New aqua N-heterocyclic carbene Ru(II) complexes with two-electron process as selective epoxidation catalysts: An evaluation of geometrical and electronic effects

Article abstract of DOI:10.1021/ic302863h

New ruthenium complexes with general formula [Ru^II(T)(CN-Me)X] ⁿ⁺ (X = Cl^- or H₂O; T = 2,2′:6′, 2″-terpyridine, trpy, or N,N-bis(2-pyridyl)ethylamine, bpea; CN-Me = N-methyl-N′-2-pyridylimidazolium) have been prepared. The complexes obtained have been characterized in solution by spectroscopic (1D- and 2D-NMR and UV-vis) techniques, mass spectrometry, and elemental analysis. The chloro complexes have also been characterized by X-ray diffraction analysis. The redox properties of all the compounds were studied by CV revealing, for the reported Ru-OH₂ complexes, bielectronic Ru(IV/II) redox processes throughout a wide pH range. The catalytic activity of aquo complexes was evaluated in the epoxidation of olefins using PhIO as oxidant, displaying in general good yields and high selectivities for the epoxide product. The influence of electronic and geometrical factors on the spectroscopic and electrochemical properties as well as on the catalytic activity is discussed.

Full text of DOI:10.1021/ic302863h

Article
pubs.acs.org/IC
New Aqua N‑Heterocyclic Carbene Ru(II) Complexes with Two-
Electron Process as Selective Epoxidation Catalysts: An Evaluation of
Geometrical and Electronic Effects
Mohamed Dakkach,†,‡ Ahmed Atlamsani,‡ Teodor Parella,§ Xavier Fontrodona,† Isabel Romero,*,†
and Montserrat Rodríguez*,†
†Departament de Química i Serveis Tec
‡Laboratoire des Mater
iaux et Systemes Interfaciaux, Dep
Morocco
§Departament de Química i Servei de RMN, Universitat Auton
̀
nics de Recerca, Universitat de Girona, Campus de Montilivi, E-17071 Girona, Spain
artement de Chimie, Faculte des Sciences, B.P. 2121, 93000 Tetouan,
́
̀
́
́
́
̀
̀
oma de Barcelona, Cerdanyola del Valles, E-08193 Barcelona, Spain
S
* Supporting Information
ABSTRACT: New ruthenium complexes with general for-
mula [RuII(T)(CN-Me)X]n+ (X = Cl− or H2O; T = 2,2′:6′,2″-
terpyridine, trpy, or N,N-bis(2-pyridyl)ethylamine, bpea; CN-
Me = N-methyl-N′-2-pyridylimidazolium) have been prepared.
The complexes obtained have been characterized in solution
by spectroscopic (1D- and 2D-NMR and UV−vis) techniques,
mass spectrometry, and elemental analysis. The chloro
complexes have also been characterized by X-ray diffraction
analysis. The redox properties of all the compounds were
studied by CV revealing, for the reported Ru−OH2 complexes,
bielectronic Ru(IV/II) redox processes throughout a wide pH
range. The catalytic activity of aquo complexes was evaluated
in the epoxidation of olefins using PhIO as oxidant, displaying
in general good yields and high selectivities for the epoxide product. The influence of electronic and geometrical factors on the
spectroscopic and electrochemical properties as well as on the catalytic activity is discussed.
challenging aspect is to tune the properties of the bounded
ligands so that a right combination of donor groups can lead to
the disproportion of oxidation state III into Ru(II) and Ru(IV),
then allowing a bielectronic RuII−OH2 ↔ RuIVO trans-
formation. This would allow the occurrence of a concerted oxo-
transfer mechanism in oxidation reactions catalyzed by this type
of systems. This is particularly interesting in olefin epoxidation
given the interest of this reaction both at an academic level and
in chemical industry,6 due to the use of epoxides as raw
materials for epoxy resins, paints, surfactants, or as
intermediates in organic syntheses.
INTRODUCTION
■
In the past few years, N-heterocyclic carbenes (NHC) have
emerged as ligated versatile building blocks for a large variety of
organometallic and coordination compounds.1,2 These ligands,
in comparison to others (e.g., phosphane, amine, alkoxy,
thioether, Schiff base), show a high propensity to act as
excellent σ-donors and to generate stable bonds with transition
metals.3 The strong bonding metal-NHC makes complexes
generally less susceptible to decomposition and increases the
catalytic performance of metal complexes.4 However, a loss of
activity may be observed if the complex formed becomes too
stable. To overcome this problem, the development of
bifunctional ligands was considered, with the idea of combining
the properties of NHC with other type of donor atoms which
form not so stable bonds with the metal center (e.g., N atoms
in amines), then relatively destabilizing the complexes and
improving their reactivity and catalytic properties.
We recently reported a bielectronic RuII−OH2 complex
containing two C-donor NHC rings, which constitutes the first
report of a Ru−OH2 carbenic complex studied as epoxidation
catalyst.7 The occurrence of a bielectronic (IV/II) wave in this
complex improves the selectivity and avoids the cis/trans
isomerization when using cis olefins as substrates. After that, a
second report describes a new Ru catalyst containing the
2,2′:6′,2″-terpyridine (trpy) ligand together with the mono-
carbene CN-Me ligand, showing that a unique NHC ring in the
complex also allows the existence of a bielectronic (IV/II) wave
The RuII−H2O complexes are of interest since the
corresponding higher oxidation states can be reached within a
relatively narrow potential range by sequential electron and
proton loss then leading to the corresponding RuIII−OH and
RuIVO species. These higher oxidation states, especially
RuIVO, are active catalysts for a variety of oxidative
reactions.5 In the chemistry of RuII−H2O type complexes, a
Received: December 31, 2012
Published: April 22, 2013
© 2013 American Chemical Society
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for the Ru−OH2 complex.8 Given the importance of
bielectronic catalysts9 in organic synthesis, we decided to
extend the study to other types of ligands in order to shed some
light into the geometrical and electronic factors that can favor
the disproportion of the Ru(III) oxidation state and
consequently allow a 2-electron redox process in newly
designed catalysts.
With all this in mind, we present here the synthesis and
complete characterization of a family of Ru complexes
containing the ligands displayed in Scheme 1. An analysis of
complex ([RuIICl2(bpea)dmso], 1,10 or [RuCl3(trpy)], 211) is
led to react with the hydrobromide form of the CN-Me ligand
in presence of triethylamine and LiCl to generate the respective
chloro complexes 3 or 6. A single trans,fac-isomer is obtained
for the bpea chloro complex 3, whereas a mixture of cis- and
trans-isomers (1:4 respectively) is attained for complex 6
containing the trpy ligand, which are separated through column
cromatography. In the case of the chloro complex 3, the
analogous trans,fac-[RuBr(CN-Me)(bpea)]+ bromo complex,
trans,fac-4, has also been isolated from the reaction mixture due
to the presence of the bromide counterions of the [HCN-Me]
Br ligand. The formation of this side product can be avoided
using 12 equiv of LiCl in the synthesis of 3.
Scheme 1. Ligands Used in This Work
The coordination of the CN-Me ligand in complex 3 can
potentially lead to seven different diastereoisomers (Scheme 3)
including those arising from a hypothetical meridional
arrangement of the bpea ligand, which has been reported
previously to coordinate in such a mode thanks to its
flexibility.12 However, the synthesized complex is isolated
exclusively as trans,fac. This can be explained in terms of (a) the
kinetic stability of the meridional coordination of bpea, which is
likely to be thermally converted into the thermodynamically
most stable facial disposition,12a and (b) the occurrence of two
hydrogen bonds between the monodentate chloro ligand and
the H atoms in the 2-position of the pyridyl rings of bpea
(depicted in orange color in Scheme 3), which can only take
place in the trans,fac isomer and which have been shown to be
determining in the isomeric ratio obtained for Ru complexes
containing bpea together with N- or P-donor didentate
ligands.12c The trans,fac geometry is also found in the bromo
complex 4 obtained as side product (see Supporting
Information for the X-ray structure of this complex).
the geometrical and electronic effects has been performed to
rationalize their spectroscopic and electronic properties,
together with the performance of the aquo complexes in the
epoxidation of various olefin substrates.
