the one of E-2
assessed in the C-NMR spectra, showing a set of five signals
in the range 120–164 ppm. In the solid state, both complexes
remained stable for months when stored in the dark at -4°C, and
(
see Figure S3), thereby evidencing Z→E photo-isomerization
process, as confirmed by the color change of the solution from
deep green to dark red. A similar behavior was observed in the
freshly prepared CH
2
Cl
2
solutions thereof could be easily
+
case of Z-6 with the apparition of two intense bands at 363 nm
handled over the course of several hours if kept at rt away from
light (Figure S1,S2).
and 515 nm (four isosbestic points located at 253, 290, 424 and
+
4
39 nm) assigned to the formation of complex E-3 . These
Surprisingly, treatment of Z-1 with [Ru(hmbz)Cl
2
]
2
did not lead to
+
experiments allowed the determination of photoisomerization
the corresponding Z-complex but rather to E-4 (in 63% yield) as
+
1
13
quantum yield (at 438 nm) of 42% and 41%, respectively for Z-5
unambiguously determined from H and C-NMR analysis.
+
+
+
+
and Z-6 . The clean and complete photoconversion of Z-6 into
Here, it is likely that the Z-complex similar to Z-5 and Z-6 is
unstable in solution and rapidly isomerizes to the E form
because of the steric hindrance of the hmbz ligand.
+
1
E-3 was also unambiguously confirmed by H-NMR analysis in
+
+
2 2
CD Cl , while the transformation Z-5 →E-2 under these
conditions showed partial loss of the benzene ligand (see Figure
+
Despite our efforts, suitable crystals for X-ray analysis of Z-5
[36]
S7,S8).
+
and Z-6 could not be obtained, thereby leading us to investigate
+
To get insight into the electronic transitions involved in Z-5 and
the geometry of putative structures using DFT calculations. Our
modeling studies revealed that both complexes could exist as
+
Z-6 , TD-DFT calculations were performed at the PBE0/6-
+
+
+
311+G(d,p)/LANL2DZ level (Figure S5,S6), followed by NTO
two distinct sets of diastereoisomers Z-5 / Z-5’ and Z-6 / Z-
+
+
analysis of relevant vertical excitations. In the case of Z-5 and
6
’ , with N=N linkage facing either the chloro or the arene ligand.
Geometry optimization performed at the B3LYP/6-
11G(d,p)/LANL2DZ level revealed an increased stability
+
Z-6 , the transitions calculated at 473 and 474 nm, respectively,
close to irradiation wavelength (438 nm), revealed a change in
electron density from the ruthenium center to the π* orbital of the
N=N azo bond (Figure 5). This suggests that irradiation induces
3
+
+
(
around 80 kJ/mol) of both Z-5 and Z-6 as compared to their
+
+
diastereoisomers Z-5’ and Z-6’ (Figure 3).
Z→E isomerization of the ligand, accompanied with
a
modification of its coordination pattern, finally leading to
+
+
complexes E-2 and E-3 .
A striking feature of this system is that no back photo-
+
+
isomerization of E-2 or E-3 to their corresponding Z-complexes
could be observed, whatever the wavelength of irradiation. While
TD-DFT analysis reveals the occurrence of vertical excitation to
the π* orbital of N=N bond at several wavelengths (Figure S9),
thus possibly leading to an equilibrium between E and Z isomers
+
upon irradiation, the higher thermodynamic stability of E-2 over
+
+
+
Z-5 (ΔE = + 87 kJ/mol) and E-3 over Z-6 (ΔE = + 92 kJ/mol)
presumably drives equilibrium towards exclusive formation of E-
isomers (Table S1).
+
+
+
+
Figure 3. DFT-optimized geometries for Z-5 / Z-5’ (top) and Z-6 / Z-6’
(
bottom) at the B3LYP/6-311G(d,p)/LANL2DZ level of theory. The hydrogen
atoms have been removed for clarity. The structures in frame correspond to
the lowest energy isomers.
