Photoswitchable Arene Ruthenium Complexes Containing o-Sulfonamide Azobenzene Ligands

Article abstract of DOI:10.1021/acs.organomet.5b00871

A series of arene ruthenium complexes containing o-sulfonamide azobenzene ligands were synthesized and found to exhibit uncommon coordination pattern with an exocyclic =N bond. Upon irradiation, these complexes cleanly undergo E → Z photoisomerization fol

Full text of DOI:10.1021/acs.organomet.5b00871

Article
pubs.acs.org/Organometallics
Photoswitchable Arene Ruthenium Complexes Containing
o‑Sulfonamide Azobenzene Ligands
Claire Deo,† Nicolas Bogliotti,*,† Remi Metivier,† Pascal Retailleau,‡ and Juan Xie*,†
́
́
†PPSM, ENS Cachan, CNRS, Universite
́
Paris-Saclay, 94235 Cachan, France
‡Institut de Chimie des Substances Naturelles, CNRS UPR 2301, Universite
91198 Gif-sur-Yvette, France
́ ́
Paris-Sud, Universite Paris-Saclay, 1, av. de la Terrasse,
S
* Supporting Information
ABSTRACT: A series of arene ruthenium complexes containing o-sulfonamide azobenzene ligands were synthesized and found
to exhibit uncommon coordination pattern with an exocyclic NN bond. Upon irradiation, these complexes cleanly undergo E
→ Z photoisomerization followed by thermal Z → E isomerization (upon resting in the dark) whose rate is dependent on the
solvent, the nature of the arene group, the sulfonamide moiety, and azobenzene substitution, as revealed by structure−property
studies.
We reasoned that structural modification of the ligand could
affect the coordination pattern in such a way that the
photoswitching ability of azobenzene would be maintained
upon coordination to the metal center. The resulting
organometallic complexes could be interesting candidates for
the photocontrol of catalytic activity39−44 and biological
applications.1,3,5,9,45−47 Here, we wish to report a new type of
photoswitchable arene ruthenium complex containing o-
sulfonamide azobenzenes as ligands (V; Figure 1) and describe
their photoisomerization properties.
INTRODUCTION
■
The photoisomerization properties of azobenzene have been
extensively exploited for the control of events at various scales
and interfacing many fields such as molecular biology,
pharmacology, chemistry, material sciences, physics, informa-
tion, and data storage technology.1−10 Azobenzene and its
derivatives such as 2-(arylazo)pyridines, 2-(arylazo)pyrimidines,
and 2-(arylazo)imidazoles are also appealing compounds in
coordination chemistry due to their interesting redox behavior
and the strong M−L bonding resulting from their potent π-
acceptor character.11 Remarkably, these ligands have been
frequently combined with ruthenium, and the resulting
complexes have found promising applications in catalysis12−16
and as cytotoxic agents toward various cancer cell lines.17−20
These compounds most commonly exhibit bidentate or
tridentate coordination patterns such as I,17−30 II,13,16,31,32
III,14,15,24,25,32,33 and IV12 (Figure 1). In each case, the NN
bond is located inside a chelate ring formed with the metal, thus
preventing azobenzene photoisomerization. Only a few
exceptions involving multimetallic species have been shown
to maintain photoisomerization ability of azobenzene upon
direct coordination of both nitrogen atoms of the NN bond
to the Ru atom.34−36 More recent examples concern the
photoisomerization of azobenzene-coupled metal complexes
without coordination of the azobenzene moiety.37,38
RESULTS AND DISCUSSION
■
Synthesis and Photophysics of Ligand 3 and Complex
4. The prototype o-tosylamide azobenzene 348 was synthesized
as described in Scheme 1 in order to investigate its behavior as
a ligand for ruthenium. Commercially available o-phenylenedi-
amine was tosylated under standard conditions (TsCl,
pyridine) to yield 91% of 1,49 which underwent reaction with
oxone to afford o-tosylaminonitrosobenzene (2) in 94% yield.
This compound was treated under Mills conditions,8 in the
presence of aniline and acetic acid in refluxing toluene, to give
the desired o-tosylaminoazobenzene ligand 3. Finally, reaction
Received: October 15, 2015
© XXXX American Chemical Society
A
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Figure 2. ORTEP representation (thermal ellipsoids drawn at 50%
probability level) of complex 4 with the atom-numbering scheme. The
hydrogen atoms have been omitted for clarity. Selected bond lengths
(Å) and angles (deg): Ru−N1 = 2.081(2), Ru−N3 = 2.101(2), Ru−
Cl1 = 2.3993(8), Ru−cent = 1.703(3), N1−N2 = 1.258(3), N3−S1 =
1.618(2); N3−Ru−Cl1 = 85.65(7), N3−Ru−N1 = 77.57(9), N1−
Ru−Cl1 = 86.46(6).
Figure 1. Common coordination pattern for ruthenium azobenzene
complexes (I−IV) and a novel type of photoswitchable organometallic
complex (V).
Scheme 1. Synthesis of Ligand 3 and Complex 4
Figure 3. Absorption spectrum of ligand (E)-3 in MeCN (C = 45 μM)
at 25 °C (black solid line) and its stepwise evolution upon 5 s
irradiation pulses at 315 nm with P = 40 mW cm−2 (black dashed
lines). The blue dashed line corresponds to PSS.
with the ruthenium dimer [Ru(p-cym)Cl2]2 and triethylamine
in methanol led to ruthenium complex 4 as a dark brown solid
(78% yield). The structure of this complex was confirmed by
1H and 13C NMR spectroscopy, HRMS, and elemental analysis.
Crystals suitable for X-ray crystallographic analysis were
obtained by slow evaporation of a solution of the complex in
CH2Cl2. The structure of complex 4 reveals a classical “three-
legged piano-stool” configuration at the ruthenium atom, and
most importantly, 3 acts as a bidentate N,N ligand forming a
five-membered ring with an exocyclic NN bond (Figure 2
and discussion below).
