Azobenzene-based ruthenium(ii) catalysts for light-controlled hydrogen generation

Article abstract of DOI:10.1039/c7dt00542c

Eight new ruthenium(ii) half-sandwich complexes containing azobenzene-appended pyridine (1), bipyridine (2-5) and phosphine (6 and 7) ligands have been synthesized and fully characterized. UV-vis spectroscopic studies showed that the trans-to-cis photoisomerization was strongly inhibited upon coordination to the metal centre in azopyridine-derived ligands 1 and 2, but it remained efficient in azobenzene-appended bipyridine (3-5) and phosphine (6 and 7) ligands. The complexes were tested as precatalysts for photo-controlled hydrogen generation by hydrolytic decomposition of ammonia-borane (AB). In situ irradiation of the reaction mixtures of compounds [Ru(p-Cym)(6)Cl]Cl and [Ru(p-Cym)(7)Cl]Cl induced a clear change in the catalytic reaction rate, serving as a proof of concept for light-controlled hydrogen generation.

Full text of DOI:10.1039/c7dt00542c

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
rsc.li/dalton

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DOI: 10.1039/C7DT00542C
Journal Name
ARTICLE
Azobenzene‐based ruthenium(II) catalysts for light‐controlled
hydrogen generation.
A. Telleria,a P.W.N.M. van Leeuwenb and Z. Freixa*a,c
Received 00th January 20xx,
Eight new ruthenium(II) half‐sandwich complexes containing azobenzene‐appended pyridine (1), bipyridine (2–5) and
phosphine (6, 7) ligands have been synthesized and fully characterized. UV‐vis spectroscopic studies showed that the
trans‐to‐cis photoisomerization was strongly inhibited upon coordination to the metal centre in azopyridine‐derived
ligands 1 and 2, but it remained efficient in azobenzene‐appended bipyridine (3–5) and phosphine (6 and 7) ligands. The
complexes were tested as precatalysts for photo‐controlled hydrogen generation by hydrolytic decomposition of
ammonia‐borane (AB). In situ irradiation of the reaction mixtures of compounds [Ru(p‐Cym)(6)Cl]Cl and [Ru(p‐Cym)(7)Cl]Cl
induced a clear change in the catalytic reaction rate, serving as proof of concept for light‐controlled hydrogen generation.
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
amine‐boranes using Rh(I) or Rh(III) precatalytsts,14 many
studies have been devoted to the development of metal‐
catalysts for a controlled hydrogen liberation from amine‐
Introduction
Traditionally, the modification of the properties of
organometallic compounds for specific applications relied on
an educated design of the appropriate ligands. An ultimate
development of such strategy consist of using ligands that do
not simply behave as spectators modifying the metal
properties, but that are capable of transforming them into
environment‐sensitive entities.1 Obviously this new generation
of responsive (i.e. to changes in pH, light, temperature, etc.)
organometallics could expand the already broad number of
possible applications of organometallic compounds.
boranes based on both, homogeneous and heterogeneous
catalysts.8, 10, 15, 16 Two different catalytic routes have been
envisioned for this purpose: dehydrocoupling and solvolysis in
protic solvents (see Scheme 1). The former process has been
widely studied; it is promoted by several metallic nanoparticles
and metallic clusters17, 18, 19 as well as by discrete and well‐
defined organometallic complexes based on Rh,20, 21, Ru,22‐24
Ir,25, 26 Os,27 Fe19 or Pd.28 Most of these systems generate only
one equivalent of hydrogen per mole of AB (according to
equation 1 in Scheme 1), and only few are able to further
dehydrogenate it releasing up to 2.8 hydrogen equivalents.24,
In the last years, we have devoted an effort to the
development of azobenzene‐based ligands (phosphines,2
29‐33
Compared to the dehydrocoupling, the solvolytic route
pyridines, bipyridines3 and phenylpyridines4,
) for the
5
presents the advantage that in most of the cases the hydrogen
evolved is close to the maximum expected 3 equivalents (even
when mono‐ and di‐N‐substituted amino‐borane are used as
substrates). This is due to the fact that, formally, the liberated
gas originates from solvolysis of a BH3 fragment “liberated” in
the protic media, and not from the coupling of protic and
hydric hydrogen atoms of the substrate.34
construction of light‐responsible organometallic compounds
for different applications. We also reported on the catalytic
activity of half‐sandwich ruthenium(II) complexes of the type
[Ru(p‐Cym)(bipy)Cl]Cl (p‐Cym = p‐cymene, bipy = 2,2’‐
bipyridine) in the hydrogen generation from ammonia‐borane
(AB) through an hydrolytic mechanism,6, 7 which is a relevant
process in terms of hydrogen economy. AB has been
considered as one of the leading hydrogen storage materials,
due to its high gravimetric and volumetric hydrogen density
(19.6 wt% and 0.145 kg H2 L‐1).8‐13 For this reason, from the
early report by Manners et al. on the dehydrocoupling of
a.
Department of Applied Chemistry, Faculty of Chemistry, University of
the BasqueCountry (UPV-EHU), 20080 San Sebastián, Spain. E-mail:
Zoraida_freixa@ehu.eus
Laboratoire de Physique et Chimie de Nano-Objets, INSA-Toulouse,
135 Avenue de Rangueil, F-31077 Toulouse, France
.
b.
c.
IKERBASQUE, Basque Foundation for Science, Bilbao, Spain
Scheme 1. Dehydrocoupling and solvolytic routes for the metal‐catalyzed
hydrogen liberation from AB. Solvolytic route has been exemplified in H2O (4)
and CH3OH (5).
Electronic Supplementary Information (ESI) available: Detailed synthetic
prodecures and characterization of the new compounds, UV‐vis absorption
photoisomerization and catalytic studies See DOI: 10.1039/x0xx00000x
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Cationic complexes of the type [Ru(p‐Cym)L2Cl]PFV6ie(wLA=rticle oOrnline
Although many heterogeneous metal‐based catalysts are
known to render very active systems for this reaction,11, 12, 34‐41
the homogeneously catalyzed process has been much less
studied. The first reports on homogeneous metal‐based
catalysts for the solvolytic dehydrogenation of AB were
published in 2010 based on Ir(III) and Ru(II) complexes.42, 43
Since then, some other examples of organometallic complexes
active for this reaction appeared in the literature.6, 7, 44‐49 Some
authors claimed that the reaction proceeded through metal‐
catalyzed B−N cleavage, and a barrierless hydrolysis of
liberated BH3 in the reaction media45 (in analogy to the
mechanism proposed for acid‐catalyzed AB solvolysis),9 and
since then it has been generally accepted. Nevertheless, to the
best of our knowledge, the actual reaction mechanism remains
virtually unexplored, and no experimental evidences
supporting this hypothesis have been published. Nevertheless,
recent results from our group revealed important electronic
effects in the reaction rate when [Ru(p‐Cym)(bipy*)Cl]Cl (bipy*
1
9)
DOI: 10.1039/C7DT00542C
were intended for the study of the chelate effect, by
comparison with compounds [Ru(p‐Cym)( )Cl]Cl and [Ru(p‐
Cym)( )Cl]Cl, respectively. These compounds were synthesized
from the neutral complexes [Ru(p‐Cym)( )Cl2] and [Ru(p‐
Cym)( )Cl2], using a second equivalent of ligand and a silver
2
8
1
9
salt (AgPF6) as halide abstractor, as described in the literature
for the latter (Scheme 2).55
=
4,4’‐disubstituted‐2,2’‐bipyridine)
were
used
as
51
precatalysts.50,
Based on this premise, and on the well
known electronic modification that the azobenzene fragment
experiences upon irradiation, we considered to address the
current problem of intermittent use of hydrogen as fuel for
locomotive applications, by developing light‐controlled
catalysts for hydrolytic decomposition of AB.
