Photoswitchable Arylazopyrazole-Based Ruthenium(II) Arene Complexes

Article abstract of DOI:10.1021/acs.organomet.7b00493

A new family of donor-functionalized photoswitchable arylazopyrazole-based ligands (3-5) was synthesized and characterized. The new ligands have been employed to prepare a series of novel photoswitchable half-sandwich ruthenium(II) cymene complexes of the

Full text of DOI:10.1021/acs.organomet.7b00493

Article
pubs.acs.org/Organometallics
Photoswitchable Arylazopyrazole-Based Ruthenium(II) Arene
Complexes
Kesete Ghebreyessus* and Stefan M. Cooper, Jr.
Department of Chemistry and Biochemistry, Hampton University, Hampton, Virginia 23668, United States
*
S Supporting Information
ABSTRACT: A new family of donor-functionalized photoswitchable arylazopyrazole-based ligands (3−5) was synthesized and
characterized. The new ligands have been employed to prepare a series of novel photoswitchable half-sandwich ruthenium(II)
6
+
cymene complexes of the type [(η -p-cymene)Ru(L)Cl] (L = 1-(2-methylenepyridyl)-4-(phenyldiazenyl)-3,5-dimethyl-1H-
pyrazole (6a), 1-(2-methylenepyridyl)-4-((4-bromophenyl)diazenyl)-3,5-dimethyl-1H-pyrazole (6b), 1-(2-benzothiazolyl)-3,5-
1
13
dimethyl-4-arylazopyrazole (7)). All of the complexes have been fully characterized by H NMR, C NMR, and UV−vis
spectroscopy and elemental analyses. In addition, the structure of complex 6a was determined by X-ray crystallography. The
UV−vis spectroscopic studies show that both the ligands and metal complexes exhibit excellent trans to cis photoisomerization of
the arylazopyrazole moiety upon irradiation with 365 nm UV light. The cis isomer of the compounds can be switched back nearly
quantitatively to the more stable trans form with 530 nm irradiation. Coordination of the metal ion has no significant influence
on the photoswitching properties of the ligands. DFT and TD-DFT calculations were performed for geometry optimization of
the ligands and to complement the experimental findings of the electronic transitions and absorption bands observed. The data
obtained from these studies were in good agreement with the experimental results. These excellent photoswitchable properties
make the new cationic Ru(II) azo compounds described in these studies interesting candidates for their potential application as
photoswitchable systems in catalytic and medicinal chemistry.
INTRODUCTION
More recently, Fuchter and co-workers have reported novel
arylazopyrazole-based molecular photoswitches, wherein the
switching properties of the azo moiety can be easily modulated
■
Photoswitchable molecules that exhibit substantial changes in
structure or functional properties upon light irradiation show
promise in a variety of applications, including molecular
switches, switchable catalysis, bioimaging, and controlled drug
20
by the substitution pattern on the pyrazole ring. In their
studies, Fuchter and co-workers have shown that arylazopyr-
azoles undergo efficient photoinduced reversible trans to cis
isomerization and exhibit superior properties in comparison to
1
−6
release. In recent years, considerable efforts have been made
in designing and preparing metal complexes that incorporate
photoswitchable ligands in order to combine the inherent
magnetic, electrochemical, catalytic, and biological properties of
the metal complexes with the photoswitching ability of the
2
0
those of their azobenzene counterparts. These novel
compounds also feature well-separated π−π* and n−π*
absorption bands and possess outstanding half-lives for the
cis isomer under ambient conditions. Isomerization of these
photoresponsive units induces a conformational change around
7
−14
photochrome.
Several such efforts have employed
azobenzene derivatives as photochromic ligands due to their
efficient reversible photoinduced trans to cis isomerization,
generating two isomers that display markedly different
20
the NN double bond, similar to azobenzene. Since
pyrazoles are widely used as ligands for a variety of metal
21−32
7
−14
complexes,
these successful findings suggest that arylazo-
properties.
However, photoswitching molecules based on
analogous heteroarene azo compounds integrating both
photochromic function and metal-binding units have been
scarcely explored.
Received: June 29, 2017
1
5−19
©
XXXX American Chemical Society
A
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pyrazoles having additional donor groups can be favorable
ligands for exploring the synthesis of hybrid metal−organic
compounds with photoswitchable properties. To the best of
our knowledge, metal complexes bearing photoswitchable
arylazopyrazole ligands have not been reported to date.
Scheme 1. General Synthetic Route for the Photoswitchable
Arylazopyrazole-Based Ligands 3−5
Half-sandwich ruthenium(II) arene complexes offer a
versatile platform for the design and synthesis of novel
compounds with potential applications in catalysis and
medicinal and supramolecular chemistry. They have been
33−40
widely used as catalysts in synthetic organic chemistry
and
are now being increasingly considered for the development of
41−45
new chemical tools and metal-based drugs.
Remarkably,
ruthenium complexes are now commonly recognized as the
second most promising type of new anticancer metal species
4
1−45
apart from the platinum family of metallo-drugs.
Varying
6
the complex framework through modification of the arene (η -
R-arene) and the other auxiliary ligands is crucial in tuning
41−45
chemical reactivity and bioactivity.
In this context, many
different supporting auxiliary ligands have been used in
combination with the Ru(II) arene scaffold to provide different
6
reactivity profiles and bioactivities. Functionalization of the η -
arene motif is possible, allowing modulation of the electronic
and steric properties of the resulting complexes and conferring
other interesting features for their use in catalysis and medicinal
4
8−51
chemistry.
However, the chemistry of ruthenium(II) arene
complexes that possess photoswitchable ligands still remains
13,14
largely unexplored.
In this article, we present the synthesis and characterization
of new photoswitchable ruthenium(II) arene complexes
appended with functionalized arylazopyrazole (AAPz)-based
ligands. AAPzs represent novel emerging molecular photo-
switches with the pyrazole structural motif providing ample
opportunities for ligand design and further modifications.
These new complexes are designed with the intent to study if
photoswitchable ruthenium(II) arene complexes can be
prepared by coupling the arylazopyrazole groups and examine
how the substitution pattern of the ligands affects their
photochromic properties. In addition, the ruthenium(II)
arene frameworks have been chosen as models for studying
the coordination chemistry of the new ligands because of their
many applications in metal-catalyzed organic transformations,
potential therapeutic activity, and well-established chemical
1
NMR, and UV−vis spectroscopy. The H NMR spectra of the
ligands recorded in CDCl are presented in the Experimental
Section. The position and intensity of the signals correspond to
the reagents used in the synthesis.
3
1
The most distinctive signals in the H NMR spectra of
ligands 4a,b are those assigned to the CH proton adjacent to
the pyridine nitrogen moiety, which appear as doublets at 8.57
ppm for 4a and 8.58 ppm for 4b. The methyl protons in the
,5-positions of the pyrazole ring appear as singlets at 2.64 and
.51 ppm (3), 2.57 and 2.54 ppm (4a), 2.56 and 2.52 ppm
4b), and 2.60 and 3.17 ppm (5), respectively. Other signals
given in the Experimental Section are in agreement with the
structures of the ligands.