Preliminary spectroscopic and photophysical studies of
ligand 3 and complex 4 were performed in MeCN. Free ligand
3 exhibits a strong absorption band at 324 nm (ε = 16650
350 L mol−1 cm−1) assigned to a π → π* transition and a weak
band at 454 nm (ε = 800 10 L mol−1 cm−1) corresponding to
a forbidden n → π* transition (black line, Figure 3). Complex 4
shows two bands of similar intensity at 319 nm (ε = 9550 20
L mol−1 cm−1) and 421 nm (ε = 6850 30 L mol−1 cm−1),
both assigned mainly to intraligand π → π* transitions, and a
weaker band in the visible region at 566 nm (ε = 2750 20 L
mol−1 cm−1) assigned mainly to Ru (4d6) → π* transition
(black line, Figure 4).20
As expected, irradiation at 315 nm of a solution of 3 in
MeCN resulted in a steady decrease of the band at 324 nm,
with two isosbestic points at 285 and 422 nm, revealing E → Z
photoisomerization of azobenzene moiety (Figure 3 and
Scheme 2). Under these conditions, the photostationary state
(PSS) was reached after ca. 35 s. Its composition was
1
determined to be E:Z = 29:71 by a combination of H NMR
and UV−vis spectroscopy (see the Supporting Information for
details). The half-life of (Z)-3 (determined upon resting the
sample in the dark at 25 °C) could not be precisely measured
due to reproducibility issues, but several independent experi-
ments indicated t1/2(Z) values ranging from 1 to 4 h in MeCN.
Similar behavior has already been reported for other
azobenzene derivatives such as azopyridines and was attributed
to their high sensitivity to traces of water or acid.50 Attempts to
buffer a solution of 3 in MeCN by addition of either 1% (v/v)
B
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Figure 4. Absorption spectrum of ruthenium complex (E)-4 in MeCN
(C = 30 μM) at 25 °C (black solid line) and its stepwise evolution
upon 5 s irradiation pulses at 406 nm with P = 9 mW cm−2 (black
dashed lines). The blue dashed line corresponds to PSS.
Figure 5. Fatigue resistance of complex 4 in MeCN (C = 60 μM)
upon alternating irradiation and resting (dark, 25 °C). Blue triangles
refer to irradiation pulses at 406 nm.
H17 (ca. 1.5 ppm) located above the aromatic ring and
deshielding of H18 (ca. 1 ppm) located on the side of the ring
(Figure 6). Irradiation of (E)-4 at 406 nm resulted in the
apparition of a new set of signals corresponding to (Z)-4. As a
consequence of isomerization of the NN bond, the aryl
group of azobenzene does not exert any ring current effect; the
p-cymene group in (Z)-4 thus appears as four well-defined
doublets between 5.42 and 5.99 ppm (green arrows).
Scheme 2. Photoswitching of Ligand 3 and Complex 4
Structure−Property Relationship in Arene Ruthenium
o-Sulfonamide Azobenzenes. Encouraged by these results,
we decided to prepare other complexes, in order to investigate
the influence of structure on photophysical properties. We
focused on (i) changing the nature of sulfonamide group, (ii)
introducing fluorine atoms on the benzene group, as such a
modification is known to result in very long lived Z
isomers,51,52 and (iii) modifying the arene ligand on ruthenium
(Figure 7).
The synthesis of complexes 7 and 9 is described in Scheme 3.
Nitrosobenzene was treated with o-phenylenediamine under
classical Mills conditions in the presence of acetic acid in
refluxing toluene, to afford the amino azobenzene 5 in 46%
yield.53 Treatment with MsCl and pyridine in dichloromethane
gave ligand 6, which was easily converted to complex 7 in 63%
yield upon reaction with [Ru(p-cym)Cl2]2 and triethylamine in
methanol. Reaction of 5 with triflic anhydride in CH2Cl2
yielded 94% of ligand 8, which was converted to complex 9
in the presence of [Ru(p-cym)Cl2]2 in MeOH.
Initial attempts to react commercially available 2,6-difluor-
oaniline with the nitroso derivative 2 did not lead to the
expected fluorinated ligand 11 (Scheme 4). However, when
2,6-difluoroaniline was transformed into the corresponding
nitroso compound 10, followed by reaction with amine 1,
ligand 11 was obtained in 24% yield. Then, formation of
difluorinated complex 12 proceeded smoothly, as described for
4 and 7.
Modification of the aryl group has been shown previously to
significantly affect the properties and reactivity of ruthenium
arene complexes.54,55,20,56−59 However, to the best of our
knowledge, its influence on the isomerization properties of
azobenzene ligands has never been investigated. Thus, we
synthesized complexes 13 and 14, analogous to 4, in which the
p-cymene ligand is replaced by benzene and hexamethylben-
zene, respectively (Scheme 5). Standard reaction conditions,
H2O at pH 7 or 1‰ (v/v) TFA greatly shortened the half-lives
of the Z isomer without improving reproducibility (t1/2 around
240 and 30 s, respectively).
Gratifyingly, complex 4 also undergoes photoisomerization
upon irradiation at 406 nm in MeCN, as revealed by a
concomitant steady decrease of the absorption band around
400 nm and increase around 515 nm, and the presence of an
isosbestic point at 456 nm (Figure 4 and Scheme 2). The PSS
was rapidly reached (in ca. 35 s), and the ratio E:Z = 56:44 was
determined by the same method as for ligand 3 (see the
Supporting Information). The rate of thermal back Z → E
isomerization was determined to be much faster than that in
the case of 3, with a reproducible t1/2 value of 6.5 0.3 min at
25 °C in MeCN. It is noteworthy that such a switching process
could be repeated over 10 times without any noticeable
degradation of the complex (Figure 5).
1
Photoisomerization Study of 4 by H NMR Spectros-
copy. Portions of the 1H NMR spectra of 4 in CD3CN before
and after irradiation are reported in Figure 6. The aromatic
protons of the p-cymene moiety appear as four distinct signals:
two doublets at 5.43 and 5.05 ppm corresponding to H14 and
H15 and two broad signals at 6.21 and 3.51 ppm corresponding
to H18 and H17, respectively (Figure 6). This significant
perturbation of chemical shifts can be explained by the
influence of the aryl group of azobenzene directly facing the
p-cymene moiety (Figure 2), resulting in a strong shielding of
C
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Figure 6. Portions of 1H NMR spectra of complex 4 in CD3CN (C = 13 mM) (1) before irradiation and (2) after irradiation at 406 nm (P = 40 mW
cm−2, t = 15 min). The red and green frames indicate the signals corresponding to the aromatic protons of the p-cymene moiety of E and Z isomers,
respectively; arrows indicate the signals corresponding to the Z isomer appearing upon irradiation.