In the current work we present the synthesis of neutral and
cationic half‐sandwich
Ru(II)
complexes
containing
azobenzene‐based pyridine, bipyridine and phosphine ligands,
together with their study as precatalysts in the hydrolytic
dehydrogenation of AB. The extent of the photoisomerization
of the azobenzene fragment present in the ligands upon
irradiation before and after metal coordination will be studied.
Eventually, the effect of irradiation on their catalytic activity,
aiming at light‐controlled hydrogen generation will be
discussed.
Results and discussion
Synthesis
Chart 1. Ru(II) complexes used in this work.
Ligands
1–6 were synthesized according to our previously
described methodologies.2,52 Ligand
7 was synthesized
following the same procedure described for ligand 6,2 but
using 3‐iodoazobenzene as starting compound. Commercially
available ligands
purposes.
8, 9 and 10 were used for comparative
All these ligands were intended for the construction of neutral
and cationic p‐cymene Ru(II) half‐sandwich complexes (Chart
1). Complexes of type [Ru(p‐Cym)(
L
)Cl]Cl, containing a
, were synthesized
chelating bipyridine ligand, L = , and 8
2–5
Scheme 2. Synthesis of complexes [Ru(p‐Cym)(
Cym)( )Cl2] (yield 89%), [Ru(p‐Cym)( )2Cl]PF6 (yield
Cym)( )2Cl]PF6. (yield = 45%). i) Acetone, reflux, 15 h. ii) AgPF6 (1 equiv.),
acetone/methanol (1/1), 1 h. iii) 4‐phenylazopyridine (1 equiv.), 15 h.
1
)Cl2] (yield = 71%), [Ru(p‐
following the standard methodology described for complexes
of type [Ru(p‐Cym)(bipy)Cl]Cl.53 Two equivalents of the
corresponding bipyridine and [Ru(p‐Cym)Cl2]2 were refluxed in
acetone to obtain the desired Ru(II) complexes in moderate to
good yields (54–93%).
The same synthetic route described above for the cationic
complexes was followed to obtain neutral complexes of
general formula [Ru(p‐Cym)LCl2] containing one monodentate
9
=
1
=
55%), and [Ru(p‐
9
In the case of [Ru(p‐Cym)(1)2Cl]PF6, a crystalline sample
suitable for X‐ray diffraction was obtained by slow diffusion of
Et2O on an acetone saturated solution of the compound
(Figure 1). As expected, in the molecular structure of [Ru(p‐
Cym)(1)2Cl]PF6, the metal centre adopts a slightly distorted
pseudo‐octahedral structure, and the azopyridine ligands are
in trans conformation and coordinating to the metal through
N (1 or 9) or P (6, 7 or 10) donor ligand (for the latter hexane
was used as solvent).53,54
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the pyridine nitrogen atoms. Each ligand adopts a nearly flat irradiated for 30 min at the wavelength attributed to the
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conformation (the angle between pyridine and benzene rings intense π→π* transition of the azobenDzOenI:e10(.1T0a3b9l/eC71D)T.0T05h4e2nC,
in the same ligand are less than 15 degrees), but the angle the solutions were irradiated for additional 30 min at the
between both ligand planes is 67 degrees. This situation is optimal wavelength individually calculated for each
quite different from the one expected for the chelate analogue compound, following Monkowius' procedure, as described
[Ru(p‐Cym)(
supposed to be co‐planar, forced by the bipyridine fragment, and after irradiation are shown in Figure S1 in the ESI. Once
and with extended conjugation through the whole bis‐ the photo stationary state (PSS) was reached, the back
azopyridine ligand isomerization was monitored by UV‐vis absorption
2
)Cl]Cl in which both azopyridine‐fragments are elsewhere.52 The spectra of all the complexes studied before
Z→E
2.
All the new compounds were fully characterized by EA, NMR, spectroscopy registering spectra at regular time‐intervals until
UV‐vis and ESI‐MS (full spectra and detailed synthetic the original spectrum was recovered (this process was run in
procedures are compiled in the ESI). For pyridine and the dark at 65 °C). The value of absorbance at λ(π→π*) was
bipyridine derivatives only one set of signals was observed in used to follow this process, and to calculate the first order rate
the corresponding 1H‐NMR spectra. Since some of the constants (k) and half‐life times (τ1/2) for this process. The
complexes contain more than one azobenzene fragment, this results obtained are presented in Table 1 (all absorbance
observation suggests that the azobenzene groups are in the versus time and first order plots obtained are compiled in the
thermodynamically most stable trans
(E
) form. For compounds ESI or published elsewhere).3
[Ru(p‐Cym)( )Cl2] and [Ru(p‐Cym)(
6
7
)Cl2], the 31P‐NMR spectra
Table 1. Kinetic data for the
Z
→E
isomerization process in CH3CN.
present an intense singlet in the region of coordinated
phosphines at (26–28 ppm) and a smaller peak in close
proximity, indicating that more than one species are present.
The most intense peak was assigned to the complex containing
all the azobenzene fragments in trans conformation (EEE),
being the thermodynamically most stable isomer. The small
peak should correspond to one of the other three possible
a
Compound
λazo π→π*
λoptimal
k
τ1/2
[nm]
[nm]
[s−1]
[min]
1b
2b
4b
5b
6c
7c
317
309
334
314
345
320
312
332
341
338
309
320
328
321
312
325
337
320
354
323
311
316
350
344
322
347
334
324
4.0 x 10−4
4.0 x 10−4
9.0 x 10−5
5.0 x 10−5
1.0 x 10−4
7.0 x 10−5
5.0 x 10−5
29
29
128
231
115
165
231
isomers, EEZ
The intensity of the small signal represents a 25% for [Ru(p‐
Cym) Cl2] and is nearly undetectable for [Ru(p‐Cym) Cl2]. The
presence of the minor isomer was also observed in the H‐
NMR spectra of [Ru(p‐Cym) Cl2]. These observations indicate
, EZZ and ZZZ (being EEZ the most probable one).