3
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+
+
Figure 5. Relevant natural transition orbitals pairs of Z-5 (top) Z-6 (bottom)
from holes (left) to particles (right) for electronic transition calculated at 473
and 474 nm respectively.
Electro-induced isomerization studies
The higher stability of the p-cym derivatives as compared to the
bz derivatives prompted us to focus our elecrochemical
+
+
investigations on E-3 and Z-6 . The redox properties of the
ruthenium complexes were investigated by cyclic voltammetry
(
CV)
in
CH
2
Cl
2
containing
tetra-n-butylammonium
hexafluorophosphate (TBAPF
6
, 0.2 M) as supporting electrolyte.
Representative curves are shown in Figures 6,7 and S10-S12.
A
*
•
5ꢀµA
+
+
Figure 4. Absorption spectra of complexes Z-5 (top) and Z-6 (bottom) in
CH Cl (µM range concentration) at 25 °C (black line) and their stepwise
2
2
evolution (step = from 1s to 30s, ttot = 150s) upon irradiation at 438 nm (grey
2
lines) with P = 12 mW/cm . The red line corresponds to the final state. Inset
shows color evolution of irradiated solutions of Z-5 and Z-6 .
+
+
B
C
5ꢀµA
•
*
•
6ꢀµA
-0.9
-0.6
-0.3
0.0
+
E (V) vs Fc /Fc
+
+
Figure 6. Cyclic voltammograms of (A) E-3 , (B) E-3 after electrolysis at Eap
=
-
+
-
0.9V (1e reduction) and (C) E-3 upon reduction at Eap = -0.9V followed by
-1
reoxidation at Eap = -0.2V. Scan rate 0.1 V.s . *indicate very weak oxidation
peaks.
4
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+
-1
The CV curve of E-3 at a scan rate of 100 mV.s is
A
•
characterized by a reversible reduction wave at E1/2 = -0.59 V vs
+
Fc /Fc with ΔE = 75 mV (Figure 6A). By analogy with previous
[37]
studies,
this signal is attributed to the reduction of the Ru
3ꢀµA
II
I
center (Ru /Ru couple). An additional irreversible peak is also
observed at Epc = -1.26V (Figure S10,S11). This signal is
ascribed to the reduction of the azopyridine ligand bound to the
metal center, close to the reduction peak of the free ligand that
is observed at Epc = -1.52 V (measured under our experimental
conditions).
B
C
5ꢀµA
+
Reduction of the E-3 complex was realized by bulk electrolysis
•
at applied potential Eap = -0.9V. Under these conditions, a brown
solution is obtained and coulometry measurements confirmed
the monoelectronic nature of the first reduction process. The CV
curves of the electrogenerated solution (Figure 6B) exhibits a
•
a c
non-fully reversible oxidation wave (ip /ip <1) at E1/2 = -0.59 V
0
attributed to the oxidation of the metal center in E-3 . An
additional irreversible peak at Epa = -0.28V is also seen. Such
irreversible oxidation is also observed in the voltammograms of
3ꢀµA
+
E-3 when scanning first toward low potentials (see * signal in
Figure 6 and S10, weak oxidation) and its relative intensity
depends on the sweep rate and on the potential of the reverse
scan. These features are clearly due to the presence of
chemical reactions coupled to the electron exchanges (possibly
-1.2
-0.9
-0.6
-0.3
0.0
+
E (V) vs Fc /Fc
+
+
Figure 7. Cyclic voltammograms of (A) Z-6 , (B) Z-6 after electrolysis at Eap
=
+
0
-
+
ECE processes): upon reduction of E-3 , E-3 is produced and
-1.1V (1e reduction) and (C) Z-6 upon reduction at E = -1.1V followed by
ap
0
-1
this complex can then lead to a new more stable species (X ),
reoxidation at Eap = 0V. Scan rate 0.1 V. s .
whose structure could not be precisely determined but in which
the coordination mode around the metal center has been
0
1.00
modified. At the CV time scale, X is barely formed whereas it is
A
the major species obtained at the electrolysis time scale.