Scheme 3. Synthesis of Ligands 6 and 8 and Their
Corresponding Complexes 7 and 9
Figure 7. Ruthenium arene complexes investigated for structure−
property relationships.
angles are given in Table 1. Complexes 4, 7, 9, and 14 share
similar structures, with ruthenium center in a pseudo-octahedral
using [Ru(C6H6)Cl2]2 or [Ru(C6Me6)Cl2]2 and ligand 3 in the
presence of Et3N in MeOH, cleanly afforded the desired
complexes in 83% and 75% yields as dark brown solids.
The molecular structures of the ruthenium complexes 4, 7, 9,
and 14 were determined by single crystal X-ray diffraction.
Their representations along with atom numbering scheme are
shown in Figure 2 and Figure 8, and selected bond lengths and
geometry in which the η6-arene ligand occupies a face. The
ruthenium−arene centroid distances (in the range between
1.702 and 1.714 Å) and Ru−N(1) distances (from 2.081(2) to
2.10(2) Å) are comparable to those reported for other arene
ruthenium complexes containing azobenzene ligands.16,20,27 It
should be noted that the (E)-azo ligands do not adopt a planar
geometry after coordination. The angles between the planes of
D
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Scheme 4. Synthesis of Fluorinated Ligand 11 and Complex
12
Table 1. Selected Bond Lengths (Å) and angles (deg) for
Complexes 4, 7, 9, and 14
4
7
9
14
Ru−N(1)
Ru−N(3)
Ru−Cl(1)
2.081(2)
2.101(2)
2.3993(8)
1.703
2.090(5)
2.104(5)
2.405(2)
1.714
2.093(2)
2.132(3)
2.398(1)
1.702
2.10(2)
2.12(1)
2.404(5)
1.710
a
Ru−cent
N(1)−N(2)
N(3)−S(1)
1.258(3)
1.618(2)
85.65(7)
77.57(9)
86.46(6)
57.38
1.255(6)
1.631(6)
85.6(1)
78.0(2)
88.7(1)
54.96
1.263(3)
1.590(3)
85.06(8)
78.0(1)
87.32(8)
68.27
1.28(3)
1.63(2)
88.7(4)
77.4(6)
94.5(5)
30.83
N(3)−Ru−Cl(1)
N(3)−Ru−N(1)
N(1)−Ru−Cl(1)
b
θ
a
b
cent = centroid of arene moiety. θ = angle between the planes of the
two aromatic rings of the azobenzene ligand.
doublets in the range 5.43−5.33 and 5.05−5.02 ppm and two
broad signals at 6.32−6.20 and 3.74−3.51 ppm, as a
consequence of the strong ring current effect exerted by the
azobenzene ligand (see Figure 6, the Supporting Information,
and the discussion above), while the i-Pr group appears as a
septuplet at 2.67−2.56 ppm and two doublets at 1.08−1.07 and
0.89−0.87 ppm and the Me group appears as a singlet at 2.15−
Scheme 5. Synthesis of Benzene and Hexamethylbenzene
Complexes 13 and 14
1
2.11 ppm. The H NMR spectrum of 12 shows broad signals
corresponding to aromatic protons above 6.5 ppm (1H),
around 5.5 ppm (2H), and around 3.7 ppm (several signals
corresponding to 1H). The i-Pr group appears as broad signals
at 2.83, 1.19, and 0.87 ppm, and the Me group shows a broad
signal at 1.99 ppm. The pattern exhibited by p-cymene in 9 is
slightly different, with seven broad signals appearing between
6.03 and 3.98 ppm (aromatic protons), around 2.50 and 1.00
ppm (i-Pr group), and in the range 2.39−2.08 ppm (Me
group). The observation of such broad signals for 12 and 9
presumably indicates their existence as slowly interconverting
conformers in solution, consistent with the disorder observed
around the i-Pr group of 9 in the solid state. The signals of the
arene group of 13 and 14 appear as singlets at 5.48 and 1.83
ppm, respectively.
The spectroscopic properties of the synthesized complexes in
MeCN were compared and are summarized in Table 2.
Compounds 4, 7, 9, 12, and 13 displayed similar absorption
spectra (Figure 4 and the Supporting Information), with two
bands around 314−319 nm and 390−454 nm that can be
assigned to the π → π* intraligand transition and one band
around 514−586 nm with a lower extinction coefficient that
the two aromatic groups of azobenzene are nearly identical in
compounds 4 and 7 (θ = 57.38 and 54.96°, respectively), while
this value is higher in 9 (68.27°) and significantly reduced in 14
(30.83°). Interestingly, the X-ray structure of 9 shows a
component of disorder around the isopropyl group of the p-
cymene moiety.
1
The H NMR spectra of 4, 7, 9, and 12−14 in CDCl3
revealed well-defined signals corresponding to the azobenzene
ligand, while signals related to the p-cymene moiety showed
great differences in complexes 4, 7, 9, and 12. In 4 and 7, p-
cymene exhibits characteristic signals for aromatic protons: two
Figure 8. ORTEP representation (thermal ellipsoids drawn at the 50% probability level) of complexes 7 (left), 9 (middle), and 14 (right) with the
atom-numbering scheme. The hydrogen atoms and solvent molecule (for 14·CH2Cl2) have been removed for clarity.