7
6
1
c
[Ru(p‐Cym)(1)2Cl]PF6
[Ru(p‐Cym)2Cl]Clc
[Ru(p‐Cym)3Cl]Clc
[Ru(p‐Cym)4Cl]Clc
[Ru(p‐Cym)5Cl]Clc
[Ru(p‐Cym)1Cl2]c
[Ru(p‐Cym)6Cl2]c
[Ru(p‐Cym)7Cl2]c
–
–
d
d
7
2.0 x 10−4
3.0 x 10−4
7.0 x 10−5
5.0 x 10−5
1.0 x 10−4
8.0 x 10−5
58
that also in these complexes most of the azobenzenes are in
trans conformation. In agreement with these observations,
UV‐vis spectra of all azobenzene‐containing compounds
showed the intense band at 300–350 nm assigned to the
characteristic π→π* transition of trans‐azobenzene (Table 1).
38
165
231
115
144
b
c
d
a Optimized light‐wavelength for the
E
→Z
photoisomerization. 55 °C. 65 °C.
The reverse process could not be analyzed due to the low degree of
photoisomerization observed for this compound. Ligand
its low solubility in all the solvents assayed.
3 was not measured due to
Although all the ligands experienced a large extent of trans‐to‐
cis photoisomerization (according to the difference in their UV‐
vis spectra), they showed a different behaviour when
coordinated to Ru(II). Irradiation of [Ru(p‐Cym)(
[Ru(p‐Cym)( )Cl]Cl produced a minimal change in their UV‐vis
spectra (Figure S1 in ESI). Therefore, an inhibition of the
isomerization upon coordination should be inferred in these
compounds. It is worth mentioning that precisely these two
ligands showed, before metal coordination, the shortest half‐
1)Cl2] and
2
Figure 1. ORTEP representation of the molecular structure of cation [Ru(p‐
Cym)(1
)2Cl]+ according to X‐ray diffraction. Hydrogens have been omitted for
clarity. Ellipsoids at 20% probability. Representative distances (Å) and angles (°:
Ru1–N1, 2.120(2); Ru1–N1A, 2.155(2); Ru1–Cl1, 2.3922(8); Ru1–C7, 2.193(3);
Ru1–C7A, 2.182(3); Ru1–C8, 2.183(3); Ru1–C8A, 2.165(3); Ru1–C9, 2.210(3);
Ru1–C10, 2.214(3); dihedral planes (N1–C6)∙(C14–C19), 14.95; (N1A–
C6A)∙(C14A–C19A), 10.55; (N1–C6)∙(N1A–C6A), 66.61
life times of the cis form (less stable
Z isomers) (see Table 1),
Photoisomerization studies
which combined with the fact that in these ligands the
azobenzene presents the strongest electronic communication
to the metal centre (occupying directly a para position of the
coordinated pyridine) could be responsible of the lack of
The light‐induced isomerization of the azobenzene‐containing
ligands and their complexes was studied by UV‐vis absorption
spectroscopy. Diluted CH3CN solutions of the compounds were
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1)2Cl]PF6 (the each complex the reaction was run with and without
photoisomerization. Surprisingly, [Ru(p‐Cym)(
non‐chelate analogue of [Ru(p‐Cym)(2
)Cl]Cl) shows some continuous irradiation (immersion lampD1O2I:51W0.1,03396/5C7nDmT0)0.5T4h2Ce
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degree of photoisomerization. In the rest of compounds experimental setup and detailed procedure is described in the
studied a clear effect of the irradiation is reflected in their UV‐ ESI.
vis spectra. This fact is remarkable, taking into account that in The substrate (0.69 mmol) and the precatalyst (0.5 mol%)
most of the examples of the literature, coordination of an were mixed for 5–10 min in 0.375 mL of freshly distilled THF in
azobenzene‐appended ligand to a transition metal centre a closed reaction vessel. The addition of 1.125 mL of H2O to
quenches this process (which is more evident when the reaction mixture was considered the starting time of the
conjugation exists between the donor atom and the reaction.
4,56,57,58
azobenzene).
The data obtained showed that the cis
)Cl]Cl confirm that light‐induced AB cleavage and hydrolysis of the
)Cl]Cl, the released BH3 was not competing with the catalyzed process
former being only slightly slower. The first order plot obtained (Figure S3 in ESI). The slight slope observed in the pressure‐
in both cases (as well as in the corresponding free ligands evolution curve observed should be attributed to an
and ), together with the presence of isosbestic points in the unavoidable temperature increase due to the continuous
series of UV‐vis spectra acquired during the process, and the irradiation (~4 °C).‡ Then, reactions using the model compound
large degree of isomerization observed for the free ligands, [Ru(p‐Cym)( )Cl]Cl, with and without irradiation, were run to
‐
Initially a blank reaction (without catalysts) was performed to
to‐trans isomerization rate of complex [Ru(p‐Cym)(
3
showed similar values as the one of [Ru(p‐Cym)(
4
3
4
8
indicated that in these ligands and their derivatives both determine the difference attributable to experimental errors
azobenzene units behave as independent molecular switches due to the heating of the reaction mixture due to irradiation
(the isomerization of the first one did not influence the (Figure S3 in ESI). Both reaction profiles were considered
isomerization of the second). This behaviour has been also comparable within the experimental error.
observed in their Ir(III) bis‐cyclometalated complexes,50 and it Initially, the influence of the irradiation on the catalytic activity
is common for molecules containing more than one of cationic complexes type [Ru(p‐Cym)(L)Cl]Cl (L = 2x1, 2–5)
azobenzene unit, unless the fragment is part of a restrained was studied. The reaction profiles obtained for these
macrocyclic ring or the azobenzene units are conjugated with a precatalysts are shown in Figures S4 and S5 in the ESI. In the
specific substitution pattern.59‐63
case of [Ru(p‐Cym)(1)2Cl]Cl and the chelate analogue [Ru(p‐
Cym)( )Cl]Cl, the irradiation showed no effect in the reaction
2
The difference between complexes [Ru(p‐Cym)(3)Cl]Cl and
[Ru(p‐Cym)(5)Cl]Cl is only the coordination point of the
azobenzene to the bipyridine ligand. The latter, containing two
azobenzene fragments at meta positions with respect to the
profiles. This result was not surprising, since Uv‐vis
experiments already indicated that the azobenzene
isomerization was strongly inhibited in these derivatives (vide
supra).
anchoring carbon, showed one order magnitude slower Z→E
The effect of irradiation on the catalytic activity was more
isomerization than the former, in which the azobenzene
fragments are in para positions. The same trend was already
observed for Ir(III) complexes containing the same ligands.3
The most remarkable behaviour was the one observed for the
significant for complexes [Ru(
Cym)( )Cl]Cl and [Ru( ‐Cym)( )Cl]Cl, the systems under
irradiation being more active.