When the previous solution was re-oxidized by electrolysis at Eap
0.75
=
-0.2V, the CV curve of the new solution (Figure 6C) was
+
similar to the signal obtained for E-3 , showing that the initial
Ru(II) complex can be fully regenerated. These results were
corroborated by UV/vis measurements (see Figure 8A): during
0
.50
+
0.25
reduction of the initial pink solution of E-3 , absorption bands
around 290 nm and 500 nm progressively increased,
corresponding to the signature of the reduced complex. Upon re-
oxidation, the initial absorption bands showing the presence of
0.00
1.00
4
00
600
800
Wavelength (nm)
+
the E-3 isomer were recovered. These observations are overall
B
consistent with the transformations reported in Scheme 3a.
0.75
0.50
0.25
0.00
+
+
1e-
1e-
- 1e-
a) E-3+
b) Z-6+
E-30
X0
E-3+
+
(
minor) (major)
- 1e-
X0
E-3+
400
600
800
+
+
Scheme 3. Transformations of species E-3 (a) and Z-6 (b) upon electrolysis
Wavelength (nm)
conditions.
+
+
Figure 8. Spectra of (A) E-3 and (B) Z-6 complexes: initial solution (black
-
-
-
line), upon 1e -reduction (red line) and upon 1e -reduction followed by 1e -
oxidation (blue line).
+
Cyclic voltammograms of a solution of Z-6 (Figure 7 and S12)
display an irreversible reduction peak at -0.95V (Ru(II) → Ru(I))
and two ill-defined oxidation peaks can be seen during the return
scan at -0.57V and -0.36V (reduction of the azopyridyl part is
also observed at lower potential, see Figure S12). Such
irreversibility of the reduction process is in accordance with
5
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strong structural changes upon reduction of the metal center in
procedures, and analyzed by the service of Structural Crystallography at
Institut de Chimie des Substances Naturelles (Gif-sur-Yvette). High-
resolution mass spectra (HRMS) were performed on a Bruker maXis
mass spectrometer by the "Fédération de Recherche" ICOA/CBM
+
-
Z-6 . Upon electrolysis of this solution at Eap = -1.1V (1e per
molecule, Figure 7B), a brown solution is formed and the
corresponding CV curve exhibits a well-defined irreversible
oxidation peak at -0.3 V coupled to a reduction peak observed
during the return scan at -0.65V. This CV curve is close to the
(
FR2708) platform.
+
Electrochemical measurements: Cyclic Voltammetry (CV) and
electrolysis experiments were performed with CH Instrument
voltammogram observed after reduction of E-3 , which seems to
0
a
indicate that complex X is formed. Upon re-oxidation of the
potentiostat (CHI 660B), using a standard one-compartment, three-
electrode electrochemical cell. Experiments were conducted at 298 K in
previous solution by electrolysis at Eap = 0V (Figure 7C), a
reversible reduction wave was seen at E1/2 = -0.60V (ΔE = 80
p
CH
TBAPF
2
Cl
2
containing
tetra-n-butylammonium
hexafluorophosphate
+
mV). These data clearly indicate that an isomerization from Z-6
to E-3 is electrochemically triggered, by reduction of Z-6
(
6
, 0.2M) as supporting electrolyte. Typically, millimolar solutions
+
+
of the complexes were used for the electrochemical studies. The counter
+
complex followed by its re-oxidation of the solution.
electrode was a platinum wire and an ALS Co. Ltd. Ag |Ag electrode
+
-2
These results were unambiguously confirmed by UV/Vis
([Ag ]=10 M in CH
separated from the solution by
3
CN) was used as reference. This electrode was
+
spectroscopy (Figure 8B): when the green initial solution of Z-6
a
bridge compartment. For CV
is first reduced by bulk electrolysis (Eapp = -1.1V), a brown
solution is generated having UV-vis absorption bands close to
measurements, the working electrode was a 1 mm diameter platinum
disk which was polished with 1 µm diamond paste before each recording.