E
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polarities (toluene, THF, CH2Cl2, acetone, MeCN, and EtOH)
were prepared. Irradiation at 406 nm cleanly triggered E → Z
photoisomerization; then the half-life of the Z isomer (rate of
thermal back-reaction) was determined by monitoring the
evolution of absorbance at the appropriate wavelength (see the
Supporting Information for details). The results are reported in
Table 3. For most complexes studied, t1/2(Z) increases with
increasing solvent polarity (as evaluated by the empirical
polarity scale ENT),60 ranging from 0.66 to 3.68 min in toluene
(ENT = 0.099) to 3.48 to 33.6 min in EtOH (ENT = 0.654). It is
worth noting the case of CH2Cl2 (ENT = 0.309), which, although
slightly less polar than acetone (ENT = 0.355), results in longer
t1/2(Z) values. The general trends observed can be understood
in term of a better solvation of Z complexes (exhibiting a higher
permanent dipole than E isomers) in polar solvents, thus
increasing their thermal stability and consequently the t1/2(Z)
values.51,61,62
While o-fluoroazobenzenes are well-known to exist as very
long lived Z isomers (with half-lives reaching more than 2
years),51,52 the t1/2(Z) value for 12 is on the time scale of
several minutes as for other Ru complexes. Although the factors
responsible for such a behavior could not yet be precisely
identified, it is likely that coordination of azobenzene to the
metal center affected the π-conjugation structure and the
increment in the dipole moment.37 In addition, the conforma-
tional change of the azobenzene ligand associated with Ru
coordination led to an increase in the angle between the planes
of the two aromatic rings (Table 1) which might facilitate the Z
→ E thermal isomerization. Interestingly, the nature of the
sulfonamide group exerts a significant effect on the rate of Z →
E thermal isomerization, with (Z)-9 (Tf group) showing a
longer half-life than (Z)-4 (Ts group) and (Z)-7 (Ms group).
The Hammett constant F, recognized as a good quantification
of the inductive withdrawing effect of a substituent, parallels
this observation: the most electron withdrawing group Tf (F =
0.7463) indeed shows a longer t1/2(Z) value than Ts (F =
0.5564) and Ms (F = 0.5435). Finally, the most striking aspect
concerning the structure−property relationship in our com-
plexes is the influence of the arene group on the isomerization.
A comparison of 4, 13, and 14 clearly revealed a strong effect
of the presence of the hexamethylbenzene ligand on the half-life
of the Z isomer. Indeed, while 4 and 13 exhibited similar
Table 2. Electronic Spectral Data and Transition Assignment
for Complexes 4−14 in CH3CN
compound
λ/nm
ε/L mol−1 cm−1
transition
4
319
421
566
315
400
560
314
390
514
317
454
586
316
444
556
319
394
485
617
9574 21
6850 24
2740 14
7729 28
5516 15
2231 27
7395 88
5437 60
1621 50
7400 28
5448 53
1847 34
8133 11
5022 26
2604 45
6842 21
8557 38
5872 15
1719 40
π → π*
π → π*
MLCT
7
π → π*
π → π*
MLCT
9
π → π*
π → π*
MLCT
12
13
14
π → π*
π → π*
MLCT
π → π*
π → π*
MLCT
π → π*
π → π*
π → π*
MLCT
can be assigned to the MLCT (metal−ligand charge transfer)
transition from the 4d orbitals of Ru(II) to the empty π* ligand
orbitals (Table 2). As a plausible consequence of the singular
geometry of 14 in comparison to other complexes, this
compound exhibited a notably different pattern with three
intense bands at 319, 394, and 485 nm assigned to π → π*
intraligand transitions and a broad band around 617 nm
corresponding to an MLCT transition (Figure 9). As previously
t
1/2(Z) values ranging from 1.50 to 7.29 min depending on the
solvent, t1/2(Z) for 14 varied between 2.33 and 33.6 min. In
order to understand the origin of such a difference, the
geometry of both E and Z isomers of complexes 4, 13, and 14
was optimized by DFT calculations using Gaussian 0965 at the
B3LYP/6-31G**(d,p);LanL2DZ[Ru] level and the CPCM
model for MeCN (Figure 10). All three E isomers exhibit
similar geometries, with angles between the planes of the two
aromatic rings of the azobenzene ligand (θ) of 55.07, 50.79,
and 33.49° for 4, 13, and 14, respectively. These results are in
good agreement with the θ values determined for 4 (57.38°)
and 14 (30.83°) in the solid state by X-ray analysis (see Table
1). Interestingly, although (Z)-4 and (Z)-13 exhibit similar
geometries (θ = 64.26 and 62.17°, respectively), the relative
orientation of phenyl rings in azobenzene (Z)-14 is reversed,
leading to a θ value of −64.62°. Such observation is presumably
the result of two phenomena: (i) steric interactions exerted by
the methyl groups of the hmbz ligand on the phenyl ring of
azobenzene and (ii) a change in the polarity of E and Z
complexes and their HOMO energy as a consequence of
electron enrichment of the metal center by the hmbz ligand.
Figure 9. Absorption spectrum of ruthenium complex (E)-14 in
MeCN (C = 30 μM) at 25 °C (black solid line) and its stepwise
evolution upon 5 s irradiation pulses at 406 nm with P = 9 mW cm−2
(black dashed lines). The blue dashed line corresponds to the PSS.
described for 4, irradiation at 406 nm of complexes 7, 9, and
12−14 in MeCN induced photoizomerization of the ligand, as
revealed by a decrease in the absorption band at 390−454 nm
and an increase around 500−550 nm (Figure 9 and the
Supporting Information).
Solvent Effects on Thermal Z → E Isomerization Rates
of Complexes. In order to assess the influence of solvent on
the isomerization properties of the synthesized complexes,
solutions of 4, 7, 9, and 12−14 in solvents with varying
F
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Table 3. Half-Lives (t1/2, min) of Z Isomers of Complexes 4−14 in Various Solvents (Ranked by Order of Increasing Polarity
a
Parameter ENT)
entry
solvent
ETN
4
7
9
12
13
14
1
2
3
4
5
6
toluene
THF
0.099
0.207
0.309
0.355
0.460
0.654
1.50 0.01
2.65 0.01
5.55 0.02
5.44 0.02
6.46 0.31
7.29 0.05
0.66 0.01
1.12 0.01
2.85 0.02
2.26 0.01
2.33 0.01
3.48 0.02
3.68 0.01
5.05 0.02
6.51 0.01
5.67 0.01
10.1 0.1
1.21 0.01
2.38 0.01
7.02 0.10
5.23 0.02
8.11 0.03
11.8 0.5
2.26 0.03
2.63 0.09
4.82 0.57
4.28 0.88
2.26 0.36
6.45 0.25
2.33 0.02
3.78 0.03
18.3 0.1
10.9 0.1
26.5 0.2
33.6 0.3
CH2Cl2
acetone
MeCN
EtOH
b
n.d.
a
Z isomers were formed by irradiation at 406 nm (P = 9 mW cm−2) for 20 s, and then thermal back-isomerization was followed by monitoring UV/
b
vis absorption in the dark at 25 °C. n.d.: not determined (unstable in EtOH).