The activity of neutral complexes containing only one ligand
or , and the effect of the irradiation on their activity was
also studied (the reaction profiles are shown in Figures S5 and
S8 in the ESI). In the case of the complex [Ru( ‐Cym)( )Cl2],
p‐Cym)(3)Cl]Cl, [Ru(p‐
4
p
5
1,
phosphine‐containing complexes [Ru(p‐Cym)(6)Cl2]Cl and
6
7
[Ru(p‐Cym)( )Cl2]Cl, which exhibited a large degree of
7
photoisomerization compared to pyridine or bipyridine
derivatives (according to the difference in their UV‐spectra
before and after irradiation). In addition to this, very similar k
values for the reverse process to the ones obtained for free
p
1
containing only one 4‐phenylazopyridine ligand, sigmoidal‐
shaped reaction profiles were obtained in both cases (with and
without irradiation), although it was clearly more pronounced
in absence of irradiation. The small extent of the azobenzene
photoisomerization observed for this compound by UV‐vis
spectroscopy makes it unlikely that the difference in these
profiles can be attributed to the presence of this azo‐fragment.
ligands
latter!). Comparing both compounds, [Ru(p‐Cym)(
containing three azobenzene groups in meta positions,
showed a slower isomerization process than the analogue
6
and
7
were obtained (identical in the case of the
7)Cl2]Cl
Z→E
with three azobenzene fragments appended through the para
position, although the difference is only marginal. Also, in both
cases, linear plots of the reverse process (see figures S64 and
S72 ESI) and the presence of isosbestic points in the UV‐vis
spectra acquired during the cis‐to‐trans isomerization suggest
that in these compounds the three azobenzene fragments
behave as independent molecular switches.
This was confirmed by studying model compound [Ru(
Cym)( )Cl2], containing a non‐substituted pyridine ligand, for
which similar results were obtained (Figure S6 in the ESI). The
similarity between the reaction profiles obtained using [Ru(
Cym)( )Cl2] and [Ru( ‐Cym)( )Cl2] indicates the need of an
p‐
9
p
‐
1
p
9
activation period; the fact that this activation can be
accelerated by light is inherent to the nature of the compound
(i.e. [Ru(p‐Cym)(pyridine)Cl2]). Similar S‐shaped curves have
been reported before for the dehydrocoupling of AB using
heterogeneous catalysts.17, 18, 64 To test if formation of active
Ru‐nanoparticles was responsible of the observed profile, a
Catalytic experiments.
In order to analyze the influence of the irradiation on the
catalytic activity, all Ru(II) complexes synthesized were studied
as precatalysts for the hydrolytic dehydrogenation of AB. For
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mercury‐test to confirm the homogeneity of the catalytic periods were performed with compounds [Ru(
ARTICLE
Vi‐eCwyAmrtic)l(e O)nCliln2e]
p
6
DOI: 10.1039/C7DT00542C
system was performed using ([Ru(p‐Cym)(9)Cl2]. At 50% of and [Ru(p‐Cym)(7)Cl2] (Figure 2). The reaction profiles
conversion ~1000 equivalents of Hg were added. This addition obtained clearly showed alternating slopes (reaction rates)
did not affect the reaction profile, confirming the homogeneity that were consistent with the slopes corresponding to
of the catalytic reaction.
irradiation and no‐irradiation experiments.
In view of these results we considered the possibility that From these results it is clear that the different activities
these profiles were due to light‐induced Ru‐pyridine bond observed upon light irradiation/no irradiation periods are due
cleavage, rendering an active form of the catalyst. This light‐ to a change in nature of the catalytically active species.
induced pyridine extrusion on Ru(II) arene complexes of type As mentioned before, there are no mechanistic studies on the
[Ru(p
‐Cym)(bipy)(py)]2+ is a well documented process.65‐67 homogeneously catalyzed solvolytic decomposition of AB. By
Eventually, the displacement of p‐cymene was also observed in analogy with the related dehydrocoupling reaction, it is
a complex of this family upon continuous irradiation.68 To supposed that it starts by an activation of AB through
analyze this hypothesis, two additional catalytic experiments coordination to the metal (presumably through its hydric B‐H
were carried out using [Ru(p‐Cym)(1
)Cl2]. In these experiments hydrogen).15 Steric differences resulting from trans‐to‐cis
irradiation and non‐irradiation periods were combined (Figure isomerization upon irradiation would be minimal for 4,4’‐
S7 in ESI). The obtained reaction profiles showed that if the substituted bipyridine ligands, due to the long distance
catalytic process was not irradiated initially and the lamp was between the azobenzene group and the putative active site of
switched on after the activation period (light‐green line), the the compound (created upon release of the coordinated
results were identical to those obtained when the process was chlorine). In the case of the azobenzene‐phosphine derivatives
not irradiated at all (dark green line). This result suggests that [Ru(p‐Cym)(
irradiation after the activation period was not producing any change in the shape of the catalytic pocket could be inferred
effect on the catalytic system. It is worth noting that after 30 from the predictable change in the cone angle of and upon
6)Cl2]Cl and [Ru(p‐Cym)(7)Cl2]Cl, a more important
6
7
minutes irradiating the lamp was switched off again. As irradiation.2 Nevertheless, if coordination of AB to the metal
described before, if the reaction was irradiated initially nearly center was RDS, the opposite trend would be expected.
no activation period was observed (red lines). If at 30 % As mentioned before, recently, we studied a family of
conversion the lamp was switched off, (light‐red line) the most complexes of the type [Ru(p‐Cym)(bipy*)Cl2]Cl (bipy* = 4,4’‐
effective results from the series of experiments were obtained. disubstituted bipyridine) as precatalysts for this reaction. The
Using this on/off procedure, the activation period time was results obtained revealed the importance of ligand electronic
shortened and the activity of the catalytic system did not modifications on the catalytic activity (being accelerated when
suffer deactivation along the reaction course as was observed EWG groups were introduced in such positions and strongly
51
for the system under continuous irradiation.
inhibited when EDG were used).50,
These results are in
According to these results, irradiation favours the initial agreement with the lower activities obtained when
activation of these systems, probably through dissociation of a azobenzene‐substituted bipyridines were used as ligands,
pyridine ligand, but a continuous irradiation is detrimental. We compared to pristine 2,2’‐bipyridine. Due to the electronic
suggest that, eventually, longer irradiation periods could changes experienced by azobenzene upon isomerization, this
promote the release of
complex, and prompt catalyst decomposition. The influence of observed upon azobenzene isomerization in the case of
the irradiation on the catalytic activity for the hydrolytic derivatives of ligands . Whether in the case of tris‐
dehydrogenation of AB using precatalysts [Ru( ‐Cym)( )Cl2] azophosphine derivatives the origin of the different activity is
and [Ru( ‐Cym)( )Cl2], incorporating azobenzene‐containing also an electronic effect or the existence of a later RDS is
p‐cymene from the ruthenium results are in agreement with the slightly faster activities
3‐5
p
6
p
7
triarylphosphines, was also studied. Both complexes showed currently under study. Nevertheless, lacking a detailed
much higher activity under continuous irradiation compared to description of the reaction mechanism for each system it is
the non‐irradiated process (Figure 2). These results are in difficult to give a definite explanation on the origin of the
accordance with the large extent of azobenzene different activities observed.
photoisomerization observed for these compounds (vide
supra).