RESULTS AND DISCUSSION
The asymmetry of the CN-Me ligand can also lead to the
formation of the cis and trans diastereoisomers in the synthesis
of complex 6, and indeed, both isomers are obtained in a 1:4
ratio, respectively. The marked preference for the trans isomer
could be a priori explained by the occurrence of a H-bond in
trans-6, involving the chloro ligand and the 2-pyridyl H atom of
■
Synthesis and Structure. Scheme 2 displays the synthetic
strategy followed for the preparation of the whole set of
complexes (the syntheses of compounds trans-6 and trans-7
have been described previously8 but are included here for
comparison purposes). The nomenclature trans or cis in the
case of complexes 6 and 7 (containing the tridentate trpy
ligand) refers to the relative position of the monodentate (Cl or
H2O) ligand with regard to the coordinating C atom of the
CN-Me carbene ligand (see Scheme 3). On the other hand, the
nomenclature trans,fac for complexes 3 and 5, containing the
facial bpea ligand, indicates the relative position of the
monodentate ligand with respect to the aliphatic N atom of
the tridentate bpea ligand.
1
CN-Me (this is clearly manifested by its chemical shift in H
NMR, 8.10 for cis-6 and 10.15 for trans-68). Also, steric
arguments could be invoked since, in cis-6, the methyl group of
the CN-Me ligand is in close proximity to the monodentate Cl
ligand. However, we described recently13 the synthesis of a pair
of analogous complexes with almost identical structure (in this
case replacing the CN-Me ligand by pypz-Me, 2-(1-methylpyr-
azolyl)-pyridine), and the corresponding trans and cis isomers
were yielded in a roughly 1:1 ratio. Then, neither H-bonding
nor steric hindrance can explain the preferential formation of
The preparation of the complexes follows a similar strategy
when using either bpea or trpy as tridentate ligand. A starting
Scheme 2. Synthetic Strategy
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a
Scheme 3. Possible Diastereoisomers for Complexes (A) 3 and (B) 6
a
The notation fac and mer refers to the facial or meridional disposition of the bpea ligand whereas up and down indicates the relative orientation of
the ethyl group of bpea upon coordination. The H atoms involved in H-bonding in the trans,fac isomer are highlighted in orange.
the trans isomer, and thus, electronic factors, related to the
strong σ-donor capacity of carbene NHC rings, must be
appealed. Kinetic reasons such as distinctive trans effect during
the coordination step can be the origin of the unequal ratio of
isomers obtained, but the thermodynamics (spectroscopic and
electrochemical properties) of the complexes also manifest the
particular influence of the carbene ligand on the metal center as
is described below.
At this level, we emphasize that both chloro complexes trans-
6 and cis-6 are stable to light irradiation in solution. However,
irradiation of the aquo complexes trans-7 and cis-7 with visible
light in aqueous solution leads in any case to a partial
isomerization reaching a steady state with a cis:trans ratio of
roughly 35:65 independently of the starting isomer as shown by
1H NMR experiments (see Supporting Information). Again, the
analogous aquo complexes with pypz-Me behave in a different
way leading in any case to a 50:50 isomeric mixture.
The corresponding Ru−OH2 complexes trans,fac-5, trans-7,
and cis-7 are easily obtained from the corresponding Ru−Cl
complexes in the presence of Ag+ in water. The geometry of the
precursor chloro complex is kept in all cases except for the
synthesis of cis-7, where a certain amount (up to 20%) of the
corresponding trans isomer has been detected despite perform-
ing the reaction at dark. The isomerization upon substitution of
the chloro ligand can be explained on the basis of the
occurrence of a dissociative mechanism leading to a
pentacoordinate intermediate species,14 which would allow
the aquo ligand to approach and bond the metal center through
any side of the trpy ligand thus leading to either of the two
isomers (see Supporting Information for a schematic
representation of this process). However, it is striking that
the mixture of isomeric Ru−OH2 complexes is found only
when starting from the cis chloro complex, manifesting again
the major thermodynamic preference for the trans isomer in the
complexes containing trpy and the CN-Me ligand. Once more,
a different behavior is detected for the analogous complexes
mentioned above that contain the pypz-Me ligand,13 where
only the cis aquo complex is obtained from the corresponding
cis Ru−Cl.
The structures of complexes trans,fac-3, trans,fac-4, and cis-6
have been solved by X-ray diffraction analysis. The main
crystallographic data for trans,fac-3 and cis-6 are summarized in
Table 1 whereas their corresponding Ortep plots are gathered
in Figure 1 (additional crystallographic information such as
selected bond distances and angles for both complexes and the
Ortep plot for the structure of trans,fac-4 can be found in the
Supporting Information). In both cases, the Ru metal centers
adopt a distorted octahedral coordination with a disposition of
the ligands according to the type of isomer obtained, as
discussed above. For trans,fac-3, two enantiomeric forms are
found within the crystal lattice, as expected from the asymmetry
and the spatial arrangement of the CN-Me ligand. Most of the
bond distances and angles are within the range of those usually
found for this type of complex,15,16 though the relatively long
Ru−Cl bond distance found for both complexes which is
around 2.44 Å is remarkable. This bond distance was
considerably longer (2.47 Å) in the previously reported trans-
6 complex,8 thus manifesting the remarkable trans influence
exerted by the imidazolic NHC carbene ring.
The molecular units in complex trans,fac-3 are organized in
linear arrangements in the direction of the crystallographic b
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Table 1. Crystal Data for Compounds trans,fac-3 and cis-6
trans,fac-3
cis-6
empirical formula
fw
C23H26ClN6F6PRu
667.99
C28H30ClF6N6OPRu
748.07
cryst syst
space group
a [Å]
orthorhombic
Pna21
orthorhombic
Pbca
16.498(5)
8.417(3)
19.454(6)
90
13.41(2)
15.63(3)
28.17(5)
90
b [Å]
c [Å]
α [deg]
β [deg]
γ [deg]
90
90
90
90
V [Å3]
2701.3(15)
4
5908(19)
8
formula units/cell
T, K
300(2)
1.642
100(2)
ρcalcd [Mg/m−3
μ [mm−1
]
1.682
]
0.805
0.749
a
final R indices [I > 2σ(I)]
R1 = 0.0477
R1 = 0.0893
wR2 = 0.1987
R1 = 0.1397
wR2 = 0.2259
wR2 = 0.1178
R1 = 0.0777
wR2 = 0.1312
R indices [all data]
a
2
2
R1 =∑∥Fo| − |Fo∥/∑|Fo|. wR2 = [∑{w(Fo − Fc )2}/
2
2
2
∑{w(Fo )2}]1/2, where w = 1/[σ2(Fo ) + (0.0042P)2] and P = (Fo
+ 2Fc2).
−
axis, with the PF6 counterions allocated in the channels
between the linear arrangements of the cationic complex (see
Supporting Information). A similar arrangement is found for
complex cis-6, in this case following the direction of the a
crystal axis. Counterions are in all cases linked to complex
cations through H-bonding with different H atoms of the
ligands.
Spectroscopic Properties. All the complexes have been
thoroughly characterized by 1D and 2D NMR techniques. The
assignment of all the resonances is detailed in the Experimental
Section, and the spectra registered are gathered in the
Supporting Information. All the resonances found for
complexes 3−7 are consistent with the structures obtained in
the solid state. For the set of complexes with trpy ligand, the
chemical shift of protons that are close to the monodentate
ligand (Cl or H2O) is clearly indicative of the isomer obtained
in each case (for instance, the singlet assigned to the methyl
protons H24 is found at 4.63 ppm in cis-6 and at 2.90 in trans-6,
due to the inductive effect of the Cl ligand in the close
proximity of the methyl group; similarly, the pyridine H16
resonance is observed at 10.15 and 8.10 for trans-6 and cis-6,
respectively).