Synthesis of Ruthenium(II) Complexes. The three types
of half-sandwich Ru(II) p-cymene complexes were readily
prepared by following the synthetic route shown in Scheme 2.
Reactions of the dimeric [(p-cymene)RuCl ] precursor with 2
equiv of the N,N-donor ligands 4a,b and 5 in dichloromethane
gave the cationic half-sandwich mononuclear ruthenium(II)
cymene complexes 6a,b and 7 as air-stable orange or brown
solids in good yield. Isolated as their chloride salts, all of the
complexes are soluble in common organic solvents such as
acetone, chloroform, dichloromethane, dimethylformamide
(DMF), dimethyl sulfoxide (DMSO), and methanol but
insoluble in water. Similar reactions of the dimeric [(p-
cymene)RuCl ] precursor with ligand 3, however, resulted in a
3
2
(
4
0−45
structures.
2
2
RESULTS AND DISCUSSION
Synthesis and Characterization. The route employed for
the synthesis of the photoswitchable arylazopyrazole-based
ligands 3−5 is outlined in Scheme 1. First, the 1,3-dione
precursor compound 1 was prepared by diazo coupling of the
■
2
,4-pentanedione with commercially available substituted
anilines according to literature methods, and subsequent
condensation reactions with hydrazine hydrate gave compound
2
20
in high yield. Treatment of compound 2 with (2-
2
2
chloroethyl)amine gave ligand 3 as an orange solid in 70%
very unstable complex that decomposed easily. The 2-
ethylamine, 2-(2-methylenepyridine), and benzothiazole de-
rivatives at the N-1 position of the pyrazole ring were mainly
selected to vary the steric bulk of the ligands and to have a
small library of ligands with varied features. The structures of
the new Ru(II) cymene complexes 6a,b and 7 were confirmed
3
1,32
yield.
chloromethyl)pyridine hydrochloride in the presence of excess
KOH and TBAB as a phase-transfer catalyst afforded ligands
Similarly, reacting compound 2 with 2-
(
31
4
a,b as sticky orange solids in good yield. After washing with
hexane and diethyl ether several times and drying, the ligands
become pure enough for the synthesis of the Ru(II) complexes.
The new ligand 5 was easily prepared in high yield by
condensation of the precursor 1 with 2-hydrazinobenzothiazole.
1
13
by H NMR and C NMR spectroscopy and elemental
analysis.
1
As expected, the H NMR spectra of the complexes 6a,b and
1
13
All of the new ligands were fully characterized by H NMR, C
7 in CDCl displayed well-defined signals that can be assigned
3
B
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Scheme 2. Synthetic Route for the Photoswitchable
Aryazopyrazole-Containing Ruthenium(II) p-Cymene
Complexes 6a,b and 7
carbons. These chemical shifts are similar to those reported for
similar ruthenium(II) arene compounds.
33−49
X-ray Crystallographic Analysis. Single crystals suitable
for X-ray crystallographic analysis were obtained by slow
diffusion of petroleum ether into a CH Cl solution of complex
2
2
6
a. Although the resulting crystal quality was poor, giving rise
to diffuse diffraction patterns, the molecular entity could be
unambiguously determined. The selected bond angles and
lengths are given in Figure 1, whereas crystallographic data and
Figure 1. ORTEP (50% probability) diagram of the half-sandwich
Ru(II) p-cymene complex 6a. The anion is omitted for clarity. Selected
bond lengths (Å) and bond angles (deg): Ru1−N1 = 2.128(3), Ru2−
N2 = 2.115(3), Ru1−Cl1 = 2.390(10), N4−N5 = 1.258(5); N1−
Ru1−N2 = 84.60(12), N1−Ru1−Cl1 = 83.39, N2−Ru1−Cl1 =
86.98(9), N2−N3−C6 = 119.4(3), N1−C5−C6 = 117.2(3).
to the chelated arylazopyrazole-based ligands and the p-cymene
moiety. Coordination of the N,N-chelating ligands to the
Ru(II) center generated nonsymmetric complexes which
1
13
induce significant modifications to the H NMR and
C
1
NMR signals of the p-cymene moiety in 6a,b. The H NMR
spectra of the complexes 6a,b exhibit four broad doublets in the
range 6.41−5.79 ppm (6a) and 6.33−5.78 ppm (6b), which are
typical for the aromatic protons of the p-cymene derivative in
structural refinement parameters are gathered in Table S1 in
the Supporting Information. The unit cell contains two slightly
different Ru(II) p-cymene moieties. The 50% probability
ORTEP view of only one of the complexes with atom
numbering is shown in Figure 1. From Figure 1, it is clear that
the complex adopts the commonly observed piano-stool
geometry as reported in similar half-sandwich Ru(II) arene
33−49
Ru(II) arene systems.
The proton resonance attributable
to the methyl groups of the isopropyl derivative appear as two
doublets at 1.32, 1.21 ppm (6a) and 1.30, 1.19 ppm (6b) and
the methyl protons as a singlet at 2.86 ppm (6a) and 2.15 ppm
50
(
6b). The CH(CH ) proton appears as a septet at 2.93 ppm
complexes, and the arylazopyrazole unit is presented in the
most thermodynamically stable trans form. The molecular
structure of complex 6a also clearly shows that the
arylazopyrazole-based ligand 4a is coordinated in a bidentate
fashion, forming a six-membered metallacycle with a bite angle
3
2
(6a) and 2.90 ppm (6b). The effect of the coordination of the
pyrazole unit to the Ru(II) center and the difference in the
proximity of the p-cymene moiety is also evident from the
significant differences in the chemical shifts of the 3,5-methyl
protons of the pyrazole ring. The methyl protons closer to the
p-cymene moiety appeared at 2.84 ppm (6a) and 2.83 ppm
of 118.3(3)°. The values of the Ru(II)−N and Ru(II)−Cl
av
distances are on average 2.122(3) and 2.390(10) Å,
respectively. Other selected bond angles and lengths are
given in Figure 1. The Ru(II)−Cl bond length is found to be
2.390(10) Å, which is in agreement with other structurally
(
6b) in comparison to methyl protons away from the Ru(II)
center that resonated at 2.19 ppm (6a) and 2.15 ppm (6b),
respectively. A similar pattern was observed in the H and
1
13
C
50
NMR spectra of complex 7.
characterized similar p-cymene Ru(II) complexes. The N−N
(1.258(5) Å) distance observed is similar to the values observed
for the free azo ligands, which conforms to the integrity of the
1
In addition, the H NMR spectra of the Ru(II) complexes
a,b contain doublet resonances at 8.93 and 8.89 ppm,
6
20
respectively attributed to the CH proton adjacent to the
pyridine nitrogen. These are shifted slightly downfield in
comparison to the free ligands 4a,b, which resonated at 8.57
ppm (4a) and 8.58 ppm (4b), respectively. The signals of the
methylene protons of the pyridine ring appear as a singlet at
azo functionality.