To confirm that the origin of the higher activity of this
precatalysts was the isomerization of the azobenzene
fragments and not the dissociation of the phosphine ligand (as
postulated for the pyridine derivatives), the catalytic activity of
the model precatalyst [Ru(p‐Cym)(10)Cl2] was studied. The
reaction profiles obtained with this precatalysts (Fig S8 in ESI)
were unaffected by irradiation, which confirms that
azobenzene isomerization was responsible for the different
activities obtained with derivatives of ligands 6 and 7.
In order to test the reversibility of the isomerization process,
additional reactions combining irradiation/no‐irradiation
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electronically monitor the variation of prVeieswsuArrtiecle Oanlninde
DOI: 10.1039/C7DT00542C
temperature of the gas phase inside a closed quartz reactor.
UV−vis absorption measurements were performed with an
Agilent 8453 diode‐array spectrophotometer utilizing 10 mm
cell‐path quartz cuvettes (110 QS).
Synthetic procedures
Tris(p‐azobenzene)phosphane (6). 4‐iodoazobenzene (1.5 g,
4.87 mmol) was azeotropically dried with toluene under a N2
atmosphere and dissolved in 30 mL of freshly distilled THF. The
solution was cooled to ‐80 °C, n‐BuLi 1.6 M in hexanes (4.5 mL,
7.3 mmol) were added and it was stirred for 30 min. PCl3 (142
μL, 1.63 mmol) were added, the reaction temperature was
gradually raised up to room temperature and it was stirred
overnight. The solvent was evaporated, the residue was
washed with EtOH and the product was obtained as an orange
solid. Yield 10%. Elemental Analysis: calculated for
(C36H27N6P∙EtOH): C, 73.53; H, 5.36; N, 13.54. Found: C, 73.88;
H, 4.98; N, 13.85. Exact Mass: ESI‐MS [C36H27N6P + H]+:
1
calculated: m/z= 575.2113, found: m/z= 575.2123. H NMR
(300 MHz, CDCl3): δ 7.88–7.79 (m, 12H), 7.49–7.38 (m, 15H).
13C APT NMR (75 MHz, CDCl3): δ 152.51 (s, 3Cquat), 152.22 (s,
3Cquat), 139.42 (d, J = 12.7 Hz, 3Cquat), 134.09 (d, J = 20.2 Hz,
6CH), 130.86 (s, 3CH), 128.68 (s, 6CH), 122.54 (s, 6CH), 122.50
(d, J = 6.0 Hz, 6CH). 31P NMR (202.5 MHz, CDCl3): δ ‐3.69 (s,
1P).
[Ru(p‐Cym)(1)(Cl)2]. Under a N2 atmosphere, Ru2(p‐Cym)2Cl4
(0.2 g, 0.32 mmol) and (4‐phenylazopyridine) (0.117 g, 0.64
mmol) were dissolved in 20 mL of acetone. The reaction
mixture was refluxed for 15 h. It was cooled to room
temperature, the solvent was evaporated and the product was
obtained as an orange solid. Yield 71%. Elemental Analysis:
calculated for (C21H23Cl2N3Ru): C, 51.54; H, 4.74; N, 8.59.
Found: C, 51.51; H, 4.71; N, 8.55. Exact Mass: ESI‐MS
[C21H23ClN3Ru]+ (M‐Cl): calculated: m/z= 454.0619, found:
m/z= 454.0618. 1H NMR (300 MHz, CDCl3): δ 9.26 (d, J = 6.9 Hz,
2H), 8.06–7.96 (m, 2H), 7.75 (d, J = 6.7 Hz, 2H), 7.64–7.55 (m,
3H), 5.52 (d, J = 5.7 Hz, 2H), 5.30 (d, J = 5.8 Hz, 2H), 3.06 (sep, J
= 6.9 Hz, 1H), 2.17 (s, 3H), 1.37 (d, J = 6.9 Hz, 6H). 13C APT NMR
(75 MHz, CDCl3): δ 157.12 (Cquat), 155.93 (2CH), 151.76 (Cquat),
132.78 (CH), 128.92 (2CH), 123.35 (2CH), 116.50 (2CH), 103.26
(Cquat), 96.88 (Cquat), 82.58 (2CH), 81.88 (2CH), 30.24 (CH),
21.86 (2CH3), 17.83 (CH3).
Figure 2. Reaction profiles, conversion (%) vs. time (min). THF/H2O = 1/3 (v/v),
[H3N∙BH3] = 2.3 mM, precatalyst 0.5 mol%, using precatalysts [Ru(p‐Cym)(
7)Cl]Cl
(top) and [Ru(p‐Cym)( )Cl]Cl (bottom). Reaction with continuous irradiation
6
(purple line), without irradiation (dark grey line), and alternating irradiation
(white squares) / no irradiation (grey squares) periods, orange line.
Experimental
General considerations
[Ru(pCym)Cl2]2, 2,2'‐bipyridine, ammonia‐borane and other
general chemicals were obtained from commercial sources
and used without further purification.
4‐phenylazopyridine,69 2,2'‐bis(4‐phenylazopyridine), 4,4'‐
bis(
p
‐azobenzene)‐2,2'‐bipyridine, 4‐(
p
‐azobenzene)‐4'‐bromo‐
[Ru(p‐Cym)(1)2(Cl)]PF6. Under a N2 atmosphere, [Ru(p‐Cym)(4‐
phenylazopyridine)(Cl2)] (0.319 g, 0.65 mmol) and AgPF6 (0.160
g, 0.63 mmol) were dissolved in 20 mL of acetone and 20 mL of
methanol. The mixture was stirred for 1 h, AgCl was removed
by filtration and 4‐phenylazopyridine (0.119 g, 0.65 mmol)
were added. The reaction mixture was stirred for 15 h and the
solvent was evaporated. The product was obtained as a red
solid after precipitation with acetone/ether. Yield 55%.