The UV−vis spectra of complexes cis-6, cis-7, trans,fac-3,
trans,fac-4, and trans,fac-5 display broad bands corresponding to
dπ−π* MLCT and intraligand π−π* absorptions typical of Ru
polypyridyl complexes.17 The spectra registered in dichloro-
methane for the chloro and aquo complexes can be found in
the Supporting Information (the bromo complex 4 displays a
spectrum, not shown, almost identical to the one registered for
the analogous trans,fac-3 chloro complex). Table 2 collects the
main absorption wavelengths and extinction coefficients
together with a tentative assignment on the basis of the λ
value and extinction coefficients.
Figure 1. Ortep plots (ellipsoids at 50% probability level) and labeling
schemes for the X-ray structure of compounds trans,fac-3 (A) and cis-6
(B).
[RuX(trpy)(bpy)]n+complexes17c (X = Cl or H2O, bpy = 2,2′-
bipyridine), thus evidencing that the change of the bipyridine
ligand by the carbene CN-Me does not have a significant
influence on the electronic spectra in these compounds. On the
other hand, in the case of complexes trans,fac-3 and 5, we
observe only bands of type dπ−π*. The intraligand π−π*
absorptions for complexes containing bpea are expected to
appear at higher energies than those of complexes containing
trpy (6 and 7) due to the lower aromatic character of the bpea
ligand when compared to trpy, but in the case of complexes
trans,fac-3 and 5 the CN-Me ligand has also a remarkable effect
in lowering the maximum wavelengths of the absorptions
detected. This is clearly manifested when comparing the
absorptions of complex trans,fac-5 with those of the analogous
complex containing bpy instead of CN-Me (entry 7),20 and the
overall effect is that, for complexes trans,fac-3/5, the dπ−π*
MLCT bands are blue-shifted and the π−π* bands appear out
of the range of λ allowed by the solvent.
On the other hand, the comparison of the MLCT bands for
Ru−Cl and Ru−H2O analogous complexes (entry 1 vs 2, or 5
vs 6) reveals a hypsochromic shift due to the replacement of the
anionic Cl− ligand by the neutral H2O, given the strong σ- and
π-donor ability of Cl− which reduces the energy difference
Complexes cis-6 and cis-7 display a typical set of absorptions
of MLCT dπ−π* and intraligand π−π* type, practically
identical to those exhibited by their trans counterparts (entries
3 and 4), and also quite similar to the spectra of the analogous
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Table 2. Main UV−Vis Spectroscopic Absorptions in CH2Cl2 for the Complexes Described
entry
1
compd
assignment
λmax, nm (ε, M−1 cm−1
)
cis-6
π → π*
dπ → π*
π → π*
dπ → π*
π → π*
dπ → π*
π → π*
dπ → π*
dπ → π*
dπ → π*
π → π*
dπ → π*
268 (19 824), 280 (sh 14742), 320 (19 331)
381 (5217), 498 (3898)
2
3
4
cis-7
271 (17 275), 314 (21 462)
364 (4996), 464 (4104)
trans-68
trans-78
267 (16 636), 279 (sh 12 768), 316 (17 314)
372 (5341), 490 (4316)
272 (18 306), 315 (21 670)
362 (5250), 463 (4494)
5
6
7
trans,fac-3
trans,fac-5
245 (sh 7326), 270 (3803), 380 (4971), 399 (4584)
272 (5929), 301 (3432), 358 (5067), 382 (4639)
246 (18 755), 292 (31 420)
trans,fac-[Ru(bpea)(bpy)(H2O)]2+,20
360 (12 257), 468 (6048)
between full d orbitals of the metal and empty π* orbitals of the
ligands.
protons upon oxidation of the complex as indicated in eq 1,21
which also makes the upper oxidation states easily accessible:
The pKa of the Ru-aqua complexes trans,fac-5 and cis-7 has
been evaluated through spectrophotometric acid−base titra-
tions in aqueous solution18 (see Supporting Information).
Upon addition of base, a red-shift of the maximum wavelength
is detected for the MLCT bands that can be explained by the
increase of the electron density coming from the anionic
hydroxyl ligand, which destabilizes the d orbitals of the metal
thus lowering the transition energy in a similar way to that
described above for chloride ligand.19 A set of isosbestic points
is found in each case (237, 251, 277, 389, and 527 nm for
trans,fac-5; 261, 331, 416, and 480 nm for cis-7), indicative of a
net RuII−OH2 ⇆ RuII−OH transformation. The pKa value
determined for cis-7 was 10.91. However, in the case of
trans,fac-5 the complete conversion to RuII−OH could not be
achieved even at pH values above 12, and consequently, the
exact pKa value cannot be calculated. The lower acidity of
trans,fac-5 when compared to cis-7 is in agreement with the
higher electron-donor capacity of the bpea ligand with regard to
trpy (which renders the metal center much less acidic)20
together with the fact that, in complex trans,fac-5, the aqua
ligand is placed trans to the aliphatic N atom of bpea thus
facilitating the transmission of the electron density.
−1H+,−1e−
−1H+,−1e−
L5RuII−OH2 HooooooooooI L5RuIII−OH HooooooooooI L5RuIV=O
+1H+,+1e−
+1H+,+1e−
(1)
The reversibility of these redox processes constitutes the
basis for the use of this kind of complex in redox catalysis. In
most cases, the two monoelectronic redox processes indicated
in eq 1, corresponding to the Ru(III/II) and Ru(IV/III) redox
couples, can be observed in cyclic voltammetry. However, some
Ru complexes have been described in the literature where the
two redox processes occur simultaneously:7−9,13
−2H+,−2e−
L5RuII−OH2 HooooooooooI L5RuIV=O
+2H+,+2e−
(2)
The complete thermodynamic information regarding the Ru-
aqua type of complex can be extracted from the Pourbaix
diagrams, displayed in Figure 2 for complexes trans,fac-5 and
Redox Properties. The redox properties of the compounds
have been determined by cyclic voltammetry (CV) and
spectrophotometric titration experiments in the case of aquo
complexes. The chloro complexes trans,fac-3 and cis-6 present a
reversible, monoelectronic Ru(III/II) wave at 0.66 and 0.77 V
versus SSCE, respectively (see Supporting Information), with a
0.11 V difference that can be attributed to the major electron-
withdrawing character of trpy ligand with regard to bpea.
However, it is interesting to compare these E1/2 values with the
one displayed by the isomeric trans-[RuCl(CN-Me)(trpy)]+
(trans-6) complex,8 which is of 0.88 V despite containing
identical ligands as cis-6. This difference reveals a particular
behavior in complexes containing carbene ligands such as CN-
Me, where the relative position of the ligands around the metal
also determines the electrochemical properties of the complex
(the analogous cis/trans isomers of chloro complexes
containing trpy and pypz-Me13 display almost identical E1/2
values). In the specific case of trans-6, the remarkably high E1/2
Ru(III/II) value probably arises from the large Ru−Cl bond
distance of 2.47 Å provoked by the presence of the carbene ring
in trans to Cl.
Figure 2. Pourbaix diagram for the aquo complexes trans,fac-5 (black
pattern) and cis-7 (gray pattern). The main proton compositions are
indicated.
cis-7. As can be observed, a unique pH-dependent redox
process is found through a wide pH range (at pH values higher
than 2.7 for cis-7 and above 1.2 for trans,fac-5). The
dependence of E1/2 versus pH is linear with a slope of
approximately −59 mV per pH unit, indicative of the transfer of
an equivalent number of protons and electrons as stated in eq
2. The change of slope to a roughly −30 mV value at a pH
The redox properties of Ru-aqua complexes are pH
dependent due to the capacity of the aqua ligand to lose
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value of around 10.8 for cis-7 is indicative of deprotonation of
the RuII−OH2 (pKa(II) value) leading to a basic pH range
where the bielectronic redox process is accompanied by the
exchange of only one proton. The pKa(II) value inferred from
the Pourbaix diagram is consistent with the one obtained from
acid−base spectrophotometric titration (10.91). For trans,fac-5,
an estimated pKa value could be suggested at a value around 12,
which is in accordance with the fact that complete
deprotonation could not be attained in acid−base titration as
stated above. Finally, the horizontal lines at acidic pH indicate
the transfer of only one electron that occurs between the RuII−
OH2 and the RuIII−OH2 forms (without proton exchange).