+
+
Potentials are referenced to the ferrocenium/ferrocene (Fc /Fc) couple,
the signal observed upon reduction of the E-3 complex,
0
which was determined by the addition of ferrocene in the solution after
each experiment. Bulk electrolyses were carried out at controlled
indicating that the same species X is formed when Z or E
-
complexes are reduced. Upon re-oxidation (1e per molecule) of
2
potential with a platinum plate (~2 cm ) as the working electrode. The
the previous solution by bulk electrolysis (Eapp = 0V), the
absorption bands of the resulting pink solution clearly shows the
auxiliary electrode was a large area platinum coil isolated from the
working compartment by a glass frit containing CH Cl + TBAPF , 0.2M.
2 2 6
+
presence of E-3 isomer, with significant bands around 360 and
In order to monitor the evolution of the species during bulk electrolysis, a
UV/Vis probe (1 mm all–quartz Helmma immersion probe) connected to
the spectrophotometer (Varian Cary 60) through an optical fiber was
dipped in the solution. Spectroelectrochemical experiments were also
conducted using a UV-vis cell (1 mm) equipped with an Ag wire (pseudo
reference electrode), a Pt wire (counter electrode) and a platinum grid
5
10 nm.
A
plausible pathway for the electro-induced
+ +
isomerization of Z-6 into E-3 is described in Scheme 3b.
(
working electrode), the cell being inserted in the UV/vis spectrometer. All
Conclusion
measurements were taken in absence of oxygen, by bubbling argon
before experiments.
A series of arene ruthenium(II) complexes derived from E and Z
Molecular modeling: Molecular geometries were optimized by the DFT
method using the B3LYP functional, the 6-311G(d,p) basis set and
LANL2DZ for the ruthenium, as implemented in the Gaussian09 software
2
,2'-azobispyridine, among which two exhibit unprecedented
coordination patterns involving the pyridine nitrogen atoms rings,
was synthesized and studied. The Z-configured complexes were
shown to undergo a complete and irreversible isomerization to
their corresponding E-isomers (with concomitant reorganization
of the coordination pattern) in response to light or
electrochemical input. The generalization of such ligand
behavior to other class of complexes, as well as their possible
applications are currently under investigation and will be
reported in due course.
[
41]
[42]
package.
Results were viewed with Avogadro 1.1.1
software and
the absence of negative frequency was checked to confirm the obtention
of an energy minimum. Oscillator strengths were calculated by TDDFT
[43]
method using PBE0 functional, the 6-311+G(d,p) basis set
LANL2DZ for the ruthenium.
and
General procedure for the determination of molar absorption
coefficient: Around 1.00 mg of the compound is precisely weighted and
dissolved in 6 mL of solvent to give a S
prepared at different concentrations by adding 20, 50, 100, 200,
00 ,400, 500 or 600 µL of S to 2.5 mL of solvent. The spectrum of each
0
solution. Eight solutions were
3
0
solution was recorded and the absorbance corresponding to a local
maximum was plotted as a function of concentration. The resulting data
were then linearized according to Beer-Lambert law.
Experimental Section
Equipment: Commercially available solvents and reagents were used
without further purification. Absorption spectra were recorded on a Cary-
Procedure for the determination of photoisomerization quantum
+
+
yield of Z-5 and Z-6 . Absorption spectra were recorded at regular time
intervals during irradiation. The resulting experimental kinetic profile was
fitted following appropriate photokinetic differential equation using a
4
000 spectrophotometer from Agilent Technologies. Photochromic
reactions were induced in situ by a continuous irradiation with Hg/Xe
lamp (Hamamatsu, LC6 Lightningcure, 200W) equipped with a narrow
band interference filter of appropriate wavelength (Semrock). The
irradiation power was measured using a photodiode from Ophir (PD300-
UV). Reactions were monitored by Thin Layer Chromatography (TLC)
using commercial TLC Silica gel 60 F254 with detection by UV light (254
nm or 365 nm). Column chromatography purification was performed on
CombiFlash-Rf with UV detection (two channels).