UV−Vis Absorption Spectra of Ligands 3−5. The UV−
vis spectra of the investigated ligands 3−5 exhibit essentially the
same features. As exemplified in Figure 2a, the electronic
spectra of the ligands 3−5 display intense bands in the 330−
346 nm range and weak bands at 435−450 nm in the visible
region as common features. Under normal conditions, the free
ligands 3−5 exist in the more thermodynamically stable trans
conformation. The absorption spectra of the high-intensity
bands of the trans isomer appear at 330 nm for 3, 337 nm for
4a, 342 nm for 4b, and 344 nm for 5 (Figure 2a). Similarly, the
absorption maxima of the low-intensity bands appear at 436 nm
for 3, 444 nm for 4a, 446 nm for 4b, and 450 nm for 5 (Figure
2a). The introduction of the bromo substituent to the arylazo
ring in 4b resulted in a slight bathochromic shift of the
6.00 ppm (6a) and 6.08 ppm (6b). Likewise, these resonances
are shifted downfield relative to those of the free ligands, which
appeared at 5.41 ppm (4a) and 5.40 ppm (4b), indicating
coordination of the Ru(II) ion to the N,N-chelating
arylazopyrazole ligands.
The 13C NMR spectrum also exhibits appropriate signals.
The methyl carbons of the p-cymene group display signals in
the range of 15 and 22 ppm. The CH carbons of the isopropyl
group appear around 29.9−30.6 ppm, while the cymene
carbons appear in the range 83.8−106.1 ppm for the C−H
C
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Figure 2. UV−vis absorption spectra of the trans isomers of (a) the arylazopyrazole-based ligands 3−5 and (b) Ru(II) complexes 6a,b and 7
−5
measured in CHCl (2.0 × 10 M).
2
Figure 3. UV−vis spectral changes of compound 4a (2.0 × 10−5 M in CH Cl ): (a) trans to cis isomerization after irradiation with 365 nm as a
2
2
function of time; (b) cis to trans photoisomerization upon irradiation at 530 nm as a function of time.
absorption maxima in comparison to the unsubstituted 4a.
There was also a slight red shift of the intense band in the
benzothiazole-based ligand 5 (344 nm) in comparison to those
of ligands 3 (330 nm) and 4a (337 nm). Analogous to reported
literature studies on arylazopyrazoles, the intense absorption
bands for the trans isomer were attributed to the π−π*
transitions, whereas the broad, weak bands at around 435−450
photoisomerization of ligand 4a is discussed in this section. The
changes in the UV−vis absorption spectra of ligand 4a in
dichloromethane solution as a function of time are depicted in
Figure 3. Upon irradiation with UV light (λ 365 nm) the trans
form of ligand 4a was easily converted to the cis isomer,
resulting in new absorption bands at 292 and 445 nm, marked
with blue arrows in Figure 3a. This was also clearly shown by
the characteristic changes in the absorbance of the band
maxima, which involved a significant decrease in the π−π*
transition of the arylazopyrazole moiety and an increase in the
n−π* transition band (Figure 3a), which were indicative of the
formation of the cis isomer. In addition, the intense band of the
π−π* transition of the trans isomer was also blue-shifted and
appeared at 292 nm upon exposure to UV light (λ 365 nm). On
the other hand, the band due to the n−π* transition showed a
slight red shift to 448 nm from 445 nm. The trans to cis
isomerization of ligand 4a reached a photostationary state
20
nm were assigned to the n−π* transitions.
UV−Vis Absorption Spectra of Complexes 6a,b and 7.
The absorption spectra of the Ru(II) complexes 6a,b and 7
show absorption bands similar to those of the free ligands 4a,b
and 5 (Figure 2b). Complexes 6a,b and 7 display a high-
intensity π−π* band at 331 nm (6a), 340 nm (6b), and 346 nm
(7) (Figure 2b). These high-intensity absorption bands are well
separated from the low-intensity n−π* bands observed at 434
nm (6a), 443 nm (6b), and 441 nm (7) (Figure 2b). Upon
coordination to the Ru(II) center, the π−π* transition of ligand
5
exhibits a minor red shift, whereas the transitions for 4a,b
(
PSS) within 2 min. Further irradiation of the sample solution
show small blue shifts in comparison to the λmax values of the
complexes 6a,b and 7. In all, the UV−vis absorption spectra of
the Ru(II) complexes are quite similar to those of the free
ligands, suggesting that coordination of the metal ion does not
significantly alter the electronic structure of the ligands.
Photoisomerization Studies of Ligands 3−5. The
reversible photoswitching properties of the new ligands 3−5
and the Ru(II) complexes 6a,b and 7 have next been examined
by UV−vis spectroscopy. As a representative example, the
beyond 2 min did not give any significant changes in the
spectral features. For ligand 4a, the cis to trans ratio at the PSS
was estimated to be 81:19 as determined by UV−vis
spectroscopy using eq 1, as previously reported by Wada and
52−54
others.
This estimation was based on the assumption that
before irradiation the high-intensity band at the absorption
maximum wavelength (337 nm) of the trans form of 4a is due
to the trans isomer only (i.e., no cis form is present).
D
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Figure 4. UV−vis absorption spectra of the ligands 3−5 before and after 2 min irradiation at 365 nm.
Similar absorption spectral changes were observed for the
other ligands, as shown in Figure 4 and Figures S1−S6 in the
Supporting Information. The estimated percent conversions,
the band maxima, and the absorption spectral data for the
ligands and Ru(II) complexes are also given in Table S2 in the
Supporting Information. The light-induced reversible photo-
switching process is described in Scheme 3.
ligands 3−5 are comparable to those of the recently reported
20,51
arylazopyrazole-based molecular photoswitches.
The back
cis to trans photoisomerizations of ligands 3, 4b, and 5 have
also been studied upon green light irradiation in a manner
similar to that for ligand 4a and are given in Figures S1−S4, S6,
and S7 in the Supporting Information.
Photoisomerization Studies of Complexes 6a,b and 7.
Although arylazopyrazoles have recently been shown to
undergo efficient trans to cis isomerization, their capacity to
isomerize when coordinated to a metal ion has not been
explored yet. We therefore performed preliminary photo-
irradiation experiments on the arylazopyrazole-conjugated
Ru(II) arene complexes 6a,b and 7. The photoisomerization
properties of the complexes have been examined by UV−vis
spectroscopy, monitoring the changes occurring in their
absorption spectra during irradiation (Figure 5). As shown in
Figure 5, the Ru(II) complexes show photoisomerization
behavior similar to that of the free ligands. Irradiating
dichloromethane solutions of the Ru(II) complexes with UV
light at 365 nm generated the cis form, characterized by
absorptions in the visible region at λmax ∼450 nm. Upon
photoisomerization, a decrease in the intensity of the π−π*
transition and an increase in the n−π* transition was observed.