The end of this area, at pH values of 1.2 and 2.7 for trans,fac-5
and cis-7, respectively, corresponds in each case to the pKa of
the oxidized RuIII−OH2 form (pKa(III)).
295, 344, and 612 nm for cis-7), thus indicating a direct, 2-
electron transformation from Ru(II) to Ru(IV).
Table 3 summarizes the electrochemical data and the pKa of
the aquo complexes described together with those of analogous
complexes for comparison purposes. In all cases, the cis or trans
configuration (relative to the aquo ligand) is determined by
considering the ligand with higher σ-donor capacity than that of
the pyridyl ring. Taking as reference the well-known [Ru-
(trpy)(bpy)(H2O)]2+ complex (entry 1), one can see that the
replacement of bpy by an anionic ligand such as picolinate
(entries 7 and 8) considerably lowers both the Ru(III/II) and
Ru(IV/III) E1/2 values as expected from the higher σ-donor
ability of the anionic ligand. It is interesting to note that a
similar effect is attained by replacing trpy by the more σ-donor
(and less π-acceptor) bpea ligand (entry 9), though both are
neutral ligands. On the other hand, the replacement of bpy in
[Ru(trpy)(bpy)(H2O)]2+ by the anionic pyrpy-O ligand
(pyrpy-O corresponds to 3-hydroxy-2,4-dimethyl-5-(2-pyridyl)-
pyrrolate, entry 2) produces a dramatic change in the redox
properties, leading to the occurrence of a bielectronic Ru(IV/
II) process. The E1/2(IV/II) value observed is virtually the same
as those for other complexes with the same trans geometry but
having one or three carbene rings in the complex structure
(entries 3 and 5, respectively). However, the analogous
complexes with cis geometry (entries 4 and 6) present also a
bielectronic process but at a potential value around 0.12−0.13
V lower than their trans counterparts. Thus, for the bielectronic
complexes, we note that the coordination of the σ-donor ligand
in trans increases the E1/2(IV/II) redox potential when
compared to the corresponding cis isomer, while in the case
of monoelectronic complexes this behavior is reversed (entries
7 and 8). Finally, the replacement of trpy in the monocarbene
complexes trans- or cis-7 by bpea (entry 10) produces a
significant decrease of the Ru(IV/II) E1/2 value still keeping the
bielectronic process, converting complex trans,fac-5 into the
one exhibiting the lowest potential value among the series of
bielectronic compounds although it contains only one carbene
ring.
To confirm the occurrence of bielectronic waves we have
performed a redox spectrophotometric titration of trans,fac-5
and cis-7 with Ce(IV) as oxidant, and the sets of spectra
obtained are shown in Figure 3 and in the Supporting
Figure 3. Redox spectrophotometric titration performed by sequential
addition of 50 μL of 10−2 M of Ce(IV) (up to 2 equiv) to 25 mL of a
10−4 M solution of trans,fac-5 in 0.1 M HCl.
Regarding the pKa(II) values gathered in Table 3, we can
observe that complexes containing carbene and/or bpea ligands
(entries 3−6, 9, and 10) present higher values (above 10.9)
than the rest of complexes (pKa(II) of 9.7−10, entries 1, 2, 7,
and 8), with independence of the type of ligands (neutral or
anionic) that they contain or whether they exhibit a bielectronic
Information, respectively. In both cases, the evolution of the
spectra along the addition of 2 equiv of Ce(IV) leads to a
featureless UV−vis spectrum characteristic of RuIV=O
species17c,22 with the occurrence of isosbestic points through-
out the whole titration (254, 308, and 467 nm for trans,fac-5;
Table 3. pKa and Electrochemical Data (pH = 7, E1/2 in V vs SSCE) for the Aquo Complexes Described in This Work and
Others Previously Reported
a
b
entry
complex (T)(D)Ru−OH2
E1/2(III/II)
E1/2(IV/III)
E1/2(IV/II)
ΔE
pKa(II)
pKa(III)
ref
1
2
(trpy)(bpy)Ru−OH2
trans-(trpy)(pyrpy-O)Ru−OH2
trans-7
0.49
0.62
0.13
≤0
≤0
≤0
≤0
9.7
1.7
1.21
3.08
2.7
4.8
3.0
2
21a
13
8
0.55
0.57
0.44
0.56
0.44
9.7
3
11.00
10.91
11.9
12.3
10.0
10.0
11.1
∼12
4
cis-7
this work
5
trans-(CNC)(CN-Bu)Ru−OH2
cis-(CNC)(CN-Bu)Ru−OH2
trans-(trpy)(pic)Ru−OH2
cis-(trpy)(pic)Ru−OH2
(bpea)(bpy)Ru−OH2
trans,fac-5
7
6
≤0
7
7
0.21
0.38
0.34
0.45
0.56
0.46
0.24
0.18
0.12
≤0
22c
8
3.7
1.2
1.19
22c
9
20
10
0.32
this work
a
Abbreviation of tridentate ligand T: CNC = 2,6-bis(butylimidazol-2-ilidene)pyridine. Abbreviation of didentate ligand D: pic =2-picolinate; bpy =
b
2,2′-bipyridine; CN-Bu = N-butyl-N′-2-pyridylimidazolium; pyrpy-O = 3,5-dimethyl-4-hydroxo-2-(2-pyridyl)pyrrolate. ΔE = E1/2(IV/III) −
E1/2(III/II) (in V).
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or two monoelectronic processes. For the case of pKa(III), the
higher values are exhibited by the carbene complexes (with the
exception of trans,fac-5, entry 10), although there is not a clear
trend since the picolinate complexes (entries 7 and 8) also
present relatively low acidic RuIII−OH2 forms.
Meyer−Lever plot as has been determined experimentally (see
Supporting Information for details of the calculation). A recent
report based on DFT calculations24 states that a correlation can
be established between the number of carbene units present in
a Ru complex and the Ru(III/II) and Ru(IV/III) redox couples
in such a way that the higher the number of carbene units, the
higher the E1/2(III/II) value and the lower the E1/2(IV/III). As
the number of carbene units increases, the point where ΔE is
equal or below zero can be more easily attained. In our case,
however, a unique carbene ring has been shown to be enough
to reach the Ru(III) disproportion in the two complexes
described.
A previous report of Meyer et al.23 establishes an empirical
correlation between ΔE1/2 (defined as the difference between
the IV/III and III/II redox couples for the Ru−OH2 type of
complexes, Table 3) vs ΣEL, which is the sum of a series of
parameters calculated by Lever23a for the nonaqua ligands
attached to the Ru metal center. The Meyer−Lever correlation,
displayed in Figure 4, shows that the family of Ru−OH2
Catalytic Epoxydation. To study the catalytic properties
and the geometric and electronic effects of tridentate
(meridional/facial) and didentate (cis/trans) ligands of
complexes trans,fac-5 and cis-7, we have applied the same
conditions used for the evaluation of the catalytic activity of
trans-7.8 Table 4 summarizes the results obtained with both
catalysts and also includes the results obtained with trans-7 for
comparison purposes. Blank experiments performed in absence
of catalyst did not yield any oxidation product.
Steric and electronic factors of both the complexes and the
substrates should be considered to rationalize the differences in
performance observed in some cases. For styrene substrate
(entry 1), a comparison between the performances of the three
complexes provides evidence of a reactivity governed
exclusively by electronic factors with a conversion degree
increasing in parallel to the redox Ru(IV/II) potential of the
catalysts (trans-7 > cis-7 > trans,fac-5). However, the selectivity
is better for trans,fac-5 that presents the lowest redox potential.