[44]
home-made Igor procedure (Wavemetrics).
3
4
trans-2,2’-azobispyridine E-1 : To an ice-cold solution of bleach (4.8%
NaOCl, 150 mL, 54.0 mmol, 5.0 eq.) was added dropwise a solution of 2-
aminopyridine (1.00 g, 10.6 mmol, 1.0 eq.) in water (10 mL). After stirring
at rt for 1 h, the reaction mixture was extracted with CH
2 2
Cl (ca. 3×150
mL), the organic layer was dried over MgSO , filtered and the solvent
4
removed under reduced pressure. Purification by silica gel
Analysis: NMR spectra were recorded using
spectrometer. Irradiation frequencies of H and C are respectively
a
JEOL ECS-400
chromatography (PE/EA: 5/5 to 3/7) afforded E-1 (448 mg, 2.44 mmol,
1
13
1
4
6%). Red solid; Rf: 0.26 (EA); H NMR (400 MHz, CDCl
3
): δ ppm 8.71
1
3
3
99.78 MHz and 100.53 MHz. When specified, C-NMR spectra of
38
(
2
1
d, J = 5.2 Hz, 2H), 7.99 (d, J = 1.6 Hz, 2H), 7.50 (dd, J = 5.2, 1.6 Hz,
H); C NMR (100 MHz, CDCl ): δ ppm 163.3, 150.5, 146.5, 126.5,
3
14.9.ꢀ
species were recorded using UDEFT sequence. Chemical shifts are
13
reported in delta (δ) units part per million (ppm) relative to the residual
[39,40]
solvent signal or Si(CH
3
)
3
.
Coupling constants are reported in Hertz
(
Hz). The following abbreviations are used: s = singlet, d = doublet, t =
cis-2,2’-azobispyridine Z-1: A solution of E-1 (353 mg, 1.90 mmol, 1.0
eq.) in CH Cl (50 mL) was irradiated for 4 h at 315 nm. The reaction
mixture was concentrated under reduced pressure and the E and Z
triplet, q = quartet, sept. = septuplet, m = multiplet, br = broad. Melting
points were measured with a Köfler bench previously calibrated. Infrared
spectra were measured using
Electron Corporation with a Nexus model equipped with an ATR-
Germanium module. Spectra were recorded using OMNI software. Single
crystals for X-Ray diffraction were obtained as described in experimental
2
2
a FTIR spectrometer from Thermo
isomers were separated by silica gel chromatography in the dark (PE/EA:
6
/4 to 4/6) to afford the starting material E-1 (210 mg, 1.14 mmol, 59%)
and Z-1 (144 mg, 0.779 mmol, 41%). The latter can be used within a few
6
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Chemistry - A European Journal
10.1002/chem.202100142
FULL PAPER
days when stored in the dark at –20°C. Orange solid; Rf: 0.61 (PE/EA:
acknowledged for an international incoming mobility fellowship
to DK. This work was performed using HPC resources from the
1
2
/8) H NMR (400 MHz, CDCl
3
): δ ppm 8.01 (d, J = 5.6 Hz, 2H), 7.56 (d,
1
3
J = 2.0 Hz, 2H), 7.09 (dd, J = 5.6, 2.0 Hz, 2H); C NMR (100 MHz,
CDCl ): δ ppm 164.9, 148.5, 145.9, 123.2, 119.9.
“
Mésocentre” computing center of CentraleSupélec and École
(
3
Normale Supérieure Paris-Saclay supported by CNRS and
Région Île-de-France (http://mesocentre.centralesupelec.fr/).