It was observed that the rate of the trans to cis isomerization of
the complexes is slightly slower than those of the free ligands.
The PSS was reached within 4 min in comparison to the 2 min
for the free ligands. The cis to trans ratio of the Ru(II)
complexes at the PSS is estimated to be 80:20 (6a), 79:21 (6b),
and 70:30 (7), as determined by using a method similar to that
Scheme 3. Photoisomerization Process of Ligand 4a and
Complex 6a
52−54
for ligand 4a.
Notably, upon coordination to the metal ion
The photoinduced reverse cis to trans photoisomerization
properties of the ligands 3−5 have next been examined by
irradiation with green (530 nm) light. To test for photo-
reversibility, UV-exposed dilute solutions of the samples were
further irradiated with 530 nm light and the process was
followed by acquiring UV−vis spectra at regular time intervals.
As depicted in Figure 3b, irradiation of a solution of ligand 4a
with 530 nm light at different time intervals led to the full
recovery of the absorption spectrum of the trans isomer. The
observed cis to trans photoisomerization properties of the
each of the ligands retained their characteristic absorption
properties. This similar behavior of the complexes and the free
ligands may suggest that coordination of the metal ions did not
significantly alter the electronic structure of the free ligands.
To ensure that the observed photoisomerization of the
complexes was fully reversible, a relaxation experiment was
carried out in a manner similar to that for the free ligands. The
reverse cis to trans photoisomerization of the Ru(II) complexes
was studied by irradiation with 530 nm light. After irradiation of
E
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Figure 5. UV−vis absorption spectra of the Ru(II) complexes 6a,b and 7 before and after 4 min irradiation at 365 nm.
solutions of the complexes with 530 nm light, spectra that
match the original spectra assigned for the trans isomer were
regenerated (Figures S4, S6, and S7 in the Supporting
Information), indicating that the process was fully reversible.
In all, the UV−vis studies clearly showed trans to cis
isomerization of the ligands 3−5 and the Ru(II) arene
complexes upon irradiation with UV light and the reverse
reaction under green light.
Thermal Cis to Trans Isomerization of 3−5 and
Complexes 6a,b and 7. The thermal cis to trans isomer-
ization rates of the arylazopyrazole-based ligands 3−5 and the
corresponding Ru(II) complexes 6a,b and 7 were measured in
The data obtained showed that, at room temperature, the
thermal cis to trans isomerization rates of the metal complexes
were much faster (less stable cis isomer) than those of the free
ligands. This kind of behavior has been observed with related
12,13
azobenzene-containing metal complexes.
The half-lives
(
(
t1/2) for the complexes were determined to be 224 min
6a), 165 min (6b), and 131 min (7). Furthermore, a
somewhat lower thermal stability was observed for the cis
isomer of complex 7 in comparison to the free ligand 5.
Presumably, this is due to the slight decomposition observed in
the thermal relaxation process of complex 7. Generally,
although a direct comparison cannot be made due to the
different natures of the ligands described here, the thermal
stabilities of the cis isomer of complexes 6a,b are comparable to
or even better than those of recently reported azobenzene-
CH CN at 25 °C by UV−vis spectroscopy. First, acetonitrile
3
solutions of ligands 3−5 and the complexes 6a,b and 7 were
irradiated for 2 min at 365 nm and kept in the dark. Then, the
reverse cis to trans isomerization process was actively
monitored by UV−vis spectroscopy, registering the spectra at
regular time intervals until the initial spectrum was restored.
The value of absorbance at the high-intensity band (π−π*) was
12,13
containing Ru(II) arene complexes.
The data obtained also showed that the thermal stabilities of
ligands 4a,b are lower than that of the recently reported benzyl-
substituted arylazopyrazole photoswitch (albeit measured with
a different solvent and temperature) but have thermal stabilities
used to calculate the first-order rate constants (k) and half-lives
8
(
t1/2) using Monkowius’ procedure, and the data for this
51
process are given in Table 1.
comparable to that of a naphthyl-substituted photoswitch.
A
higher thermal stability of the cis isomer was observed for
ligand 3 (2310 min) and ligand 5 (1386 min). As previously
observed for other pyrazole ring substituted azopyrazoles, the
nature of the substituents on the pyrazole nitrogen has a
significant influence on the thermal stabilities of the new
Table 1. Thermal Kinetic Data for the Cis to Trans
Isomerization at 25 °C
compound
λmax (nm)
k (min−1)
half-life (min)
3
330
337
342
344
331
342
340
3.00 × 10−
2.20 × 10−
4
3
3
4
3
3
3
2310
315
20,51
photoswitches described here.
4
4
5
6
6
7
a
The photostability of the reversible trans to cis photo-
isomerization of the complexes was investigated using 6a as a
representative example. To test for photostability, the UV−vis
absorption spectra of a dilute solution of 6a was monitored by
alternate irradiation of the sample with 365 nm (2 min)
followed by 530 nm light (10 min). Complex 6a was stable
under these conditions (Figure S5 in the Supporting
−
b
1.77 × 10
392
5.00 × 10−
1386
224
3.10 × 10−
a
4.20 × 10−
b
165
a
5.30 × 10−
∼131
a
Approximate value due to slight decomposition in solution.
F
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Figure 6. H NMR spectral changes in the aromatic region of complex 6a (2.0 × 10−3 M) in CD CN solution: (a) spectrum of trans-6a before
1
3
irradiation (trans-6a only); (b) spectrum of 6a after irradiation with UV light at 365 nm for 30 min.
Figure 7. Optimized calculated structures of the trans and cis forms of (a) ligand 4a and (b) Ru(II) complex 6a.
1
Information), and no signs of photodegradation were detected
during the 10 cycles of irradiation.
photoirradiation. Likewise, the doublet signal in the H NMR
spectra of the CH proton adjacent to the pyridine nitrogen
moiety was shifted upfield from 9.04 to 8.97 ppm. These shifts
in NMR signals could be ascribed to the alteration of the
chemical environment in the structure after the photo-
isomerization reactions. The signal for the CH proton adjacent
to the pyridine nitrogen was then integrated to calculate the
percent composition of the cis isomer produced. The percent
conversion of the cis isomer was determined to be 82% for
Investigation of the Photoisomerization Process of
1
Complex 6a by H NMR Spectroscopy. The photo-
isomerization process of the complexes is further evidenced
1
by H NMR spectroscopy, following the changes occurring in
the spectra of the Ru(II) cymene complexes subjected to UV
light irradiation. As a representative example, Figure 7 shows
1
changes in the portions of the H NMR spectra of complex 6a
1
in CD CN before and after irradiation. In the trans form of
complex 6a by H NMR, which is similar to data obtained using
3
complex 6a, the aromatic protons of the p-cymene moiety
display four distinct signals: two doublets at 5.85 and 5.25 ppm
corresponding to H2 and H3 and two doublet signals at 5.60−
UV−vis spectrscopy.