According to the literature, the epoxide selectivity is affected by
steric effects25 but also by the acidity of catalysts,26 which lead
to a rearrangement of the epoxide thus producing a decrease in
selectivity. In the case of styrene, the improvement in selectivity
is in agreement with the increasing pKa(II) value in the order
cis-7 < trans-7 < trans,fac-5. (see Table 3).
Figure 4. Meyer−Lever plot of observed ΔE1/2 vs ∑EL. Molecular
formulas for the complexes represented: (A) [Ru(trpy)(acac)(OH2)]+
(acac is acetylacetonate); (B) cis-[Ru(trpy)(pic)(OH2)]+ (pic is
picolinate); (C) [Ru(trpy)(tmen)(OH2)]2+ (tmen is N,N,N,N-
tetramethylethylenediamine); (D) cis-[Ru(CNC)(CN-Bu)(OH2)]2+
(CNC is 2,6-bis(butylimidazol-2-ilidene)pyridine; CN-Bu is N-butyl-
N′-2-pyridylimidazolium); (E) -[Ru(CNC)(bpy)(OH2)]2+ (bpy is
2,2′-bipyridine); (F) [Ru(trpy)(bpy)(OH2)]2+; (G) [Ru(bpy)2(OH2)-
(PPh3)]2+; (H) [Ru(trpy)(dppene)(OH2)]2+ (dppene is cis-1,2-
bis(diphenylphosphino)ethylene).
On the other hand, focusing the attention in the initial
conversion rates vi, one can observe that complex trans,fac-5
presents considerably higher rates for almost all the substrates
studied, whereas cis-7 is the slowest catalyst as average. This fact
is in agreement with the higher σ-donor and lower π-acceptor
capacity of the bpea ligand in comparison to trpy, which could
lead to a more nucleophilic active RuO species, but structural
factors can also have an influence. Indeed, the structural
arrangement of the ligands in the three catalysts seems to be
more favorable for trans-7 and trans,fac-5, where only C−H
pyridyl atoms are located in the surroundings of the RuO
complexes lay into two different lines depending on the σ-
donating or π-accepting character of the ligands attached to the
Ru metal center, and that the right ligand combination, in order
to reach the ΔE ≤ 0 zone (which involves the spontaneous
disproportion of the Ru(III) oxidation state, i.e. a bielectronic
redox process), should have an ΣEL value between approx-
imately 0.95 and 1.2. The ΣEL parameters for trans,fac-5 and cis-
7 have been determined to be of 1.09 and 1.14, respectively,
nicely placing both aquo complexes into the ΔE ≤ 0 area of the
a
Table 4. Catalytic Epoxidation of Different Alkenes with trans,fac-5, trans-7, and cis-7 Using PhI(OAc)2 as Oxidant
trans,fac-5
conv
trans-7
cis-7
b
entry
complex alkene
sel
vi
conv
sel
vi
conv
60
sel
vi
1
2
3
4
5
6
styrene
48
93.4
99
>99
90
61
89
94
94
80
97
50
66
92
98
c
trans-stilbene
cis-β-methylstyrene
cyclooctene
>99
85
250
38.5
25.4
13.9
7.8
13.6
6.1
90
7.7
9.9
5.5
1.2
8.6
94
81
>99
85
>99
96
99.5
99.6
97.2
>99
98
8.4
d
d
1-octene
8.6
15 (70)
96.5
98 (96)
93
4-vinylcyclohexene
80.5
99
10.9
a
Conditions: alkene (50 mM), complex (0.5 mM), oxidant PhI(OAc)2 (100 mM), CH2Cl2 (2.5 mL), 25 °C, 24 h, biphenyl (15 mM) as internal
b
standard. Conversion (conv) and selectivity (sel) values are given in %, and initial reaction rates vi in 10−6 mol h−1 Selectivity for epoxide, [yield/
conversion] × 100. Byproducts have been determined in most cases to be the corresponding ketones or aldehydes. Analysis performed after 3 h. In
c
d
parentheses, analysis performed after 48 h.
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(trpy)(H2O)](PF6)2, trans-7,8 were prepared as described in the
literature. All synthetic manipulations were routinely performed under
nitrogen atmosphere using Schlenk tubes and vacuum line techniques.
trans,fac-[RuIICl(CN-Me)(bpea)](PF6) and trans,fac-[RuIIBr(CN-
Me)(bpea)](PF6), trans,fac-3·0.5CH2Cl2 and trans,fac-
4·2.5CH2Cl2. Method 1. A sample of 1 (100 mg, 0.209 mmol) was
dissolved in 20 mL of diethyleneglycol, and it was stirred at room
temperature for 30 min under N2 atmosphere. After that the mixture
was heated to 60 °C, and then, 50.18 mg (0.209 mmol) of [HCN-Me]
Br dissolved in 2 mL of diethyleneglycol and 0.09 mL (0.627 mmol) of
NEt3 were added. The reaction mixture was heated at 150 °C for 3 h.
After this time, it was cooled to room temperature, and then, 1 mL of a
saturated aqueous NH4PF6 solution and 20 mL of water were
subsequently added. A yellow precipitate of [RuIIX(CN-Me)(bpea)]-
(PF6) (X = Cl− or Br−) appeared which was filtered, washed with
water and ether, and dried under vacuum. In order to separate the
chloro complex from the bromo complex, the resulting precipitate was
purified over silica column using dichloromethane/acetone (9/1) as
eluent. The first orange fraction corresponds to [RuIIBr(CN-
Me)(bpea)](PF6), trans,fac-4, and the second yellow fraction
corresponds to [RuIICl(CN-Me)(bpea)](PF6), trans,fac-3. The sol-
utions of both complexes were evaporated to dryness, and the resulting
solid was recrystallized from a mixture of dichloromethane and
pentane (1:1, v/v), washed with ether and then pentane, and dried
under vacuum. Yield of trans,fac-3: 48 mg (34.4%). Yield of trans,fac-4:
20 mg (13.4%).
group, in contrast with cis-7, which presents a methyl group
that can convey steric hindrance (then producing a decrease in
the reaction rates for this catalyst, as is observed in most
substrates). Moreover, trans,fac-5 presents the pyridyl rings of
bpea in a perpendicular fashion with respect to the equatorial
plane of the complex, and this arrangement could favor the
approach of aromatic substrates through π-interactions27 (see
Supporting Information for a schematic representation of this
effect). Indeed, the two aromatic substrates (trans-stilbene and
cis-β-methylstyrene, entries 2 and 3) are epoxidized by trans,fac-
5 with the highest vi values. It is particularly noticeable in the
case of trans-stilbene, which is completely epoxidized within 3 h
reaction with excellent selectivity. Finally, in trans,fac-5 the
atom in trans position with regard to the RuO active site is
the aliphatic N atom of bpea that can in principle generate a
more nucleophilic oxo group28 thanks to the more effective
transmission of the electron density, which would also be in
agreement with the higher rates displayed by this complex
especially for activated substrates. A similar argument (besides
the steric hindrance mentioned above) could explain the
slightly better performance of complex trans-7 with regard to
cis-7, since in trans-7 the RuO group is in trans with regard to
the more σ-donor carbene ring whereas in cis-7 this position is
occupied by a pyridyl ring. The low reaction rates displayed by
cis-7 are in some cases accompanied by a decrease in the
selectivity for the epoxide (entries 1 and 2).
Finally, it is worth mentioning here that the epoxidation of
cis-β-methylstyrene (entry 3) is stereospecific for the cis epoxide
with all the catalysts. This is in agreement with the occurrence
of bielectronic redox processes on the catalysts, which can lead
to a concerted mechanism of O atom transfer from the RuO
active group.4,29 Catalysts that operate by a radical mechanism
as Mn-salen complexes30 and some other ruthenium oxo
complexes5b have been described to generate a mixture of cis/
trans epoxides. The design of bielectronic catalysts is thus of
great importance to achieve good selectivity degrees. The
epoxidation of 4-vinylcyclohexene (entry 6) is also regiospe-
cific, with the epoxidation taking place exclusively at the
cyclohexene ring.