General procedure for the synthesis of [Ru(Ar)(azpy)Cl]PF
synthesis: To a solution of [Ru(Ar)Cl (0.08 mmol, 0.5 eq.) in MeOH (5
mL) under argon was added dropwise a solution of 1 (30 mg, 0.16 mmol,
.0 eq.) in MeOH (1.5 mL). After stirring at rt for 30 min, NH PF (132
mg, 0.80 mmol, 5.0 eq.) was added and the mixture was stirred for 10
min. The precipitate was filtered and washed with Et O to afford
6
2 2
]
Keywords: azobispyridine • arene ruthenium • photochemistry •
1
4
6
electrochemistry • coordination pattern
2
[
6
Ru(Ar)(azpy)Cl]PF .
+
[
(
9
8
(
Ru(bz)(trans-azpy)Cl]PF
CH Cl /MeOH: 95/5); m.p. > 270°C; H NMR (400 MHz, CD
.48 (d, J = 5.6 Hz, 1H), 8.90 (d, J = 4.0 Hz, 1H), 8.73 (d, J = 7.6 Hz, 1H),
.35 (td, J = 7.6, 1.2 Hz, 1H), 8.14-8.05 (m, 2H), 7.86 (m, 1H), 7.79-7.75
6
E-2 : 92% yield, black red solid; Rf: 0.29
1
2
2
2 2
Cl ): δ ppm
1
3
m, 1H), 6.32 (s, 6H); C UDEFT NMR (100 MHz, CD
49.6, 141.6, 139.8, 131.3, 130.0, 129.5, 116.4, 91.7, 2Cq not visible;
IR (ATR): 2986; 1467; 1439; 1390; 1239; 1154; 1050; 963; 835; 799; 744
cm ; ε (λ= 363 nm, CH Cl ): 11800 L·mol ·cm ; HRMS (ESI) m/z:
2 2
+
4
Calcd for C16H14ClN Ru [M] : 398.9945, found 398.9952.
2 2
Cl ): δ ppm 155.0,
1
–
1
−1
−1
+
+
[35]
[
0
Ru(p-cym)(trans-azpy)Cl]PF
.23 (CH Cl /MeOH: 95/5); m.p. 182°C; H NMR (400 MHz, CD
ppm 9.48 (dd, J = 5.6, 1.2 Hz, 1H), 8.89 (dt, J = 4.4, 0.8 Hz, 1H), 8.72
dd, J = 7.6, 0.8 Hz, 1H), 8.33 (td, J = 8.0, 1.6 Hz, 1H), 8.11-8.05 (m, 2H),
.89 (td, J = 6.0, 1.6 Hz, 1H), 7.77 (m, 1H), 6.38 (d, J = 6.8 Hz, 1H), 6.33
d, J = 6.8 Hz, 1H), 6.04 (d, J = 6.4 Hz, 2H), 2.57 (sept., J = 6.8 Hz, 1H),
6
E-3
:ꢀ 71% yield, black red solid; Rf:
1
2
2
2 2
Cl ): δ
(
7
(
2
1
1
1
3
2 2
.27 (s, 3H), 0.99 (t, J = 6.8 Hz, 6H); C NMR (100 MHz, CD Cl ): δ ppm
63.0, 162.4, 154.2, 148.8, 140.6, 139.0, 130.0, 129.3, 128.4, 115.7,
11.1, 110.1, 92.0, 90.7, 88.4, 87.6, 31.0, 21.5, 21.4, 18.9; IR (ATR):
973; 2899; 1466; 1436; 1375; 1248; 1054; 963; 834; 794; 744; 694 cm
ε (λ= 363 nm, CH Cl ): 13400 L·mol ·cm ; HRMS (ESI) m/z: Calcd
–
2
1
−1
−1
;
2 2
+ +
for C20
H
22ClN
4
Ru [M] : 455.0576, found 455.0577.ꢀ
+
[
(
Ru(hmbz)(trans-azpy)Cl]PF
CH Cl
.82 (dd, J = 6.0, 1.2 Hz, 1H), 8.78 (dd, J = 4.0, 0.8 Hz, 1H), 8.71 (dd, J =
6
E-4 : 86% yield, black red solid; Rf: 0.26
1
2
2 2 2
/MeOH: 95/5); m.p. > 270°C; H NMR (400 MHz, CD Cl ): δ ppm
8
8
1
1
1