Figure 10. DFT-optimized geometries for E (top) and Z isomers (bottom) of complexes 13 (left), 4 (middle), and 14 (right).
CONCLUSION
EXPERIMENTAL SECTION
■
■
General Procedures. Unless otherwise stated, all reagents were
used as received without further purification. Manipulations under
anhydrous conditions were carried out under an atmosphere of argon
in dried glassware. Solvents were dried with a mBraun MB-SPS-800
purification system. 1H and 13C{1H} NMR spectra were recorded on a
JEOL 400 spectrometer and referenced to the resonances of the
solvent residual peak. Infrared (IR) spectra were recorded with a
Nicolet Nexus FT-IR spectrometer equipped with an ATR-
Germanium unit and are reported as wavenumbers (cm−1). Melting
points were determined using a Kofler bench. High-resolution mass
spectra (HRMS) were obtained on a Bruker maXis Q-TOF mass
A novel series of arene ruthenium complexes containing o-
sulfonamide azobenzene ligands was synthesized and charac-
terized. The unusual coordination pattern exhibited by the N
N bond of the azobenzene ligand, which was found to be
exocyclic, allowed the preservation of its photoisomerization
properties, thus leading to photoswitchable organometallic
complexes. In comparison to uncoordinated azobenzene
ligands, the corresponding Ru complexes favored the Z → E
thermal isomerization. The influence of solvent and structural
parameters, such as substituents on the phenyl ring of
azobenzene and the nature of the sulfonamide group and
arene ligand, on the rate of thermal Z → E back-isomerization
was investigated, and the presence of hexamethylbenzene group
on ruthenium was found to significantly increase the half-life of
the corresponding Z isomer. On the basis of DFT calculations,
this effect was rationalized by steric interactions between the
arene ligand and the phenyl ring of azobenzene. With these
novel photoswitchable organometallic complexes with well-
defined properties in hand, our current efforts are focused on
the study of their reactivity.
́ ́
spectrometer by the “Federation de Recherche” ICOA/CBM
(FR2708) platform. Elemental analyses were carried out by the
Laboratory for Microanalysis at ICSN (Gif-sur-Yvette, France).
Crystals suitable for X-ray analysis were obtained either by slow
evaporation of a solution of the complexes in CH2Cl2 or by slow vapor
diffusion of petroleum ether in a solution of the complexes in CH2Cl2.
Spectroscopic Measurements. Acetonitrile, acetone, dichloro-
methane, ethanol, toluene, and THF used for absorption measure-
ments were of spectrometric grade. Dichloromethane was passed
through a neutral alumina column prior to use to avoid any acidic
catalysis. UV/vis absorption spectra were recorded on a Cary5000
spectrophotometer from Agilent Technologies. Photoisomerization
G
DOI: 10.1021/acs.organomet.5b00871
Organometallics XXXX, XXX, XXX−XXX

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was induced by a continuous irradiation Hg/Xe lamp (Hamamatsu,
LC6 Lightningcure, 200 W) equipped with narrow-band interference
filters of appropriate wavelengths (Semrock FF01-315/15-25 for λirr
315 nm; FF01-406/15-25 for λirr 406 nm). The irradiation power was
measured using a photodiode from Ophir (PD300-UV).
2) to afford 6 (57 mg, 0.21 mmol, 82%): orange crystalline solid; Rf =
0.34 (petroleum ether/ethyl acetate 8/2); mp 100 °C; IR 3270, 1595,
1484, 1375, 1335, 1166, 1152, 971, 773, 732, 690 cm−1; 1H NMR (400
MHz, (CD3)2CO) δ 9.52 (br s, 1H), 8.00 (m, 2H), 7.87 (dd, J = 8.0,
1.6 Hz, 1H), 7.79 (m, 1H), 7.62−7.56 (m, 4H), 7.31 (ddd, J = 8.4, 7.2,
1.4 Hz, 1H), 3.15 (s, 3H); 13C{1H} NMR (100 MHz, (CD3)2CO) δ
153.2, 141.8, 137.3, 133.7, 132.6, 130.2, 125.1, 123.9, 121.4, 120.4,
40.2; HRMS (ESI) calcd for C13H13N3O2SNa+ [M + Na]+ m/z
298.0626, found m/z 298.0629.
N-(2-Nitrosophenyl)-4-methylbenzenesulfonamide (2). To a
solution of 149 (100 mg, 0.362 mmol) in CH2Cl2 (5 mL) was added
dropwise a solution of oxone (281 mg, 0.457 mmol, 1.2 equiv) in 5
mL of H2O. The resulting mixture was vigorously stirred at room
temperature for 24 h. The layers were separated, and the aqueous layer
was extracted with CH2Cl2 (2 × 5 mL). The organic layers were
combined, washed with brine, dried over anhydrous sodium sulfate,
and evaporated under vacuum to afford 2 (99 mg, 0.36 mmol, 94%):
green crystalline solid; Rf = 0.35 (petroleum ether/ethyl acetate 8/2);
mp 135 °C; IR 3662, 3227, 2986, 2902, 1624, 1597, 1492, 1320, 1164,
Complex 7. To a solution of 6 (19 mg, 0.066 mmol) in 4 mL of
MeOH were added Et3N (18 μL, 0.13 mmol) and [Ru(p-cym)Cl2]2
(20 mg, 0.033 mmol). After it was stirred for 24 h, the mixture was
evaporated to dryness and the product was purified by flash
chromatography (silica gel, CH2Cl2/MeOH 250/2 to 250/4) to
afford 7 (23 mg, 0.042 mmol, 63%): black powder; Rf = 0.58
(CH2Cl2/MeOH 95/5); mp 189 °C; IR 3477, 1593, 1468, 1300, 1241,
1
1127, 915, 819, 763, 684 cm−1; H NMR (400 MHz, (CD3)2CO) δ
1
10.81 (br s, 1 H), 7.93 (d, J = 8.2 Hz, 1H), 7.85 (d, J = 8.2 Hz, 2H),
7.76 (m, 1H), 7.33 (d, J = 8.2 Hz, 2H), 7.15 (t, J = 7.6 Hz, 1H), 6.71
(d, J = 7.3 Hz, 1H), 2.33 (s, 3H); 13C{1H} NMR (100 MHz,
(CD3)2CO) δ 158.1, 145.5, 139.7, 139.5, 137.7, 130.8, 128.4, 124.3,
122.3, 114.0, 21.5; HRMS (ESI) calcd for C13H12N2O3SK+ [M + K]+
m/z 315.0200, found m/z 315.0198.