Method 2. In order to synthesize only the trans,fac-3 complex, it
was prepared in a manner identical to the previous method except that
the reaction was performed in the presence of an excess of LiCl (12
equiv per Ru). The yellow solid obtained after the addition of 1 mL of
a saturated aqueous NH4PF6 solution corresponds to complex
trans,fac-3. Yield: 57%. Anal. Found (Calcd) for
C23H26N6ClRuPF6·0.5CH2Cl2: C, 39.51 (39.73); N, 11.85 (11.82);
H, 4.10 (3.83). 1H NMR (600 MHz, acetone-d6, 25 °C) δ (ppm): 9.61
(ddd, J = 5.4, 1.6, 0.8 Hz, H1), 9.49 (ddd, J = 5.4, 1.6, 0.8 Hz, H14),
8.34 (d, J = 2.3 Hz, H17), 8.25 (ddd, J = 5.7, 1.6, 0.7 Hz, H23), 8.06
(dt, J = 8.3, 0.9 Hz, H20), 7.94 (td, J = 8.3, 7.4, 1.6 Hz, H21), 7.89 (td,
J = 7.7, 1.6 Hz, H3), 7.70 (td, J = 7.7, 1.6 Hz, H12), 7.54 (d, J = 7.8
Hz, H4), 7.49 (td, J = 7.8, 1.2 Hz, H2), 7.46 (d, J = 2.3 Hz, H16), 7.40
(d, J = 7.6 Hz, H11), 7.26 (t, J = 5.8 Hz, H13), 7.20 (td, J = 5.9, 1.2
Hz, H22), 4.51 (d, J = 16.5 Hz, H9a), 4.37 (d, J = 16.4 Hz, H6a), 4.27
(t, J = 15.8 Hz, H9b), 4.25 (t, J = 15.8 Hz, H6b), 3.46 (s, H18, 3H),
2.55 (dd, J = 13.4, 7.2 Hz, H7b), 2.31 (dd, J = 13.4, 7.1 Hz, H7a), 0.99
(t, J = 7.2 Hz, H8, 3H). 13C NMR (151 MHz, acetone-d6, 25 °C) δ
(ppm): 204.78 (C15), 162.82 (C10), 161.03 (C5), 156.21 (C19),
152.94 (C14), 152.76 (C23), 150.81 (C1), 137.84 (C21), 137.68
(C3), 136.00 (C12), 125.88 (C16), 124.83 (C2), 124.28 (C13),
122.56 (C4), 121.95 (C22), 121.72 (C11), 116.98 (C17), 111.33
(C20), 68.96 (C9), 67.13 (C6), 63.14 (C7), 36.19 (C18), 8.77 (C8).
NOEs: H1 with H23, H23 with H6b, H4 with H6a, H17 with H20,
and H11 with H9a. For the NMR assignment we have used the same
numbering scheme as for the X-ray structure displayed in Figure 1. IR
(νmax, cm‑1): 2368 (s), 1610 (m),1490 (m), 1349 (m), 1255 (w), 836
(s) 769 (m), 555 (s). E1/2(III/II) (CH2Cl2 + 0.1 M TBAH): 0.66 V vs
SSCE. UV−vis (CH2Cl2): λmax, nm (ε, M−1 cm−1) 245 (7326), 270
(3803), 380 (4971), 399 (4584). ESI-MS (m/z): 523.1 [M − PF6]+.
Data for trans,fac-4·2.5CH2Cl2 follow. Anal. Found (Calcd) for
C23H26N6BrRuPF6·2.5CH2Cl2: C, 33.33 (33.12); N, 8.87 (9.08); H,
In conclusion, we have carried out the synthesis and
characterization of new examples of the scarce Ru(II)−OH2
complexes displaying bielectronic processes, where the
presence of carbene NHC ligands seems to be relevant for
the attainment of a Ru(IV/II) redox couple but with
independence of the cis or trans geometry. The comparison
of complexes bearing the meridional trpy or the facial bpea
ligand provide evidence that both electronic and steric factors
seem to be more favorable for the bpea ligand as epoxidation
catalyst, revealing higher reaction rates and selectivities for the
trans,fac-5 complex. The occurrence of a σ-donor ligand in trans
fashion with regard to the aqua/oxo ligand also favors the
catalytic performance.
1
3.81 (3.38). H NMR (200 MHz, acetone-d6, 25 °C) δ (ppm): 9.77
EXPERIMENTAL SECTION
(d, H1), 9.63 (d, H14), 8.34 (d, H18), 8.25 (d, H23), 8.06 (d, H20),
7.94 (d, H21), 7.89 (t, H3), 7.70 (t, H12), 7.54 (d, H4), 7.49 (t, H2),
7.46 (d, H17), 7.40 (d, H11), 7.26 (t, H13), 7.20 (t, H22), 4.51 (d,
H9a), 4.37 (d, H6a), 4.27 (t, H9b), 4.25(t, H6b), 3.46 (s, H16, 3H),
2.55 (d, H7b), 2.31 (d, H7a), 0.99 (t, H8, 3H). For the NMR
assignment we have used the same numbering scheme as for the X-ray
structure displayed in Figure 1. E1/2(III/II) (CH2Cl2 + 0.1 M TBAH):
0.67 V versus SSCE. UV−vis (CH2Cl2): λmax, nm (ε, M−1 cm−1) 269
(3295), 377 (4915). ESI-MS (m/z): 567.2 [M − PF6]+.
■
Materials. All reagents used in the present work were obtained
from Aldrich Chemical Co. and were used without further purification.
Reagent grade organic solvents were obtained from SDS, and high
purity deionized water was obtained by passing distilled water through
a nanopure Mili-Q water purification system. RuCl3·2H2O was
supplied by Johnson and Matthey Ltd. and was used as received.
Preparations. Ligands CN-Me (N-methyl-N′-2-pyridylimidazo-
lium bromide)31 and bpea (N,N-bis(2-pyridylmethyl)ethylamine)),32
and the complexes [RuIICl2(bpea)dmso], 1,10 [RuIIICl3(trpy)], 2,11
trans-[RuCl(CN-Me)(trpy)]PF6, trans-6,8 and trans-[Ru(CN-Me)-
trans,fac-[RuII(CN-Me)(bpea)OH2](PF6)2, trans,fac-5. A 25 mg
(0.037 mmol) sample of trans,fac-3 and a 12.57 mg (0.074 mmol)
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−
sample of AgNO3 were placed in a round-bottom flask together with
15 mL of H2O. The resulting mixture was refluxed for 3 h in darkness.
After this time the reaction was cooled to room temperature (RT) and
then in an ice bath, and the AgCl formed was removed by filtration
through Celite. Afterward, a 1 mL sample of saturated aqueous
solution of NH4PF6 was added to the filtrate, and the solvent was
removed slowly in a rotary evaporator until precipitation of a green
solid that was filtered, washed with ether and then pentane, and dried
under vacuum. Yield: 20 mg (68%). Anal. Found (Calc) for
C23H28N6ORuP2F12: C, 34.75 (34.73); N, 10.01 (10.5); H, 3.87
− PF6 ]+. For the NMR assignment we have used the same
numbering scheme as for the X-ray structure displayed in Figure 1.
cis-[RuII(CN-Me)(trpy)OH2](PF6)2·H2O, cis-7·3.5H2O. A 17 mg
(0.025 mmol) sample of cis-6 and an 8.57 mg sample of AgNO3 were
placed in a round-bottom flask together with 15 mL of H2O. The
resulting mixture was refluxed for 3 h in darkness. After this time the
reaction was placed in an ice bath, and the AgCl formed was removed
by filtration over Celite. Afterward, a 1 mL sample of saturated
aqueous solution of NH4PF6 was added, and the solvent was removed
slowly in a rotary evaporator until precipitation of a brown solid that
was filtered, washed with ether and then pentane, and dried under
vacuum. Yield: 15 mg (74.9%). [Note that in this synthesis we have
observed the formation of approximately 20% of the trans isomer.]