Elemental Analysis: calculated for (C32H32ClN6RuPF6): C, 49.14;
H, 4.12; N, 10.75. Found: C, 49.23; H, 4.01; N, 10.59. Exact
Mass: ESI‐MS [C21H23ClN3Ru]+ (M‐L‐PF6): calculated: m/z=
454.0619, found: m/z= 454.0618. 1H NMR (300 MHz, CDCl3): δ
9.27 (d, J = 6.9 Hz, 4H), 7.97 (dd, J = 1.6 Hz, J = 7.5 Hz, 4H), 7.87
2,2'‐bipyridine, 4,4'‐bis(
m
‐azobenzene)‐2,2'‐bipyridine,3 tris(
m‐
azobenzene)phosphane,2 [Ru(p‐Cym)(2,2'‐bipyridine)(Cl)]Cl,53
[Ru(p‐Cym)(pyridine)(Cl)2],70 [Ru(p‐Cym)(PPh3)(Cl)2],54 were
synthesized according to published methodologies or slightly
modified.
All solvents were dried and purified by known procedures and
freshly distilled under nitrogen from appropriate drying agents
prior to use. All manipulations and reactions involving air
and/or moisture‐sensitive organometallic compounds were
performed under an atmosphere of dry nitrogen using
standard Schlenk techniques.
H2 evolution was measured using the kinetic kit (MOTM) series
X102 kit from manonthemoontech71 which permits to
6 | J. Name., 2012, 00, 1‐3
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(d, J = 6.9 Hz, 4H), 7.57 (brd, J = 7.3 Hz, 6H), 6.03 (d, J = 6.1 Hz, 1H), 2.17 (s, 3H), 0.99 (d, J = 6.8 Hz, 6H). 13C APT NMR (75 MHz,
View Article Online
DOI: 10.1039/C7DT00542C
2H), 5.78 (d, J = 6.1 Hz, 2H), 2.67 (sep, J = 6.9 Hz, 1H), 1.86 (s, MeOD d4): δ 157.24 (Cquat), 157.11 (CH), 156.98 (CH), 155.79
3H), 1.23 (d, J = 6.9 Hz, 6H). 13C APT NMR (75 MHz, CDCl3): δ (Cquat), 155.11 (Cquat), 153.96 (Cquat), 152.07 (Cquat), 138.53
157.66 (2Cquat), 155.29 (4CH), 151.74 (2Cquat), 133.09 (2CH), (Cquat), 138.05 (Cquat), 132.97 (CH), 132.11 (CH), 130.41 (2CH),
128.94 (4CH), 123.47 (4CH), 118.22 (4CH), 102.59 (Cquat), 129.85 (2CH), 128.83 (CH), 126.43 (CH), 124.77 (2CH), 124.08
101.80 (Cquat), 88.49 (2CH), 81.77 (2CH), 30.45 (CH), 21.84 (2CH), 122.93 (CH), 106.64 (Cquat), 105.62 (Cquat), 88.21 (CH),
(2CH3), 17.39 (CH3).
88.00 (CH), 85.87 (CH), 85.68 (CH), 32.41 (CH), 22.37 (2CH3),
[Ru(p‐Cym)(2)(Cl)]Cl. Under a N2 atmosphere, Ru2(p‐Cym)2Cl4 18.97 (CH3).
(0.1 g, 0.16 mmol) and 2,2'‐bis(4‐phenylazopyridine) (0.119 g, [Ru(p‐Cym)(5)(Cl)]Cl. Under a N2 atmosphere, Ru2(p‐Cym)2Cl4
0.326 mmol) were dissolved in 10 mL of acetone. The reaction (0.100 g, 0.163 mmol) and 4,4'‐bis(m‐azobenzene)‐2,2’‐
mixture was refluxed for 15 h. It was cooled to room bipyridine (0.168 g, 0.326 mmol) were dissolved in 10 mL of
temperature and the solid was filtered. The desired compound acetone. The reaction mixture was refluxed for 15 h and ,4‐4'‐
was obtained after precipitated with CH2Cl2/Et2O as a dark red bis(m‐azobenzene)‐2,2’‐bipyridine (0.168 g, 0.326 mmol) were
solid. Yield 73%. Elemental Analysis: calculated for added. It was refluxed for another 15 h. It was cooled to room
(C32H30Cl2N6Ru∙CH2Cl2): C, 52.46; H, 4.27; N, 11.12. Found: C, temperature and the solid was filtered. The desired compound
52.33; H, 4.30; N, 11.09. Exact Mass: ESI‐MS [C32H30ClN6Ru]+: was obtained after precipitated with CH2Cl2/Et2O as a brown
1
calculated: m/z= 635.1264, found: m/z= 635.1282. H NMR solid. Yield 54%. Elemental Analysis: calculated for
(300 MHz, MeOD‐d4): δ 9.69 (d, J = 6.1 Hz, 2H), 8.98 (s, 2H), (C44H38Cl2N6Ru∙CH2Cl2): C, 59.54; H, 4.44; N, 9.26. Found: C,
8.19–8.04 (m, 6H), 7.77–7.61 (m, 6H), 6.24 (d, J = 6.2 Hz, 2H), 58.94; H, 4.43; N, 9.03. Exact Mass: ESI‐MS [C44H38ClN6Ru]+:
1
6.00 (d, J = 6.2 Hz, 2H), 2.75 (sep, J = 6.8 Hz, 1H), 2.34 (s, 3H), calculated: m/z= 787.1890, found: m/z= 787.1920. H NMR
1.13 (d, J = 6.9 Hz, 6H). 13C APT NMR (75 MHz, MeOD‐d4): δ (300 MHz, CDCl3): δ 9.98 (s, 2H), 8.51 (s, 2H), 8.26 (s, 2H),
160.37 (2Cquat), 158.42 (2CH), 157.91 (2Cquat), 153.73 (2Cquat), 8.13–7.86 (m, 10H), 7.71 (t, J = 7.2 Hz, 2H), 7.55–7.45 (m, 6H),
135.13 (2CH), 130.76 (4CH), 125.16 (4CH), 120.49 (2CH), 6.38 (s, 2H), 6.23 (s, 2H), 2.79 (m, 1H), 2.32 (s, 3H), 1.12 (d, J =
118.74 (2CH), 107.02 (Cquat), 106.04(Cquat), 88.47 (2CH), 86.13 6.4 Hz, 6H). 13C APT NMR (75 MHz, CDCl3): δ 156.88 (2CH),
(2CH), 32.34 (CH), 22.38 (2CH3), 19.02 (CH3).