Computational Studies. To gain further insight into the
isomerization behavior and optical properties of the ligands and
the Ru(II) complexes, computational modeling of both the cis
and trans forms of the compounds was performed by the DFT
method at the B3LYP/6-311++G(d,p) level using the Gaussian
5
.55 ppm corresponding to H5 and H6, respectively (Figure 6).
Upon irradiation with green (530 nm) light for 30 min the
aromatic protons resonated as four doublets in the upfield
region at 4.80 (H2), 5.45 (H3), 5.65 (H5), and 5.70 (H6) ppm,
providing evidence in favor of the formation of the cis isomer.
Analogously, the methylene protons located on the pyridine
moiety (H7) shifted upfield from 5.30 to 5.00 ppm after
55
09 package. As representative examples, the lowest-energy
conformers of the gas-phase optimized geometries for the trans
and cis forms of ligand 4a and its Ru(II) p-cymene complex 6a
are shown in Figure 7. The results from the calculations reveal
G
DOI: 10.1021/acs.organomet.7b00493
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
that the trans form of the azopyrazole unit in both the ligands
and the Ru(II) complexes adopts a nearly planar geometry with
C−NN−C dihedral angles of ∼180°(Figure 7 and Figure S8
in the Supporting Information). On the other hand, the
computed geometry of the cis isomers of both the ligands and
the Ru(II) complexes indicates significant twisting of the
phenyl ring of the azo unit in order to minimize the steric
interaction between the methyl group of the pyrazole ring
CONCLUSION
■
In summary, we report herein the synthesis, characterization,
and photoswitching behavior of new benzothiazole- and
pyridine-functionalized arylazopyrazole-based ligands and their
half-sandwich ruthenium(II) p-cymene complexes. One of the
complexes (6a) formed crystals suitable for X-ray structural
determination, exhibiting a typical piano-stool coordination
geometry around the Ru(II) center. The detailed photo-
isomerization studies reveal that the ligands and the Ru(II)
complexes undergo fast trans to cis isomerization upon
irradiation with 365 nm UV light, reaching favorable photosta-
tionary states. The reverse cis to trans isomerization of the
compounds can be nearly quantitatively restored by irradiation
with 530 nm light. The results also show that coordination of
the Ru(II) does not significantly alter the reversible trans to cis
isomerization properties of the ligands, with ligands and metal
complexes displaying essentially similar photoswitching behav-
ior. Furthermore, variation of the substituents on the N-1
position of the pyrazole ring has a minor influence on the
photoisomerization properties of the ligands and metal
complexes. However, the presence of different substituents on
the pyrazole ring has a significant influence on the thermal
stabilities of the compounds investigated. Since Ru(II) arene
complexes have known diverse catalytic applications and have
recently been increasingly considered for the development of
metal-based pharmaceutical agents, we believe that adding a
light-switchable moiety in their ligand structure may enable
modulating their functionality and enhance their potential in
the domains of catalysis and biological applications.
(Figure 7 and Figure S8). The optimized geometries of the
ligands are similar to those for previously reported arylazopyr-
azole-based molecular switches reported by Fuchter and co-
2
0
workers. The predicted structure is quite similar to that of the
experimentally determined X-ray crystal structure of complex
6a. The optimized bond lengths of complex 6a are in the range
of those observed for Ru−N(1) (2.205 Å vs 2.128 Å), Ru−
N(2) (2.172 Å vs 2.115 Å), Ru−Cl (2.428 Å vs 2.390 Å), and
N(4)−N(5) (1.258 Å vs 1.259 Å).
The calculated energy values for both the ligands and Ru(II)
p-cymene complexes also predict that the cis form of the
arylazopyrazole is higher in energy than the trans configuration.
For instance, the results from the computations predict that the
trans isomer of ligand 4a is 16.4 kJ/mol more stable than the cis
isomer. Likewise, the trans form of the Ru(II) p-cymene
complex 6a is calculated to be 18 kJ/mol more stable than the
cis form. The computed energy difference between the free
ligand 4a and its Ru(II) complex 6a is minor, indicating that
the incorporation of the Ru(II) p-cymene group has very little
effect on the thermodynamics of the trans to cis isomerization.
This minor calculated energy difference is in good agreement
with the observed experimental findings that both the ligands
and Ru(II) p-cymene complexes show similar isomerization
behavior in solution. A similar trend is observed for the other
ligands and Ru(II) complexes. The lowest-energy conformers
for the trans and cis forms of the other ligands (3, 4b, znd 5)
and the Ru(II) p-cymene complexes (6b and 7) are depicted in
Figure S8 in the Supporting Information.
Using the optimized structures, a comparative analysis of the
absorption spectra of the ligands was also performed by time-
dependent (TD) DFT calculations. These calculations were
conducted in dichloromethane in a manner similar to the
conditions under which the experimental absorption profiles
were measured. The computed values of the electronic
transitions giving rise to the main absorption features for the
arylazopyrazole-based ligands are compiled in Table S3 in the
Supporting Information. In agreement with the experimental
results, the UV−vis absorption data predicted from the TD-
DFT calculations reveal the presence of two bands for the
π−π* transitions and one band for the n−π* transitions in all
of the compounds studied (Table S3). However, the calculated
absorption wavelength values of the ligands were significantly
red shifted in comparison to the experimentally observed
values. For instance, the main absorption spectra (π−π*
transition) of ligand 4a show the presence of an intense band
centered at λcalcd 355.73 nm which is ascribed to a highest
occupied molecular orbital (HOMO) → lowest occupied
EXPERIMENTAL SECTION
■
All reactions were carried out under nitrogen. All chemicals and
solvents were purchased from commercial sources and used as
6
56
received. [(η -cymene)RuCl ] and 3-(2-phenylhydrazono)pentane-
2
2
20
1
2
,4-dione (1) were prepared according to literature methods.
H
13
1
NMR and C{ H} NMR spectra were recorded on a JEOL Eclipse3
400 MHz spectrometer using solvent resonances as internal references.
Spectroscopic Measurements. Dry acetonitrile and dichloro-
methane solvents were used for all UV−vis measurements. UV−vis
absorption spectra were recorded on a Varian Cary 50 BIO
spectrometer. Photoisomerization was induced by irradiating dichloro-
methane solutions of the chelating ligands or ruthenium(II) cymene
complexes in a quartz cuvette at room temperature with a hand-held 6
W 365 nm UV lamp, followed immediately by recording the
absorption spectrum. For the reverse isomerization process, irradiation
at 530 nm was carried out using a Shimadzu RF-5301 PC
spectrofluorometer equipped with a 150 W Xe lamp.
Determination of the Percentage of Cis by UV−Vis
Spectroscopy. The percent conversion of the cis isomer of the
ligands and Ru(II) complexes at the photostationary state formed by
52−54
UV irradiation was estimated on the basis of the equation
cis = Atrans
− Acis
Atrans
%
× 100
(1)
where Atrans is the absorbance for the trans isomer at λmax before light
irradiation and Acis is the absorbance for the cis isomer at the same
wavelength measured at the photostationary state.
