The spectroscopic, electrochemical and structural characterization of a family of Ru complexes containing the C2-symmetric didentate chiral 1,3- oxazoline ligand and their catalytic activity

Article abstract of DOI:10.1002/ejic.200700368

The structural, spectroscopic, and electrochemical characterization and a preliminary catalytic investigation of a family of Ru^II complexes with general formula [Ru(Y) (terpy)(phbox-R)]ⁿ⁺ [where terpy is 2,2′:6′,2″-terpyridine, phbox

Full text of DOI:10.1002/ejic.200700368

FULL PAPER
DOI: 10.1002/ejic.200700368
The Spectroscopic, Electrochemical and Structural Characterization of a
Family of Ru Complexes Containing the C2-Symmetric Didentate Chiral 1,3-
Oxazoline Ligand and Their Catalytic Activity
Xavier Sala,[a] Naiara Santana,[a] Isabel Serrano,[a] Elena Plantalech,[a] Isabel Romero,[a]
Montserrat Rodríguez,*[a] Antoni Llobet,*[b,c] Susanna Jansat,[d] Montserrat Gómez,[d] and
Xavier Fontrodona[e]
Keywords: Ruthenium / N ligands / Epoxidation / Chiral ligands / Atropisomerism
The structural, spectroscopic, and electrochemical charac-
terization and a preliminary catalytic investigation of a family
of RuII complexes with general formula [Ru(Y)(terpy)(phbox-
hindered atropisomer, which is determined by the conforma-
tion of the stereogenic centers in the bidentate ligand. The
catalytic properties of the aquo complexes have been tested
R)]n+ [where terpy is 2,2Ј:6Ј,2ЈЈ-terpyridine, phbox-R is a C2- in epoxidation reactions, and moderate selectivity for the ep-
symmetric bidentate oxazoline ligand (R = Et or iPr), and Y
is a monodentate ligand], are discussed. The X-ray structure
of [RuCl(terpy)(phbox-iPr)]2+ has been solved and confirms,
as we described recently, the predicted generation of the less
oxides is obtained for styrene and trans-stilbene when using
PhI(OAc)2 as co-oxidant.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
Introduction
Transition metal complexes bearing oxazoline ligands
have shown good activities and enantioselectivities in a
myriad of catalytic organic syntheses involving metals such
as Cu or Fe.[10] However, the performance of oxazoline-con-
taining ruthenium complexes as catalysts has been devel-
oped only very recently, and the number of contributions
in this area is still limited.[11] Their performance as asym-
metric epoxidation catalysts with clean oxidants such as
H2O2, where they show good results in terms of both
chemo- and enantioselectivity, has been widely studied by
Beller and co-workers.[12]
Interest in polypyridylruthenium complexes stems from
their multiple applications in scientific fields such as pho-
tophysics and photochemistry,[13] catalysis,[14] and bioinor-
ganic chemistry.[15] One of the main goals for this type of
complex is the design of structures that allow the fine-tun-
ing of their redox, absorption, and emission properties.
Among redox properties, and especially from a catalytic
perspective, one of the major aims is the design of Ru=O
functional groups and analogues capable of reversibly ac-
cepting multiple electrons and protons within a relatively
narrow potential range.[16]
C2-Symmetric bis(1,3-oxazolines) have received an in-
creasing amount of attention over the last few years, as can
be seen from the increasing number of publications dealing
with this type of ligand.[1] Since the first reports of the use
of chiral oxazoline ligands in asymmetric catalysis,[2] a great
variety of oxazoline-based compounds have been synthe-
sized and successfully tested as catalysts,[3] and their coordi-
nation chemistry studied,[4] mainly because of their ease of
preparation and versatility.
Atropisomerism, where chirality is generated due to rota-
tional restriction between two isomers containing axially
chiral biaryl compounds,[5,6] is a good way of obtaining ef-
ficient tools in asymmetric synthesis[6a,7] (for example the
rigid molecular frameworks obtained from chiral ligands
like BINAP[8]), and has found important applications in di-
verse areas such as liquid crystals, pharmaceuticals, nano-
scale information storage, or biomimetic catalysis.[9]
[a] Departament de Química, Universitat de Girona, Campus de
Montilivi,
17071 Girona, Spain
[b] Institut Català d’Investigació Química (ICIQ),
Av. Paisos Catalans 16, 43007 Tarragona, Spain
[c] Departament de Química, Universitat Autònoma de Barcelona,
Cerdanyola del Vallès, 08193 Barcelona, Spain
[d] Departament de Química Inorgànica, Universitat de Barcelona,
Martí i Franquès 1–11, 08028 Barcelona, Spain
[e] Serveis Tècnics de Recerca, Universitat de Girona, Edifici P II,
Campus Montilivi,
With all this in mind, we present here the complete char-
acterization and catalytic activity of a family of ruthenium
complexes bearing C2-symmetric oxazoline ligands, includ-
ing two aquo complexes that are potential pre-catalysts for
redox processes involving the above mentioned Ru=O
group. The chirality of the complexes prepared is deter-
mined by the stereogenic centers in the oxazoline entities
17071 Girona, Spain
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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but also by the chiral axes generated upon coordination of terpy ligand coordinating in a meridional fashion through
the ligand, which leads to atropisomeric forms.
its N atoms. Two of the remaining coordination sites are
occupied by the didentate oxazoline ligand, whereas the
sixth coordinative position is occupied by a chlorido ligand.