[Ru(p‐Cym)(3)(Cl)]Cl. Under a N2 atmosphere, Ru2(p‐Cym)2Cl4 135.76 (2Cquat), 131.19 (2CH), 130.10 (2CH), 129.50 (2CH),
(0.100 g, 0.163 mmol) and 4,4'‐bis( ‐azobenzene)‐2,2’‐ 128.75 (4CH), 125.47 (2CH), 124.69 (2CH), 122.61 (4CH),
154.36 (2Cquat), 152.64 (2Cquat), 151.84 (2Cquat), 150.30 (2Cquat),
p
bipyridine (0.168 g, 0.326 mmol) were dissolved in 10 mL of 121.02 (2CH), 120.24 (2CH), 104.47 (Cquat), 103.84 (Cquat), 87.18
acetone. The reaction mixture was refluxed for 15 h. It was (2CH), 84.50 (2CH), 30.76 (CH), 21.86 (2CH3), 18.72 (CH3).
cooled to room temperature and the dark red solid was [Ru(p‐Cym)(6)(Cl)2]. Under a N2 atmosphere, Ru2(p‐Cym)2Cl4
filtered. Yield 70%. Elemental Analysis: calculated for (0.11 g, 0.17 mmol) and tris(p‐phenylazobenzene)phosphine
(C44H38Cl2N6Ru): C, 64.23; H, 4.66; N, 10.21. Found: C, 63.79; H, (0.2 g, 0.348 mmol) were dissolved in 15 mL of hexane. The
4.70; N, 10.13. Exact Mass: ESI‐MS [C44H38ClN6Ru]+: calculated: reaction mixture was refluxed for 15 h. It was cooled to room
1
m/z= 787.1890, found: m/z= 787.1917. H NMR (300 MHz, temperature and the red solid was filtered. Yield 95%.
CDCl3): δ 9.88 (brd, J = 5.5 Hz, 2H), 8.62 (s, 2H), 8.00 (m, 10H), Elemental Analysis: calculated for (C46H41Cl2N6PRu): C, 62.73;
7.91 (brd, J = 7.9 Hz, 4H), 7.58–7.45 (m, 6H), 6.29 (d, J = 5.6 Hz, H, 4.69; N, 9.54. Found: C, 62.93; H, 4.90; N, 9.36. Exact Mass:
2H), 6.16 (d, J = 5.4 Hz, 2H), 2.72 (sep, J = 6.8 Hz, 1H), 2.28 (s, ESI‐MS [M‐2Cl+H]: calculated: m/z= 811.2252, found: m/z=
3H), 1.06 (d, J = 6.8 Hz, 6H). 13C APT NMR (75 MHz, CDCl3): δ 811.2229. 1H NMR (300 MHz, CDCl3): δ 7.97 (t, J = 8.5 Hz, 6H),
156.48 (2CH), 154.32 (2Cquat), 153.08 (2Cquat), 151.98 (2Cquat), 7.82 (m, 12H), 7.42 (m, 9H), 5.21 (d, J = 6.1 Hz, 2H), 4.98 (d, J =
149.70 (2Cquat), 136.44 (2Cquat), 131.20(2CH), 128.70 (4CH), 5.6 Hz, 2H), 2.83 (sep, J = 6.9 Hz, 1H), 1.85 (s, 3H), 1.06 (d, J =
128.11 (4CH), 125.02 (2CH), 123.35 (4CH), 122.66 (4CH), 6.9 Hz, 6H). 13C APT NMR (75 MHz, CDCl3): δ 153.58 (s, 3Cquat),
120.27 (2CH), 104.46 (Cquat), 103.47 (Cquat), 86.87 (2CH), 84.38 152.76 (s, 3Cquat), 136.22 (d, J = 45 Hz, 3Cquat), 135.47 (d, J = 9.7
(2CH), 30.66 (CH), 21.79 (2CH3) 18.59 (CH3).
Hz, 6CH), 131.78 (s, 3CH), 129.33 (s, 6CH), 123.27 (s, 6CH),
[Ru(p‐Cym)(4)(Cl)]Cl. Under a N2 atmosphere, Ru2(p‐Cym)2Cl4 122.47 (d, J = 10.5 Hz, 6CH), 112.07 (s, Cquat), 96.65 (s, Cquat),
(0.100 g, 0.163 mmol) and 4‐(p‐azobenzene)‐4'‐bromo‐2,2’‐ 89.37 (brd, J = 2.2 Hz, 2CH), 87.60 (d, J = 5.2 Hz, 2CH), 30.63 (s,
bipyridine (0.135 g, 0.326 mmol) were dissolved in 10 mL of CH), 22.10 (s, 2CH3), 18.13 (s, CH3). 31P NMR (202 MHz, CDCl3):
acetone. The reaction mixture was refluxed for 15 h. It was δ 25.79 (s, 1P).
cooled to room temperature and the solid was filtered. The [Ru(p‐Cym)(7)(Cl)2]. Under a N2 atmosphere, Ru2(p‐Cym)2Cl4
desired compound was obtained after precipitated with (0.100
g,
0.163
mmol)
and
tris(m‐
CH2Cl2/Et2O as an orange solid Yield 65%. Elemental Analysis: phenylazobenzene)phosphine (0.206 g, 0.359 mmol) were
calculated for (C32H29BrCl2N4Ru∙CH2Cl2): C, 49.15; H, 3.87; N, dissolved in 27 mL of hexane. The reaction mixture was
6.95. Found: C, 49.04; H, 3.80; N, 6.77. Exact Mass: ESI‐MS refluxed for 15 h. It was cooled to room temperature and the
[C32H29BrClN4Ru]+: calculated: m/z= 685.0308, found: m/z= solvent was evaporated. The desired compound was obtained
685.0334. 1H NMR (300 MHz, MeOD d4): δ 9.39 (d, J = 6.0 Hz, after precipitation with CH2Cl2/ether as a red solid. Yield 90%.
1H), 9.20 (d, J = 6.1 Hz, 1H), 8.92 (brd, J = 1.8 Hz, 1H), 8.76 (brd, Elemental Analysis: calculated for (C46H41Cl2N6PRu): C, 62.73;
J = 1.5 Hz, 1H), 8.12–7.94 (m, 5H), 7.93–7.80 (m, 3H), 7.52– H, 4.69; N, 9.54. Found: C, 62.72; H, 4.96; N, 9.28. Exact Mass:
7.40 (m, 3H), 6.04 (m, 2H), 5.80 (m, 2H), 2.59 (sep, J = 6.9 Hz, ESI‐MS [M‐2Cl+H]: calculated: m/z= 811.2252, found: m/z=
This journal is © The Royal Society of Chemistry 20xx
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1
811.2248. H NMR (300 MHz, CDCl3): δ 8.48 (d, J = 11.0 Hz, of this process was reflected in their catalytic activity for the
View Article Online
DOI: 10.1039/C7DT00542C
3H), 8.11 (t, J = 8.7 Hz, 3H), 7.90 (d, J = 7.0 Hz, 3H), 7.79 (m, hydrolytic dehydrogenation of AB.