Determination of the Percentage of Cis by NMR for 6a. A
concentrated solution of 6a (2.0 × 10 M) in CD CN was prepared
in an NMR tube, and the initial proton spectrum was recorded before
irradiation. Then, the solution was irradiated with 365 nm light for 30
min in an NMR tube. After the irradiation was completed, the solution
was quickly placed in the NMR spectrometer and the proton spectrum
was acquired at 25 °C. The CH proton adjacent to the pyridine
nitrogen was then integrated to calculate the percent composition of
the cis isomer produced.
52−54
−3
3
(
LUMO) transition. A minor HOMO-1 → LUMO transition
centered at λcalcd 306.65 nm was also observed. The computed
value of the high-intensity band (λcalcd 355.73 nm) is red-shifted
by ∼20 nm in comparison to the experimentally determined
λ
value at 337 nm.
max
H
DOI: 10.1021/acs.organomet.7b00493
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
X-ray Structure Determination. Single crystals of complex 6a
were grown by layering a dichloromethane solution with petroleum
ether. Crystals were mounted on glass fibers. All measurements were
made using graphite-monochromated Cu Kα radiation on a Bruker-
AXS three-circle diffractometer, equipped with a SMART Apex II
CCD detector. Initial space group determination was based on a
matrix consisting of 120 frames. The data were reduced using SAINT
(CH Py), 14.0 (CH Pz), 13.6 (CH Pz). Anal. Calcd for C H N , :C,
64.17; H, 7.04; N, 28.78. Found: C, 63.92; H, 7.01; N, 28.35.
2
3
3
13 17
5
General Procedure for the Synthesis of Ligands 4a,b. 3,5-
Dimethyl-4-(phenyldiazenyl)-1H-pyrazole (2; 1.0 g, 10.0 mmol) was
dissolved in 25 mL of anhydrous acetonitrile. To this solution were
added 0.50 g of 2-chloromethylpyridine hydrochloride, 0.20 g of KOH,
and 0.10 mg of TBAB. The mixture was refluxed for 24 h. The
resulting mixture was filtered, and the solvent was removed by rotary
evaporation, giving a sticky brown solid product in 80% yield. The
solid was washed several times with hexane and diethyl ether solvents
to obtain a dry solid.
57
58
+
,
and empirical absorption correction was applied using SADABS.
The X-ray structure of 6a was solved using direct methods. Least-
2
squares refinement was carried out on F . Two crystallographically
independent molecules were present. The non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were placed in riding
positions and refined isotropically. Structure solution, refinement,
and the calculation of derived results were performed using the
SHELXTL package of computer programs and Shelxle. Despite
attempts to crystallize 6a from a variety of solvents, in each case the
resulting crystals were found to be solvates in which the included
solvent was highly disordered and could not be modeled. Therefore,
Platon-Squeeze was used to mask the solvent-based reflection data
1-(2-Methylenepyridyl)-4-(phenyldiazenyl)-3,5-dimethyl-1H-pyra-
1
zole (4a). H NMR (CDCl
, 400 MHz): δ 8.57 (d, J = 6.00 Hz, 1H
3
Py), 7.79 (d, J = 8.7 Hz, 2H, Py), 7.64 (t, J = 7.8 Hz, 1H, Py), 7.46 (m,
2H, Py), 7.39 (m, 1H, Py), 7.20 (d, J = 8.2 Hz, 1H, Py), 6.95 (d, J =
7.8 Hz, 1H, Py), 5.40 (s, 2H, CH Py), 2.57 (s, 3H, CH , Pz), 2.54 (s,
59
60
2
3
13
3H, CH , Pz). C NMR (CDCl , 100 MHz): δ 156.0 (Pz), 152.2
3
3
(Py), 149.4 (Pz), 143.0 (Pz), 140.0 (Py), 137.1 (Py), 135.4 (Py),
132.0 (2C, Py), 123.3 (2C, py), 122.7 (Py), 121.1 (Py), 54.7 (CH Py),
2
61
during final refinement. A total potential solvent-accessible void
14.0 (CH Pz), 13.6 (CH Pz). Anal. Calcd for C H N : C, 70.07; H,
3
3
17 17
5
3
−
volume of 880.4 Å was found, corresponding to 339 e /unit cell. It is
likely that this volume corresponds to two molecules of hexane, which
was used as the cocrystallization solvent. Details of the X-ray
experiment and crystal data are summarized in Table S1 in the
Supporting Information. Selected bond lengths and bond angles are
given in Figure 1.
5.90; N, 24.03. Found: C, 69.84; H, 5.84; N, 23.67.
1-(2-Methylenepyridyl)-4-((4-bromophenyl)diazenyl)-3,5-dimeth-
yl-1H-pyrazole (4b). Ligand 4b was synthesized following similar
1
procedures as in 4a. H NMR (CDCl
, 400 MHz): δ 8.58 (d, J = 6.8
3
Hz, 1H Py), 7.65 (m, 3H, Py), 7.57 (m, 2H, Py), 7.21 (d, J = 6.8 Hz,
1H, Py), 6.97 (d, J = 7.8 Hz, 1H, Py), 5.41 (s, 2H, CH Py), 2.56 (s,
3H, CH , Pz), 2.52 (s, 3H, CH , 100 MHz): δ
, Pz). 13C NMR (CDCl
2
Computational Details. All density functional theory (DFT)
3
3
3
calculations were performed with the Gaussian09 package at the
156.0 (Pz), 152.2 (Py), 149.4 (Pz), 143.0 (Pz), 140.0 (Py), 137.1 (Py),
135.4 (Py), 132.0 (2C, Py), 123.3 (2C, py), 122.7 (Py), 121.1 (Py),
55
B3LYP level. LanL2DZ and 6-311G**(d,p) basis sets were used for
ruthenium(II) and all other atoms, respectively. All structures are gas-
phase optimized geometries. The absence of imaginary frequencies was
checked on all calculated structures to confirm that they were true
54.7 (CH
Py), 14.0 (CH Pz), 13.6 (CH Pz). Anal. Calcd for
3 3
2
C H N Br: C, 55.15; H, 4.36; N, 18.92. Found: C, 54.91; H, 4.31;
17
16
5
N, 18.55.
Synthesis of 1-(2-Benzothiazolyl)-4-(phenyldiazenyl)-3,5-di-
5
5
minima. Computed structures were illustrated using Avogadro.
Calculations were performed on the Comet cluster at SDSC via
Extreme Science and Engineering Discovery Environment (XSEDE)
Comet Rapid Access Project (to S.M.C.), with the GridChem 2.0.0
methyl-1H-pyrazole (5). 2-Hydrazinobenzothiazole (1.0 g, 1 equiv)
was added to a solution of 3-(2-phenylhydrazino)-pentane-2,4-dione
(1, 1 equiv) dissolved in ethanol, and the mixture was refluxed for 5 h.