Anal. Found (Calcd) for C24H22N6ORuP2F12·3.5H2O: C, 33.79
(34.05); N, 9.90 (9.92); H, 2.73 (3.20). 1H NMR (600 MHz,
acetone-d6/10% D2O, 25 °C) δ (ppm): 8.84 (d, J = 8.1 Hz, H7, H10),
8.65 (d, J = 8.0 Hz, H4, H13), 8.52 (d, J = 2.3 Hz, H24), 8.44 (t, J =
8.1 Hz, H9), 8.22 (dd, J = 7.4, 0.7 Hz, H1, H16), 8.03 (td, J = 7.8, 1.5
Hz, H3, H14), 7.99 (d, J = 8.3 Hz, H17), 7.88 (d, J = 2.3 Hz, H23),
7.75 (tq, J = 8.2, 7.0, 1.4 Hz, H18), 7.41 (tq, J = 7.5, 5.9, 1.1 Hz, H2,
H15), 7.39 (d, J = 5.1 Hz, H20), 6.89 (tq, J = 7.3, 6.1, 1.1 Hz, H19),
4.44 (s, H25, 3H). 13C NMR (151 MHz, acetone-d6/10% D2O, 25 °C)
δ (ppm): 199.51 (C22), 160.11 (C5, C12), 158.50 (C6, C11), 157.75
(C1, C16), 157.41 (C21), 152.97 (C17), 138.99 (C19), 138.82 (C3,
C14), 138.51 (C9), 128.68 (C2, C15), 126.96 (C23), 125.05 (C4,
C13), 124.02 (C7, C10), 122.52 (C18), 117.53 (C24), 112.00 (C20),
37.19 (C25). IR (νmax, cm−1): 3660 (w), 3500 (s), 1620 (m), 1610
(w), 1494 (m), 1247 (m), 1024 (m), 835 (s), 763 (s), 555 (s).
E1/2(IV/II), phosphate buffer pH = 7: 0.44 V versus SSCE. UV−vis
phosphate buffer pH = 7: λmax, nm (ε, M−1 cm−1) 225 (12 647), 268
(8293), 312 (10 774), 361 (2428), 456 (2098). CH2Cl2: 271 (17 275),
314 (21 462), 364 (4996), 464 (4104). ESI-MS (m/z): 513.1 [M −
H+− 2PF6]+.
Instrumentation and Measurements. Fourier transform IR
(FTIR) spectra were taken in a Mattson-Galaxy Satellite FT-IR
spectrophotometer containing a MKII Golden Gate Single Reflection
ATR system. UV−vis spectroscopy was performed on a Cary 50 Scan
(Varian) UV−vis spectrophotometer with 1 cm quartz cells. Cyclic
voltammetric (CV) experiments were performed in a IJ-Cambria IH-
660 potentiostat using a three electrode cell. Glassy carbon electrodes
(3 mm diameter) from BAS were used as working electrode, platinum
wire as auxiliary, and SSCE as the reference electrode. Electrochemical
experiments were performed under either N2 or Ar atmosphere with
degassed solvents. All E1/2 values estimated from cyclic voltammetry
were calculated as the average of the oxidative and reductive peak
potentials (Epa + Epc)/2 at a scan rate of 100 mV/s whereas they were
directly taken from the maximum of the peak in differential pulse
voltammetry experiments. Unless explicitly mentioned the concen-
tration of the complexes was approximately 1 mM. In aqueous
solutions the pH was adjusted from 0 to 2 with HCl. Potassium
chloride was added to keep a minimum ionic strength of 0.1 M. From
pH 2 to 10, 0.1 M phosphate buffers were used, and from pH 10 to 12
diluted, CO2 free, NaOH. Bulk electrolyses were carried out in a three-
compartment cell using carbon felt from SOFACEL as the working
electrode.
1
(3.54). H NMR (600 MHz, acetone-d6/10% OD2, 25 °C) δ (ppm):
9.01 (d, J = 5.1 Hz, H1), 8.81 (d, J = 5.1 Hz, H14), 8.42 (d, H17), 8.40
(d, J = 5.6 Hz, H23), 8.15 (d, J = 8.2 Hz, H20), 8.06 (t, J = 7.7 Hz,
H21), 7.95 (t, J = 7.4 Hz, H3), 7.78 (t, J = 7.8 Hz, H12), 7.60 (d, J =
8.0 Hz, H4), 7.55 (t, J = 6.3 Hz, H2), 7.53 (s, H16), 7.46 (d, J = 8.0
Hz, H11), 7.35 (t, J = 5.9 Hz, H13), 7.31 (t, J = 6.0 Hz, H22), 4.48 (d,
J = 17.0 Hz, H9a), 4.37 (d, J = 16.4 Hz, H6a), 4.22 (d, J = 16.7 Hz,
9Hb), 4.22 (d, J = 16.7 Hz, H6b), 3.55 (s, H18, 3H), 2.33 (m, H7b),
2.13 (m, H7a), 0.92 (t, J = 7.1 Hz, H8, 3H). 13C NMR (151 MHz,
acetone-d6/10% D2O, 25 °C) δ (ppm): 200.87 (C15), 162.41 (C10),
160.57 (C5), 156.31 (C19), 153.51 (C14), 150.34 (C23), 149.30
(C1), 139.69 (C21), 138.47 (C3), 136.99 (C12), 126.64 (C16),
125.35 (C2), 124.82 (C13), 123.14 (C4), 122.76 (C22), 122.23
(C11), 117.79 (C17), 112.02 (C20), 69.28 (C9), 67.92 (C6), 63.44
(C7), 36.40 (C18), 8.58 (C8). IR (νmax, cm‑1): 3507 (w), 3200 (s),
2362 (m), 1625 (m), 1450 (m), 1340 (m), 1200 (m), 831 (s) 767 (s),
551 (s). E1/2(IV/II), phosphate buffer pH = 7: 0.32 V versus SSCE.
UV−vis phosphate buffer pH = 7: λmax, nm (ε, M−1 cm−1) 266 (13
787), 347 (10 847). CH2Cl2: 272 (5929), 301 (3432), 358 (5067),
−
382 (4639). ESI-MS (m/z): 505.1 [M − H+− 2PF6 ]+.
cis-[RuIICl(CN-Me)(trpy)](PF6), cis-6·0.45CH2Cl2. A sample of 2
(100 mg, 0.218 mmol) and LiCl (18.48 mg, 0.436 mmol) were
dissolved in a 3:1 EtOH/H2O mixture (40 mL) under N2 atmosphere.
Then, NEt3 (0.06 mL, 0.436 mmol) was added, and the brown mixture
was stirred for 30 min at room temperature, upon which it
progressively became a dark-green solution. At this point, the
[HCN-Me]Br ligand (52.33 mg, 0.218 mmol) dissolved in 2 mL of
degassed 3:1 EtOH/H2O was added, and the resulting solution was
refluxed overnight. Upon cooling to room temperature, the solution
was filtered on a frit to eliminate small amounts of a black solid.