6H), 7.48 (ddd, J = 2.5 Hz, J = 7.7 Hz, J = 10.2 Hz, 3H), 7.38 (m, Cationic Ru(II) complexes containing the azopyridine fragment
9H), 5.27 (d, J = 6.1 Hz, 2H), 5.09 (d, J = 5.6 Hz, 2H), 2.81 (sep, J (ligands
= 6.9 Hz, 1H), 1.88 (s, 3H), 1.04 (d, J = 6.9 Hz, 6H). 13C APT NMR irradiation in the catalytic activities.
(75 MHz, CDCl3): δ 151.94 (s, Cquat), 151.57 (d, J = 9.7 Hz, Cquat), The neutral complex [Ru(p‐Cym)(
1 and 2) did not exhibit significant differences upon
1)Cl2] showed a peculiar S‐
136.70 (d, J = 9.7 Hz, 3CH), 134.33 (d, J = 45.0 Hz, Cquat), 130.89 shaped reaction profile, less evident under irradiation
(s, 3CH), 128.98 (s, 3CH), 128.85 (d, J = 4.5 Hz, 3CH), 128.59 (s, conditions. Additional experiments with model compound
6CH), 123.44 (s, 3CH), 122.57(s, 6CH), 110.91 (s, Cquat), 96.54 (s, [Ru(p‐Cym)(
quat), 88.63 (s, 2CH), 87.07 (d, J = 5.2 Hz, 2CH), 29.87 (s, CH), activation period that was shortened with irradiation, probably
21.54 (s, 2CH3), 17.42 (s, CH3). 31P NMR (202 MHz, CDCl3): δ due to light‐induced ligand dissociation.
9)Cl2] showed that these precatalysts required an
C
28.18 (s, 1P).
The
complexes
incorporating
azobenzene‐containing
[Ru(p‐Cym)(9)2(Cl)]PF6. Under
a N2 atmosphere, [Ru(p‐ triarylphosphines showed the most significant changes in their
Cym)(pyridine)(Cl2)] (0.218 g, 0.566 mmol) and AgPF6 (0.142 g, catalytic activities upon irradiation, in accordance with results
0.566 mmol) were dissolved in 15 mL of acetone and 15 mL of obtained for the photoisomerization studies; these complexes
methanol. The mixture was stirred for 1 h, AgCl was removed showed important changes in their absorption spectra through
by filtration and pyridine (0.05 mL, 0.623 mmol) were added. irradiation and presented longer half‐life time of the cis form.
The reaction mixture was stirred for 4 h and the solvent was To confirm that the improved catalytic activity observed for
evaporated. The product was obtained as a yellow solid after these complexes under irradiation was due to
precipitation with CH2Cl2/ether. Yield 45%. Elemental Analysis: photoisomerization of the azobenzene fragment, complex
calculated for (C20H24ClN2RuPF6∙CH2Cl2): C, 38.28; H, 3.98; N, [Ru(p‐Cym)(10)Cl2] was synthesized and used as precatalyst for
4.25. Found: C, 38.14; H, 4.00; N, 4.21. Exact Mass: ESI‐MS the hydrolytic dehydrogenation of AB. The irradiation did not
[C15H19ClNRu]+ (M‐L‐PF6): calculated: m/z= 350.0250, found: affect its catalytic activity, proving that the catalytic activity of
m/z= 350.0243. 1H NMR (300 MHz, CDCl3): δ 9.09 (brdd, J = 1.6 [Ru(p‐Cym)(
6)Cl2] and [Ru(p‐Cym)(7)Cl2] was improved
Hz, J = 6.6 Hz, 4H), 7.87 (tt, J = 1.5 Hz, J = 7.6 Hz, 2H), 7.54–7.47 because of the isomerization of the azobenzenes, and light‐
(m, 4H), 5.94 (d, J = 6.1 Hz, 2H), 5.66 (d, J = 6.1 Hz, 2H), 2.59 induced displacement of the PPh3 ligand was discarded. An
(sep, J = 6.9 Hz, 1H), 1.76 (s, 3H), 1.19 (d, J = 6.9 Hz, 6H). 13C additional experiment was carried out combining irradiation
NMR (75 MHz, CDCl3): δ 154.14 (4CH), 139.03 (2CH), 126.23 and no irradiation periods, confirming this hypothesis and the
(4CH), 102.72 (Cquat), 101.96 (Cquat), 88.71 (2CH), 81.97 (2CH), reversibility of the process. Consequently, these Ru(II) systems
30.83 (CH), 22.25 (2CH3), 17.68 (CH3).
containing azobenzene‐appended phosphine ligands behave as
photo‐controlled hydrogen generation catalysts.
Procedure for the solvolytic dehydrogenation of AB. Catalytic
reactions were carried out in a 35 mL quartz reactor immersed
in a water bath. For the non‐irradiated reactions, the bath was
thermostated at 25 °C by means of an external water‐
circulation jacket. For the reactions under irradiation, the
water bath that was thermostated at 21−25 °C by inserting
into it the irradiation lamp (refrigerated by a quartz‐made
cooling jacket) (see ESI for a detailed scheme). In both cases,
the reactor was connected to an electronic pressure
transducer (MOTM).71 0.69 mmol (21.3 mg) of AB and 0.5
mol% of the ruthenium precatalyst were stirred inside the
reactor in 0.375 mL of freshly distilled THF for 10 min. Addition
of 1.250 mL of distilled water to this mixture was considered
initial reaction time. For the irradiation periods the 125W (365
nm) immersion lamp was switched on. The evolution of the
reaction was monitored by electronic measurement of the
pressure inside the reactor, and converted to moles by using
the ideal gas law.
Acknowledgements
Financial support from Ikerbasque, Spanish MINECO
(CTQ2013‐23333), MINECO/FEDER (CTQ2015‐65268‐C2‐1‐P)
and UPV‐EHU (GIU13/06) is gratefully acknowledged. PvL
thanks the Université Fédérale Toulouse Midi‐Pyrénées for an
IDEX Chaire d´Attractivité. AT thanks ICIQ for a scholarship in
2014.
UPV‐EHU SGIker is acknowledged for technical assistance in
the NMR and HR‐MS analyses. Prof. M.A. Garralda is kindly
acknowledged.
Notes and references
‡ Our experimental setup permitted us to control the
temperature of the reaction in the range 23 ± 2.5 °C under
continuous irradiation and at 25 ± 0.5 °C without irradiation. The
reaction temperature without irradiation was deliberately set at
25 °C to guarantee that any catalytic rate improvement upon
irradiation was not attributable to temperature effects.
Conclusions
Eight new Ru(II) complexes with azobenzene‐containing
bipyridine, pyridine and phosphine ligands were synthesized
and fully characterized. The photoisomerization of these
complexes was studied by UV‐vis spectroscopy, and the extent
8 | J. Name., 2012, 00, 1‐3
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