.0, 0.8 Hz, 1H), 8.31 (td, J = 8.0, 1.6 Hz, 1H), 8.12 (td, J = 8.0, 2.0 Hz,
H), 8.01-7.98 (m, 1H), 7.96-7.91 (m, 1H), 7.73-7.68 (m, 1H), 2.01 (s,
8H); C UDEFT NMR (100 MHz, CD
41.2, 139.9, 129.8, 129.4, 128.0, 118.2, 103.4, 16.1, 1Cq not visible; IR
1
3
2 2
Cl ): δ ppm 164.4, 151.2, 149.6,
(
6
ATR): 2985; 2892; 1457; 1432; 1367; 1243; 1072; 899; 836; 780; 741;
–
1
−1
−1
99; 679 cm ; ε (λ= 311 nm, CH Cl ): 10000 L·mol ·cm ; HRMS (ESI)
2 2
+ +
m/z: Calcd for C22
H
26ClN
4
Ru [M] : 483.0889, found 483.0889.
+
[
(
Ru(bz)(cis-azpy)Cl]PF
6
Z-5 :ꢀ 89% yield, dark green solid; Rf: 0.37
1
2 2 2 2
CH Cl /MeOH: 95/5); H NMR (400 MHz, CD Cl ): δ ppm 9.04 (d, J =
5
.2 Hz, 2H), 8.08 (td, J = 7.6, 1.2 Hz, 2H), 7.57 (d, J = 7.6 Hz, 2H), 7.41
13
(
m, 2H), 5.91 (s, 6H); C UDEFT NMR (100 MHz, CD
2 2
Cl ): δ ppm 163.6,
1
54.4, 142.3, 125.9, 119.8, 86.7; IR (ATR): 1598; 1441; 833; 784; 741
–
1
cm .ꢀ
+
[
(
5
(
Ru(p-cym)(cis-azpy)Cl]PF
6
Z-6 : 82% yield, yellow solid; Rf: 0.23
/MeOH: 95/5); H NMR (400 MHz, CD Cl ): δ ppm 8.98 (d, J =
.6 Hz, 2H), 8.09 (td, J = 8.0, 1.2 Hz, 2H), 7.60 (d, J = 7.6 Hz, 2H), 7.42
td, J = 6.4, 0.8 Hz, 2H), 5.74 (d, J = 6.0 Hz, 2H), 5.58 (d, J = 6.0 Hz, 2H),
1
CH
2
Cl
2
2
2
1
3
2
.66 (sept., J = 6.8 Hz, 1H), 2.04 (s, 3H), 1.22 (d, J = 6.8 Hz, 6H);
Cl ): δ ppm 163.8, 153.5, 142.3, 126.1,
19.9, 106.6, 101.3, 85.3, 84.5, 31.4, 22.3, 18.8; IR (ATR): 2971; 1598;
C
UDEFT NMR (100 MHz, CD
1
1
2
2
–
1
466; 1429; 1058; 1026; 836; 783; 762 cm
.
Acknowledgements
We thank Nolwenn Mahieu and Sylvain Badin for preliminary
synthesis of Ru-complexes, Dr. Stéphane Maisonneuve for
assistance with NMR measurements, and Nicolas Fabre and Dr.
Rémi Métivier for their help in the determination of
photoisomerization quantum yield. ENS Paris-Saclay is
7
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Chemistry - A European Journal
10.1002/chem.202100142
FULL PAPER
Entry for the Table of Contents
One shot switch in response to light or electrochemical stimuli of 2,2’-azobispyridine involved in a 7-membered chelate ring with a
Ru(II) metal center is disclosed. The molecular pathways at work in the course of this transformation were investigated by a
combination of analytical and computational techniques, thereby providing a rational for the efficient and irreversible nature of the
process.
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