1126, 960, 926, 845, 733, 679 cm−1; H NMR (400 MHz, CDCl3) δ
8.09 (d, J = 8.7 Hz, 1H), 7.95 (dd, J = 8.5, 1.1 Hz, 1H), 7.75 (d, J = 7.8
Hz, 2H), 7.61 (t, J = 7.8 Hz, 2H), 7.50 (m, 1H), 7.34 (m, 1H), 6.76
(m, 1H), 6.20 (br s, 1H), 5.33 (d, J = 6.0 Hz, 1H), 5.02 (d, J = 5.5 Hz,
1H), 3.51 (br s, 1H), 3.12 (s, 3H), 2.56 (sept, J = 6.9 Hz, 1H), 2.11 (s,
3H), 1.07 (d, J = 6.9 Hz, 3H), 0.89 (d, J = 6.9 Hz, 3H); 13C{1H} NMR
(100 MHz, CDCl3) δ 156.4, 152.4, 149.3, 134.1, 129.0, 128.6, 122.1,
119.8, 119.0, 83.0, 82.8, 41.0, 31.0, 23.3, 21.5, 19.2; HRMS (ESI) calcd
for C23H27N3O2RuS+ [M − Cl]+ m/z 510.0789, found m/z 510.0789.
Anal. Calcd for C23H27N3O2RuSCl, C, 50.68; H, 4.81; N, 7.71. Found:
C, 50.35; H, 4.73; N, 7.34.
(E)-2-(4-Methylphenylsulfonamido)azobenzene (3).66 To a
solution of aniline (100 μL, 1.10 mmol) in toluene (20 mL) were
added acetic acid (260 μL, 4.40 mmol, 4 equiv) and 2 (305 mg, 1.10
mmol, 1 equiv). The resulting mixture was stirred under reflux for 48
h, and H2O (20 mL) was added. The layers were separated, and the
aqueous layer was extracted with toluene (2 × 10 mL). The organic
layers were combined, washed with a saturated aqueous solution of
NaHCO3 (10 mL), dried over anhydrous sodium sulfate, and
evaporated under vacuum. The crude product was purified by flash
chromatography (silica gel-petroleum ether/ethyl acetate 8/2) to
afford 3 (350 mg, 1.00 mmol, 90%): orange crystalline solid; Rf = 0.30
(petroleum ether/ethyl acetate 8/2); mp 105 °C (lit.66 mp 129 °C);
IR 3251, 1597, 1478, 1389, 1336, 1167, 1092, 911, 772, 693 cm−1; 1H
NMR (400 MHz, (CD3)2CO) δ 9.71 (br s, 1H), 7.89 (m, 2H), 7.76
(d, J = 9.2 Hz, 1H), 7.69−7.66 (m, 3H), 7.60−7.58 (m, 3H), 7.53 (t, J
= 7.8 Hz, 1H), 7.25 (t, J = 7.8 Hz, 1H), 7.16 (d, J = 7.8 Hz, 2H), 2.22
(s, 3H); 13C{1H} NMR (100 MHz, (CD3)2CO) δ 153.1, 144.7, 142.9,
137.6, 136.7, 133.3, 132.5, 130.3, 130.1, 127.9, 125.9, 123.9, 123.5,
119.8, 21.3.
(E)-2-(Trifluoromethanesulfonamido)azobenzene (8). To a
solution of 5 (208 mg, 1.05 mmol) in dry CH2Cl2 (20 mL) under
argon was added trifluoromethanesulfonic acid (196 μL, 1.16 mmol,
1.1 equiv). The mixture was stirred at room temperature for 10 min
before addition of H2O (20 mL). The layers were separated, and the
aqueous layer was extracted with CH2Cl2 (3 × 20 mL). The organic
layers were combined, washed with brine (10 mL), dried over
anhydrous sodium sulfate, and finally evaporated under vacuum. 8
(325 mg, 94%) was used without further purification. A small portion
for analysis was purified by flash chromatography (silica gel, petroleum
ether/ethyl acetate 98/2): orange crystalline solid; Rf = 0.58
(petroleum ether/ethyl acetate 8/2); mp 76 °C; IR 3264, 1597,
1
1485, 1415, 1214, 1193, 1140, 957, 770, 684 cm−1; H NMR (400
MHz, (CD3)2CO) δ 10.71 (br s, 1H), 8.03 (dd, J = 7.8, 1.8 Hz, 2H),
7.86 (dd, J = 8.0, 1.6 Hz, 1H), 7.75 (m, 1H), 7.67 (m, 1H), 7.62−7.60
(m, 3H), 7.53 (m, 1H); 13C{1H} NMR (100 MHz, (CD3)2CO) δ
153.4, 146.0, 134.5, 133.3, 132.9, 130.2, 128.9, 127.1, 124.2, 117.8;
HRMS (ESI) calcd for C13H11F3N3O2S+ [M + H]+ m/z 330.0519,
found m/z 330.0519.