Afterward, 2 mL of a saturated NH4PF6 aqueous solution was added to
the solution, and the volume was reduced at low pressure until a
precipitate appeared. The resulting solid was filtered on a frit, washed
with water and ether, and dried in vacuum. The dark-brown solid
obtained was purified by chromatography over alumina using
dichloromethane as eluent and an increasing gradient of methanol
(0−1%). A first violet fraction corresponding to cis-6 and a second
brown fraction corresponding to its trans isomer were obtained,
together with an orange fraction corresponding to [Ru(trpy)2]2+
byproduct. After column chromatography, cis-6 was recrystallized
from a mixture of dichlomethane and ether (1:1, v/v). Anal. Found
(Calcd) for C24H20N6ClRuPF6·0.45CH2Cl2: C, 41.54 (41.23); N,
1
11.40 (11.79); H, 3.01 (2.96). H NMR (600 MHz, acetone-d6, 25
°C) δ (ppm): 8.74 (d, J = 8.1 Hz, H7, H10), 8.57 (d, J = 8.0 Hz, H4,
H13), 8.50 (d, J = 2.4 Hz, H24), 8.29 (t, J = 8.1 Hz, H9), 8.10 (d, J =
5.4 Hz, H1, H16), 7.99 (d, J = 8.3 Hz, H20), 7.92 (td, J = 7.8, 6.3 Hz,
H3, H14), 7.83 (d, J = 2.3 Hz, H23), 7.75 (tq, J = 8.7, 7.4, 1.5 Hz,
H19), 7.43 (dt, J = 5.8, 0.9 Hz, H17), 7.31 (tq, J = 7.4, 6.0, 1.3 Hz, H2,
H15), 6.88 (tq J = 7.2, 5.9, 1.2 Hz, H18), 4.63 (s, H25, 3H). 13C NMR
(151 MHz, acetone-d6, 25 °C) δ (ppm): 200.04 (C22), 159.83 (C5,
C12), 157.59 (C6, C11), 157.34 (C1, C16), 156,91 (C21), 152.44
(C17), 138.04 (C19), 137.47 (C3, C14), 136.38 (C9), 127.97 (C2,
C15), 126.96 (C23), 124.45 (C4, C13), 123.07 (C7, C10), 122.29
(C18), 117.09 (C24), 111.73(C20), 38.21 (C25). NOEs: H25 with
H16, H4 with H7, and H20 with H24. IR (νmax, cm−1): 1739 (m),
1610 (m), 1494 (m), 1251 (w), 1124 (m), 1091 (w), 836 (s), 771 (s),
555 (s). E1/2(III/II) (CH2Cl2 + 0.1 M TBAH): 0.77 V versus SSCE.
UV−vis (CH2Cl2): λmax, nm (ε, M−1 cm−1) 268 (19 824), 280 (14
742), 320 (19 331), 381 (5217), 498 (3898). ESI-MS (m/z): 529.1 [M
The 1H NMR spectroscopy was carried out on a a Bruker DPX 200
MHz or a Bruker AVANCE 600 MHz spectrometers. Samples were
run in acetone-d6 or a mixture of acetone-d6 and D2O, with internal
references (residual solvent protons and/or tetramethylsilane).
Elemental analyses were performed using a CHNS-O elemental
analyzer EA-1108 from Fisons. ESI-MS experiments were performed
on a Navigator LC/MS chromatograph from Thermo Quest Finnigan,
using acetonitrile as a mobile phase.
The 1H NMR photoisomerization study on complex cis-7 was
performed by dissolving the initial cis-7:trans-7 1:4 mixture in d6-
acetone containing 10% of D2O and irradiating the tube with an 80 W
lamp. A first spectrum was registered after 2 h, and the mixture was left
to evolve under light for 24 h, after which a final spectrum was
registered. For acid−base spectrophotometric titrations, 3−4 × 10−5
M buffered aqueous solutions of the complexes were used. The pH of
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Inorganic Chemistry
Article
the different solutions was adjusted by adding small volumes
(approximately 10 μL) of 4 M NaOH in order to produce a negligible
overall volume change. Redox spectrophotometric titrations were
performed by sequential addition of a (NH4)2[CeIV(NO3)6] 0.1 M
solution in HCl to an aqueous solution of the complex.
Weskamp, T.; Bohm, V. P. W. Adv. Organomet. Chem. 2002, 48, 1−
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X-ray Structure Determination. Measurements of the crystals
were performed on a Bruker Smart Apex CCD diffractometer using
graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å) from an
X-ray tube and examined using the following software: data collection,
Smart V.5.631 (BrukerAXS 1997-02); data reduction, Saint+ Version
6.36A (Bruker AXS 2001); absorption correction, SADABS version
2.10 (Bruker AXS 2001); and structure solution and refinement,
SHELXTL Version 6.14 (Bruker AXS 2000−2003). The crystallo-
graphic data as well as details of the structure solution and refinement
procedures are reported in Table 1. Data were deposited with the
CCDC for trans,fac-3, trans,fac-4, and cis-6. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.uk/data_request/cif.
Catalytic Studies. Experiments have been performed in anhydrous
dichloromethane at room temperature. In a typical run, Ru catalyst
(0.5 mM), alkene (50 mM), and PhI(OAc)2 (100 mM) were stirred at
room temperature in dichloromethane (2.5 mL) for 24 h. After the
addition of biphenyl (15 mM) as an internal standard, an aliquot was
taken for gas chromatography (GC) analysis. The oxidized products
were analyzed in a Shimadzu GC-17A gas chromatography apparatus
with a TRA-5 column (30 m × 0.25 mm diameter) incorporating a
flame ionization detector. GC conditions: initial temperature, 80 °C
for 10 min; ramp rate, 10 °C min−1; final temperature, 220 °C;
injection temperature, 220 °C; detector temperature, 250 °C; carrier
gas, He at 25 mL min−1. All catalytic oxidations were carried out under
a N2 atmosphere.
(6) (a) Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed Oxidations of
Organic Compounds; Academic Press: New York, 1981. (b) Hill, C. L.
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Washington, DC, 1990.
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T.; Benet-Buchholz, J.; Poyatos, M.; Llobet, A. J. Am. Chem. Soc. 2006,
128, 5306−5307.
ASSOCIATED CONTENT
* Supporting Information
Additional details, figures, and tables. Crystallographic data in
CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: montse.rodriguez@udg.edu (M.R.).
■
(8) Dakkach, M.; Fontrodona, X.; Parella, T.; Atlamsani, A.; Romero,
I.; Rodríguez, M. Adv. Synth. Catal. 2011, 353, 231−238.
(9) (a) Concepcion, J. J.; Jurss, J. W.; Templeton, J. L.; Meyer, T. J. J.
Am. Chem. Soc. 2008, 130, 16462−16463. (b) Gerli, A.; Reedijk, J.;
Lakin, M. T.; Spek, A. L. Inorg. Chem. 1995, 34, 1836−1843. (c) Hua,
X.; Shang, M.; Lappin, G. Inorg. Chem. 1997, 36, 3735−3740.
(10) (a) Evans, I. P.; Spencer, A.; Wilkinson, J. J. Chem. Soc., Dalton
Trans. 1973, 2, 204−209. (b) Alessio, E.; Mestroni, G.; Nardin, G.;
Attia, W. M.; Calligaris, M.; Sava, G.; Zorzet, S. Inorg. Chem. 1988, 27,
4099−4106. (c) Alessio, E. Chem. Rev. 2004, 104, 4203−4242.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research has been financed by MINECO of Spain through
Projects CTQ2010-21532-C02-01, CTQ2012-32436, and
CTQ2009-08328. Johnson & Matthey LTD are acknowledged
for a RuCl3·nH2O loan. M.D. thanks AECID (MAEC) of Spain
for the allocation of a MAEC-AECID grant.
(12) (a) Mola, J.; Romero, I.; Rodríguez, M.; Bozoglian, F.; Poater,
A.; Sola,
Inorg. Chem. 2007, 46, 10707−10716. (b) Mola, J.; Rodríguez, M.;
Romero, I.; Llobet, A.; Parella, T.; Poater, A.; Duran, M.; Sola, M.;
Benet-Buchholz, J. Inorg. Chem. 2006, 45, 10520−10529. (c) Sala, X.;
Poater, A.; von Zelewsky, A.; Parella, T.; Fontrodona, X.; Romero, I.;
̀
M.; Parella, T.; Benet-Buchholz, J.; Fontrodona, X.; Llobet, A.
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