The high distortion revealed in this cation is partly ex-
plained by the structural restrictions imposed by the terpy
ligand coordination, which narrows the N1–Ru–N3 bond
angle to 157.3°, far from the ideal value of 180° for a regu-
lar octahedral geometry.
Results and Discussion
Synthesis and Solid-State Structure
The complexes described here have the general formula
[Ru(Y)(terpy)(phbox-R)]n+, where phbox-R is one of the
two oxazoline ligands depicted in Figure 1 and Y is a
monodentate ligand [Cl, H2O, MeCN, pyridine (py), or 2-
hydroxypyridine (2-OH-py)]. The chlorido complexes
(RcRc,SaRa)-2a and (ScSc,RaSa)-2b were obtained by re-
duction of [RuCl3(terpy)] (1) in the presence of the appro-
priate bidentate oxazoline ligand, and the remaining com-
pounds were derived from these two complexes or their
aquo analogs (RcRc,SaRa)-3a and (ScSc,RaSa)-3b. The no-
menclature of the complexes, as explained in the Experi-
mental Section, involves a variety of chiral configurations
arising from (1) the stereogenic centers inherent to each ox-
azoline ligand (either R,R or S,S; see Figure 1), which are
represented by the first two letters having a “c” subscript,
and (2) the chiral axes generated upon coordination of the
ligand to the ruthenium metal center, as symbolized by the
two remaining letters of the nomenclature used, with an “a”
subscript. The two latter letters correspond to the chiral
axis involving the oxazolinyl ring trans to the terpy ligand
and that involving the ring trans to the monodentate ligand
Y, respectively. The formation of one or other isomer is
determined by the configuration of the oxazoline ligand
(see below).
Figure 2. ORTEP plot (ellipsoids at 50% probability) of the cat-
ionic moiety of complex (ScSc,RaSa)-2b, including the atom num-
bering scheme.
Table 1. Selected bond lengths and angles for complex (ScSc,RaSa)-
2b.
Bond lengths [Å]
Bond angles [°]
N1–Ru1–N2
Ru1–N1
2.042
1.935
2.126
2.162
2.035
2.416
79.26
157.34
98.95
90.97
78.87
170.06
87.31
103.62
93.86
82.93
87.28
92.71
87.90
96.98
178.21
Ru1–N2
Ru1–N3
Ru1–N4
Ru1–N5
Ru1–Cl1
N1–Ru1–N3
N1–Ru1–N4
N1–Ru1–N5
N2–Ru1–N3
N2–Ru1–N4
N2–Ru1–N5
N3–Ru1–N4
N3–Ru1–N5
N4–Ru1–N5
Cl1–Ru1–N1
Cl1–Ru1–N2
Cl1–Ru1–N3
Cl1–Ru1–N4
Cl1–Ru1–N5
Coordination of the phbox-iPr ligand, as described pre-
viously for analogous complexes,[17] generates a seven-mem-
bered ring which is unlikely to keep a planar conformation
because of structural restrictions. The ligand is thus forced
Figure 1. Atropisomers generated upon phbox-R ligand coordina-
tion.
The X-ray crystal structure for the cationic moiety of to bend in order to accommodate itself upon coordination,
complex (ScSc,RaSa)-2b is shown in Figure 2; Table 1 lists and this bending leads to two possible atropisomeric config-
selected bond lengths and angles. Further crystallographic urations depending on the final orientation of the central
information for this structure can be found in the Experi- phenyl ring of the phbox-iPr ligand, in other words over
mental Section. This cation presents a highly distorted octa- the N1 or N3 pyridyl rings of the terpy ligand (see Figure 1
hedral environment around the Ru metal center, with the for schematic drawings of the two possible conformations).