Complex 4. To a solution of 3 (80 mg, 0.23 mmol) in 10 mL of
MeOH were added Et3N (62 μL, 0.46 mmol) and [Ru(p-cym)Cl2]2
(70 mg, 0.11 mmol). After the mixture was stirred for 24 h, the
precipitate was filtered and washed with Et2O to afford 4 (112 mg,
0.180 mmol, 78%): brown powder; Rf = 0.73 (CH2Cl2/MeOH 95/5);
mp 244 °C; IR 2969, 1596, 1474, 1305, 1142, 1082, 932, 851, 763, 697
Complex 9. A solution of 8 (108 mg, 0.328 mmol) in 10 mL of
MeOH and [Ru(p-cym)Cl2]2 (100 mg, 0.215 mmol) was stirred for 48
h and evaporated to dryness. The product was purified by flash
chromatography (silica gel, CH2Cl2/MeOH 250/2 to 250/5) to afford
9 (58 mg, 0.097 mmol, 29%): dark red powder; Rf = 0.77 (CH2Cl2/
MeOH 95/5); mp 245 °C; IR 3057, 1598, 1476, 1353, 1203, 1179,
1162, 1134, 949, 838, 767, 694 cm−1; 1H NMR (400 MHz, CDCl3) δ
8.11−7.77 (br s, 1H), 7.97 (d, J = 8.7 Hz, 1H), 7.86 (d, J = 7.8 Hz,
2H), 7.65 (m, 2H), 7.54 (m, 1H), 7.37 (ddd, J = 8.6, 7.0, 1.4 Hz, 1H),
6.91 (m, 1H), 6.03−3.98 (7 br s, 1H), 2.49 (br s, 1H), 2.39−2.08 (m,
3H), 1.07−0.83 (m, 6H); 13C{1H} NMR (100 MHz, CDCl3) δ 155.8
(br), 149.6, 149.1, 133.7, 129.3, 129.2, 122.1, 121.9, 120.7 (br), 118.7,
108.5 (br), 102.5 (br), 92.1 (br), 89.0 (br), 86.0 (br), 85.1 (br), 83.5−
82.5 (br), 66.0, 31.1, 30.0, 22.9−21.6 (br), 19.2, 15.4; HRMS (ESI)
calcd for C23H23F3N3O2RuS+ [M − Cl]+ m/z 564.0507, found m/z
564.0519. Anal. Calcd for C23H23ClF3N3O2RuS, C, 46.12; H, 3.87; N,
7.01. Found: C, 46.36; H, 4.13; N, 7.05.
2,6-Difluoronitrosobenzene (10). To a solution of 2,6-
difluoroaniline (500 μL, 4.65 mmol) in CH2Cl2 (15 mL) was added
dropwise a solution of oxone (5.71 g, 9.29 mmol, 2 equiv) in 15 mL of
H2O. The resulting mixture was vigorously stirred at room
temperature for 15 h. The layers were separated, and the aqueous
layer was extracted with CH2Cl2 (2 × 5 mL). The organic layers were
combined, washed with a saturated aqueous solution of NaHCO3 (10
1
cm−1; H NMR (400 MHz, CDCl3) δ 8.09 (d, J = 8.2 Hz, 2H), 7.86
(m, 3H), 7.64 (t, J = 7.8 Hz, 2H), 7.52 (m, 1H), 7.38 (d, J = 8.7 Hz,
1H), 7.15 (d, J = 8.2 Hz, 2H), 7.04 (t, J = 8.4 Hz, 1H), 6.60 (t, J = 7.6
Hz, 1H), 6.32 (br s, 1H), 5.43 (d, J = 6.0 Hz, 1H), 5.05 (d, J = 5.5 Hz,
1H), 3.74 (br s, 1H), 2.67 (sept, J = 6.9 Hz, 1H), 2.31 (s, 3H), 2.15 (s,
3H), 1.08 (d, J = 6.9 Hz, 3H), 0.87 (d, J = 6.9 Hz, 3H); 13C{1H} NMR
(100 MHz, CDCl3) δ 156.0, 151.6, 148.5, 142.1, 137.9, 133.3, 129.3,
128.9, 128.7, 122.5, 119.6, 119.0, 118.4, 82.9, 82.5, 31.0, 23.6, 21.6,
21.2, 19.3; HRMS (ESI) calcd for C29H30N3O2RuS+ [M − Cl]+ m/z
586.1104, found m/z 586.1106. Anal. Calcd for C29H30N3O2RuSCl, C,
56.08; H, 4.87; N, 6.76. Found: C, 56.11; H, 4.95; N, 6.61.
(E)-2-(Methanesulfonamido)azobenzene (6). To a solution of
553 (50 mg, 0.25 mmol) in pyridine (5 mL) was added
methanesulfonyl chloride (22 μL, 0.28 mmol, 1.1 equiv). Stirring at
room temperature for 12 h was followed by addition of another
portion of methanesulfonyl chloride (22 μL, 0.28 mmol, 1.1 equiv).
After completion of the reaction, the mixture was evaporated to
dryness. CH2Cl2 (20 mL) and H2O (20 mL) were added, the layers
were separated, and the aqueous layer was extracted with CH2Cl2 (3 ×
10 mL). The organic layers were combined, washed with a saturated
aqueous solution of NH4Cl (10 mL), dried over anhydrous sodium
sulfate, and evaporated under vacuum. The crude product was purified
by flash chromatography (silica gel, petroleum ether/ethyl acetate 8/
H
DOI: 10.1021/acs.organomet.5b00871
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
mL), dried over anhydrous sodium sulfate, and evaporated under
vacuum to afford 10 (660 mg, 4.61 mmol, 99%): light brown powder;
Rf = 0.55 (petroleum ether/ethyl acetate 8/2); mp 119 °C; IR 3662,
3225, 2986, 2901, 1608, 1478, 1283, 1163, 1086, 1019, 914, 788 cm−1;
1H NMR (400 MHz, (CD3)2CO) δ 7.92 (m, 1H), 7.37 (t, J = 8.9 Hz,
Computational Details. All density functional theory (DFT)
calculations were performed with the Gaussian09 package at the
B3LYP level.65 LanL2DZ and 6-31G**(d,p) basis sets were used for
Ru and all other atoms, respectively. The effect of solvent
(acetonitrile) was taken into account by continuum CPCM single-
point calculations on gas-phase optimized geometries. The absence of
imaginary frequencies was checked on all calculated structures to
confirm they are true minima.
2H); 13C{1H} NMR (100 MHz, (CD3)2CO) δ 154.2 (d, J = 267 Hz),
139.6 (t, J = 11.4 Hz), 114.1 (d, J = 23.8 Hz).