(http://www.gridchem.org/) computational gateway, which is sup-
The resulting solution was rotary evaporated to give a bright yellow
1
ported by the NSF.
solid product in 90% yield. H NMR (CDCl
3
, 400 MHz): δ 7.93 (d, J
Photochemical Studies. Stock solutions of the chelating ligands
= 7.8 Hz, 1H), 7.85 (d, J = 7.8 Hz, 2H), 7.49 (d, J = 7.8 Hz, 2H),
1
3
3
×
, 4a,b, and 5 and ruthenium(II) p-cymene complexes 6a,b and 7 (2.0
7.35−7.46 (m, 4H), 3.17 (s, 3H, CH
3
, Pz), 2.60 (s, 3H, CH , Pz). C
3
−5
10 M) were prepared in dichloromethane. Electronic absorption
NMR (CDCl , 100 MHz): δ 162.6 (Pz), 154.8 (Py), 153.0 (Pz), 147.3
3
spectral experiments were performed in a quartz cuvette of 1 cm path
length. Solutions of both the ligands and metal complexes were
irradiated by UV light (365 nm).
(Pz), 144.3 (Pz), 139.1 (Py), 134.3 (Py), 131.7 (Py), 130.5 (2C, Py),
127.9 (Py), 126.3 (Py), 124.1 (Py), 123.6 (2C, Py), 122.8 (Py), 16.4
(CH
Pz), 13.7 (CH Pz). Anal. Calcd for C18H N S: C, 64.84; H,
3
3 15 5
Half-lives for the thermal cis to trans back-isomerization reactions
4.53; N, 21.01. Found: C, 65.08; H, 4.68; N, 21.06.
were determined by following procedures similar to those described in
General Procedure for the Synthesis of Ru(II) p-Cymene
Complexes 6a,b and 7. The appropriate ligands dissolved in
dichloromethane (5.0 mL) were added to a solution of the [(η -p-
8
,20
the literature. The rate constant k for the reaction is determined by
plotting ln{(Atrans − A )/(A − A )} vs time (A = absorbance of
6
pss
trans
t
trans
the pure trans isomer; Apss = absorbance at the photostationary state;
cymene)RuCl ] dimer in 10.0 mL of dichloromethane. The mixture
2 2
8
At = absorbance at the interval time t). Assuming the photo-
was stirred for 24 h at room temperature. The solution was
concentrated to ∼3.0 mL, and the corresponding product was
precipitated with cold diethyl ether and filtered off. The solid product
was dissolved in dichloromethane and recrystallized by layering with
hexane or petroleum ether.
isomerization process obeys a first-order reaction, the slope of the
best-fit line gives the rate k for the reverse reaction. The half-life of the
complexes and ligands is then calculated using the following equation:
t1/2 = ln(2)/k.
Synthesis of 1-(2-Aminoethyl)-4-(phenyldiazenyl)-3,5-di-
methyl-1H-pyrazole (3). 3,5-Dimethyl-4-(phenyldiazenyl)-1H-pyr-
azole (2; 1.0 g, 10.0 mmol) was dissolved in 25 mL of anhydrous
acetonitrile, 1.80 g (45.0 mmol) of sodium hydroxide was added to
this solution, and the mixture was stirred for 30 min at ambient
temperature. The reaction mixture was refluxed, and to this mixture
was slowly added a suspension of 2-chloroethylamine. After the
solution was refluxed for 12 h, it was cooled to room temperature and
the precipitate that formed was removed by filtration. The solvent of
6
[(η -p-Cymene)](1-(2-methylenepyridyl)-4-(phenyldiazenyl)-3,5-
1
dimethyl-1H-pyrazole)chlororuthenium(II) Chloride (6a). H NMR
(CDCl , 400 MHz): δ 8.93 (d, J = 5.5 Hz, 1H, Py), 7.98 (d, J = 7.32
3
Hz, 1H, Py), 7.92 (t, J = 6.9 Hz, 1H, Py), 7.75 (d, J = 7.32 Hz, 1H,
Py), 7.39−7.48 (m, 4H, Pz), 6.41 (d, J = 6.0 Hz, 1H, Ar), 6.20 (d, J =
5.9 Hz, 1H, Ar), 6.0 (s, 2H, CH
Py), 5.97 (d, J = 6.0 Hz, 1H, Ar), 5.79
, Ar), 2.86
, Pz), 1.32
2
(d, J = 6.0 Hz, 1H, Ar), 2.94 (sept, J = 6.8 Hz, 1H, H(CH
(s, 3H, CH , Ar), 2.84 (s, 3H, CH , Pz), 2.19 (s, 3H, CH
(d, J = 6.9 Hz, 3H, CH , Ar), 1.21 (d, J = 6.9 Hz, 3H, CH
NMR (CDCl , 100 MHz): δ 156.3 (Pz), 155.8 (Py), 153.0 (Py), 149.6
)
3
3 2
3
3
1
3
, Ar). C
3
3
the filtrate was removed by rotary evaporation to give a yellow solid in
3
1
7
5% yield. H NMR (CDCl , 400 MHz): δ 7.79 (d, J = 7.8 Hz, 2H),
(Pz), 142.0 (Pz), 140.1 (Py), 135.8 (Py), 130.7 (2C, Py), 129.1 (2C,
Pz), 126.7 (Pz), 125.2 (Pz), 122.2 (2C, Pz), 106.1 (Ar), 101.5 (Ar),
86.3 (Ar), 84.8 (Ar), 84.1 (Ar), 83.8 (Ar), 30.7 (CH, Ar), 22.7 (CH3,
Pz), 22.5 (CH , Pz), 18.7 (CH , Ar), 16.3 (CH , Ar), 11.0 (CH , Ar).
3
7.47 (t, J = 8.12 Hz, 2H), 7.38 (t, J = 7.32 Hz, 2H), 4.35 (t, J = 5.96
Hz, 2H, CH ), 3.92 (t, J = 5.96 Hz, 2H, CH ), 2.64 (s, 3H, CH , Pz),
2
2
3
1
3
2
.51 (s, 3H, CH , Pz). C NMR (CDCl , 100 MHz): δ 156.0 (Pz),
3
3
3
3
3
3
152.2 (Py), 149.4 (Pz), 143.0 (Pz), 140.0 (Py), 137.1 (Py), 135.4
Anal. Calcd for C H N RuCl : C, 54.27; H, 5.23; N, 11.72. Found:
27 31 5 2
(
Py), 132.0 (2C, Py), 123.3 (2C, py), 122.7 (Py), 121.1 (Py), 54.7
C, 54.61; H, 5.29; N, 11.66.