(E)-2-(4-Methylphenylsulfonamido)-2′,6′-difluoroazoben-
zene (11). To a solution of 1 (92 mg, 0.35 mmol) in acetic acid (5
mL) was added 10 (50 mg, 0.35 mmol, 1 equiv). The resulting mixture
was stirred under reflux for 16 h and evaporated to dryness under
vacuum. The crude product was purified by flash chromatography
(silica gel, petroleum ether/ethyl acetate 9/1) to afford 11 (33 mg,
0.085 mmol, 24%): orange crystalline solid; Rf = 0.39 (petroleum
ether/ethyl acetate 8/2); mp 140 °C; IR 3299, 1613, 1594, 1477,
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b00871.
1
1390, 1338, 1165, 1092, 1028, 926, 759 cm−1; H NMR (400 MHz,
NMR spectra, additional UV−vis absorption spectra,
procedures for the determination of half-lives and Z/E
ratios in the PSS, and details concerning X-ray structure
determination (PDF)
All computed molecule Cartesian coordinates (XYZ)
Crystallographic data for 4, 7, 9, and 14 (CIF)
CDCl3) δ 9.20 (s, 1H); 7.79 (d, J = 8.2 Hz, 1H), 7.72−7.69 (m, 3H),
7.44 (m, 1H), 7.40 (ddd, J = 8.5, 6.0, 2.5 Hz, 1H), 7.17−7.06 (m, 5H),
2.33 (s, 3H); 13C{1H} NMR (100 MHz, CDCl3) δ 156.3 (d, J = 259
Hz), 144.2, 141.0, 136.2, 136.1, 133.9, 131.4 (t, J = 10.5 Hz), 129.8,
127.4, 124.2, 119.9, 119.8, 112.9, 21.6; HRMS (ESI) calcd for
C19H15F2N3O2SNa+ [M + Na]+ m/z 410.0751, found m/z 410.0748.
Complex 12. To a solution of 11 (97 mg, 0.26 mmol) in 10 mL of
MeOH were added Et3N (70 μL, 0.52 mmol) and [Ru(p-cym)Cl2]2
(80 mg, 0.13 mmol). After it was stirred for 24 h, the mixture was
evaporated to dryness and the product was purified by flash
chromatography (silica gel, CH2Cl2/MeOH 250/3) to afford 12 (93
mg, 0.14 mmol, 57%): black powder; Rf = 0.65 (CH2Cl2/MeOH 95/
5); mp 245 °C; IR 3738, 2967, 1712, 1595, 1469, 1400, 1308, 1245,
1139, 1081, 1007, 924, 849, 787 cm−1; 1H NMR (400 MHz, CDCl3) δ
8.08 (d, J = 7.8 Hz, 2H), 7.69 (d, J = 8.2 Hz, 1H), 7.43−7.37 (m, 2H),
7.20−7.05 (m, 5H), 6.91 (m, 1H), 6.60 (t, J = 7.8 Hz, 1H), 5.51−5.46
(m, 2H), 3.66 (br s, 1H), 2.83 (m, 1H), 2.27 (s, 3H), 1.99 (s, 3H),
1.19 (br s, 3H), 0.87 (br s, 3H); 13C{1H} NMR (100 MHz, CDCl3) δ
151.0 (d, J = 237 Hz), 142.2, 137.4, 134.2, 133.6, 129.3, 129.0, 128.6
(t, J = 91 Hz), 119.3, 119.2, 118.8, 111.7 (br), 100.1, 97.3, 83.2, 81.8,
78.7, 53.6, 31.2, 31.1, 24.5, 21.6, 20.5, 19.8; HRMS (ESI) calcd for
C29H28F2N3O2RuS+ [M − Cl]+ m/z 622.0915, found m/z 622.0918.
Anal. Calcd for C29H28ClF2N3O2RuS, C, 53.01; H, 4.29; N, 6.39.
Found: C, 52.22; H, 4.59; N, 5.70.
Complex 13. To a solution of 3 (71 mg, 0.20 mmol) in 5 mL of
MeOH were added Et3N (55 μL, 0.40 mmol) and [Ru(bz)Cl2]2 (50
mg, 0.10 mmol). After the mixture was stirred for 2 h, the precipitate
was filtered and washed with MeOH and Et2O to afford 13 (94 mg,
0.13 mmol, 83%): brown powder; Rf = 0.42 (CH2Cl2/MeOH: 95/5);
mp >270 °C; IR 3070, 1596, 1472, 1299, 1155, 1135, 1084, 939, 908,
848, 821, 767, 695 cm−1; 1H NMR (400 MHz, CDCl3) δ 8.09 (d, J =
8.2 Hz, 2H), 7.88−7.85 (m, 3H), 7.66 (t, J = 7.6 Hz, 2H), 7.54 (m,
1H), 7.41 (d, J = 8.7 Hz, 1H), 7.17 (d, J = 7.8 Hz, 2H), 7.07 (t, J = 7.8
Hz, 1H), 6.63 (t, J = 7.8 Hz, 1H), 5.48 (s, 6H), 2.32 (s, 3H); 13C{1H}
NMR (100 MHz, CDCl3) δ 155.7, 152.0, 148.7, 142.3, 137.7, 133.5,
129.3, 129.0, 128.9, 122.4, 119.6, 119.2, 118.7, 87.2, 21.6; HRMS
(ESI) calcd for C25H22N3O2RuS+ [M − Cl]+ m/z 530.0477, found m/
z 530.0476. Anal. Calcd for C25H22ClN3O2RuS, C, 53.14; H, 3.92; N,
6.27. Found: C, 52.59; H, 4.11; N, 7.37.
AUTHOR INFORMATION
Corresponding Authors
*E-mail for N.B.: nicolas.bogliotti@ppsm.ens-cachan.fr.
*E-mail for J.X.: joanne.xie@ens-cachan.fr.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank Dr. Gilles Clavier for helpful discussions concerning
DFT calculations.
■
REFERENCES
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1
850, 767, 679 cm−1; H NMR (400 MHz, CDCl3) δ 8.56 (d, J = 7.8
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(m, 3H), 6.61 (m, 1H), 2.24 (s, 3H), 1.83 (s, 18H); 13C{1H} NMR
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129.6, 129.2, 128.9, 124.8, 120.6, 118.9, 117.7, 96.2, 21.5, 16.1; HRMS
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DOI: 10.1021/acs.organomet.5b00871
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DOI: 10.1021/acs.organomet.5b00871
Organometallics XXXX, XXX, XXX−XXX