I
DOI: 10.1021/acs.organomet.7b00493
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
6
[
(η -p-Cymene)](1-(2-methylenepyridyl)-4-((4-bromophenyl)-
tometer facility and helping with X-ray data analysis and solving
of the crystal structure.
diazenyl)-3,5-dimethyl-1H-pyrazole)chlororuthenium(II) Chloride
1
(
6b). H NMR (CDCl , 400 MHz): δ 8.89 (d, J = 5.0 Hz, 1H, Py),
3
8
.02 (d, J = 7.32 Hz, 1H, Py), 7.88 (t, J = 7.32 Hz, 1H, Py), 7.55−7.62
REFERENCES
(
6
m, 4H, Pz), 7.39 (t, J = 6.4 Hz, 1H, Pz), 6.33 (d, J = 5.5 Hz, 1H, Ar),
.16 (d, J = 5.5 Hz, 1H, Ar), 6.08 (s, 2H, CH Py), 5.96 (d, J = 5.5 Hz,
■
(
1) Xiong, Y.; Rivera-Fuentes, P.; Sezgin, E.; Jentzsch, A. V.;
2
1
H, Ar), 5.78 (d, J = 5.3 Hz, 1H, Ar), 2.90 (sept, J = 6.9 Hz, 1H,
Eggeling, C.; Anderson, H. L. Org. Lett. 2016, 18, 3666−3669.
(2) Lv, G.; Sun, A.; Wei, P.; Zhang, N.; Lan, H.; Yi, T. Chem.
Commun. 2016, 52, 8865−8868.
(3) Roubinet, B.; Weber, M.; Shojaei, H.; Bates, M.; Bossi, M. L.;
Belov, V. N.; Irie, M.; Hell, S. W. J. Am. Chem. Soc. 2017, 139, 6611−
6620.
H(CH ) , Ar), 2.83 (s, 3H, CH Pz), 2.82 (s, 3H, CH Ar), 2.15 (s, 3H,
CH Ar), 1.30 (d, J = 6.9 Hz, 3H, CH , Ar), 1.19 (d, J = 6.9 Hz, 3H,
CH , Ar). C NMR (CDCl , 100 MHz): δ 156.1 (Pz), 155.5 (Py),
3
2
3
3
3
3
13
3
3
1
51.6 (Py), 149.5 (Pz), 142.4 (Pz), 140.0 (Py), 135.6 (Py), 132.1 (2C,
Py), 126.8 (Pz), 125.1 (Pz), 124.6 (Pz), 123.5 (2C, py), 105.9 (Ar),
1
3
01.4 (Ar), 86.1 (Ar), 84.6 (Ar), 84.0 (Ar), 83.6 (Ar), 54.5 (CH Py),
(4) Pianowski, Z. L.; Karcher, J.; Schneider, K. Chem. Commun. 2016,
2
0.6 (CH, Ar), 22.5 (CH , Pz), 22.3 (CH , Pz), 18.5 (CH , Ar), 16.2
52, 3143−3146.
3
3
3
(
CH , Ar), 10.9 (CH , Ar). Anal. Calcd for C H N RuBrCl : C,
(5) Pizzolato, S. F.; Collins, B. S. L.; van Leeuwen, T.; Feringa, B. L.
Chem. - Eur. J. 2017, 23, 6174−6184.
3
3
27 30
5
2
47.94; H, 4.47; N, 10.35. Found: C, 48.36; H, 4.52; N, 10.80.
6
[
(η -p-Cymene)](1-(2-benzothiazolyl)-4-(phenyldiazenyl)-3,5-di-
(6) Neilson, B. M.; Bielawski, C. W. Organometallics 2013, 32, 3121−
3128.
1
methyl-1H-pyrazole)chlororuthenium(II) Chloride (7). H NMR
CDCl , 400 MHz): δ 7.83 (t, J = 7.8 Hz, 2H), 7.76 (t, J = 7.8 Hz,
(
2
(7) Moustafa, M. E.; McCready, M. S.; Puddephatt, R. J.
Organometallics 2013, 32, 2552−2557.
3
H), 7.04−7.39 (m, 5H), 5.89 (d, J = 6.4 Hz, 1H, Ar), 5.83 (d, J = 6.4
Hz, 1H, Ar), 5.70 (d, J = 5.96 Hz, 1H, Ar), 5.55 (d, J = 6.4 Hz, 1H,
(8) Kaiser, M.; Leitner, S. P.; Hirtenlehner, C.; List, M.; Gerisch, A.;
Monkowius, U. Dalton Trans. 2013, 42, 14749−14756.
(9) Bandara, H. M. D.; Burdette, S. C. Chem. Soc. Rev. 2012, 41,
1809−1020.
Ar), 2.93 (sept, J = 6.7 Hz, 1H, H(CH ) , Ar), 2.60 (s, 6H, CH , Pz),
3
2
3
2
6
1
1
.06 (s, 3H, CH , Ar), 1.87 (d, J = 6.8 Hz, 3H, CH , Ar), 1.32 (d, J =
3
3
13
.8 Hz, 3H, CH , Ar). C NMR (CDCl , 100 MHz): δ 153.1 (Pz),
3
3
51.5 (Pz), 145.8 (Pz), 142.8 (Pz), 137.5 (Py), 132.8 (Py), 130.2 (Py),
29.0 (Py), 128.7 (2C, Py), 126.4 Py), 124.8 (Py), 122.6 (Py), 122.1
(10) Kume, S.; Nishihara, H. Dalton Trans. 2008, 3260−3271.
(11) Han, M.; Hirade, T.; Hara, M. New J. Chem. 2010, 34, 2887−
891.
(
2C, Py), 121.3 (Py), 101.2 (Ar), 96.7 (Ar), 81.3 (2C, Ar), 80.5 (2C,
2
(
Ar), 30.6 (Cym), 22.1 (2C, Cym), 18.9 (Cym), 14.8 (Pz), 12.2 (Pz).
Anal. Calcd for C H N SRuCl : C, 52.91; H, 3.96; N, 11.02. Found:
C, 53.27; H, 4.04; N, 10.88.
12) Segarra-Maset, M. D.; van Leeuwen, P. W. N. M.; Freixa, Z. Eur.
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ASSOCIATED CONTENT
Supporting Information
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*
S
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00493.
(
2
(
UV−vis absorption spectra from photoisomerization
studies, orbital assignments of the most intense
transitions obtained in the TD-DFT calculations,
calculated structures, NMR spectra, and X-ray crystallo-
graphic data for 6a (PDF)
(
(
(
Accession Codes
CCDC 1556869 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
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AUTHOR INFORMATION
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Corresponding Author
K.G.: e-mail, Kesete.Ghebreyessus@hamptonu.edu, tel, 757-
27-5475.
*
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Kesete Ghebreyessus: 0000-0003-4273-7556
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ACKNOWLEDGMENTS
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We thank the U.S. National Science Foundation (NSF) for
financial support through the PREM (Award Number
1
523620) and CREST (HRD-1137747) grants. The authors
also thank Dr. Robert D. Pike of the College of William and
Mary for providing access to a single-crystal X-ray diffrac-
J
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DOI: 10.1021/acs.organomet.7b00493
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