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Olefin Epoxidation Catalysts
The coordination of the ligand also fixes its two oxazolinyl Spectroscopic Properties
rings in a twisted position with regard to the central phenyl
ring plane, thereby generating the two chiral axes depicted
The UV/Vis spectra of complexes 2a,b–5a,b and 6b were
in Figure 1. The formation of one or the other atropisomer recorded in dichloromethane and/or phosphate buffer solu-
is governed by the configuration of the stereogenic centers tion. The main absorption wavelengths, together with their
(C20 and C31) of the oxazolinyl rings in such a way that assignments, are listed in Table 2. The assignment of the
only one atropisomer (the one that keeps the iPr groups spectral absorptions of RuII octahedral complexes has al-
farthest away from the terpy plane) is found. The X-ray ready been well documented[18] and, accordingly, the set of
structural analysis of complex (ScSc,RaSa)-2b shows that highly energetic bands at wavelengths below 350 nm can be
the atropisomer generated is indeed the ScSc,RaSa one, as assigned to πǞπ* intraligand transitions. Several charge-
expected for an S,S configuration of the oxazoline ligand. transfer dπ(Ru)Ǟπ* MLCT absorptions in the visible re-
The N4 and N5 oxazolinyl rings of the phbox-iPr ligand gion of the spectra are also present for this family of com-
are rotated by 51.47° and 40.26°, respectively, with regard pounds. These probably involve the π antibonding orbitals
to the central phenyl ring, and the N5–Ru–N4 angle is of terpy as the higher aromatic character of this ligand
82.93°.
lowers its orbitals’ energies with regard to the phbox-R li-
The terpy ligand is not perfectly planar and experiments gand.
a slight twofold bending, with the two peripheral N1 and
No significant differences can be found in Table 2 be-
N3 pyridyl rings pointing away from the plane of the central tween analogous complexes containing the phbox-Et or the
N2 ring (the torsion angles are 8.64° and 7.7°, respectively, phbox-iPr oxazoline ligands. However, the electronic nature
which gives an overall bending of 15.53° for the ligand). of the monodentate Y ligand influences the energies of the
The rotation of the N1 ring of the terpy ligand with regard transitions involving orbitals with d(Ru) character to some
to the central pyridyl ring in terpy places it in an almost extent. Thus, as expected, the π-donor character of the
parallel disposition with regard to the central phenyl ring chlorido ligand increases the energy of the d(Ru) filled orbit-
of the phbox-iPr ligand (the angle between these two rings als, with complexes (RcRc,SaRa)-2a and (ScSc,RaSa)-2b be-
is 19.91°).
ing the ones that show the most red-shifted MLCT bands.
Finally, the cationic moieties in the crystal interact with Complexes 5 and 6, which contain py and 2-OH-py ligands,
one another through a series of hydrogen bonds that involve respectively, show absorption energies similar to those
the chlorido ligand of one molecule and H7 and H13 of found for the corresponding aquo complexes 3a,b, therefore
two neighboring molecules, with contact distances of 2.65 it can be concluded that the electron-withdrawing π effects
and 2.85 Å, respectively. These interactions arrange the of the corresponding aromatic systems in the pyridine-type
complex molecules such that they form pseudo-square ligands are minimal. This is consistent with the rapid ligand
channels in the direction of the crystallographic b axis, rotation determined in solution from NMR experiments.[17]
which also allow a certain degree of π-stacking between the
Finally, a comparison of these complexes with structur-
terpy ligands of adjacent entities (see the Supporting Infor- ally similar Ru complexes also containing terpy reveals that,
mation for diagrams of these 3D interactions).
for complexes with bidentate oxazoline ligands,[19] MLCT
Table 2. UV/Vis spectroscopic data for complexes 2a,b–5a,b and 6b.
Compound
Solvent
CH2Cl2
λmax, nm (ε, –1 cm–1)
Assignment
(RcRc,SaRa)-2a
282 (20360), 324 (26359)
370 (5617), 524 (5534), 570 (5202)
284 (23139), 322 (29822)
368 (6468), 530 (5342), 562 (5585)
282 (20215), 318 (28681)
475 (sh), 518 (4935)
280 (21538), 316 (25538)
470 (sh), 518 (4745)
278 (28320), 318 (29792)
486 (4989), 520 (4950)
264 (17983), 316 (23760)
467 (3296), 525 (3948)
280 (19150), 315 (18540), 327 (17340)
383 (5970), 412 (5980), 541 (4730), 567 (sh)
276 (22429), 318 (30269)
370 (sh), 475 (sh), 518 (5162)
278 (26080), 318 (35425)
370 (sh), 470 (sh), 512 (5690)
276 (26120), 316 (30050)
377 (sh), 494 (3920), 534 (sh)
πǞπ*
dπǞπ*
πǞπ*
dπǞπ*
πǞπ*
dπǞπ*
πǞπ*
dπǞπ*
πǞπ*
dπǞπ*
πǞπ*
dπǞπ*
πǞπ*
dπǞπ*
πǞπ*
dπǞπ*
πǞπ*
(ScSc,RaSa)-2b
(RcRc,SaRa)-3a
CH2Cl2
CH2Cl2
phosphate buffer pH 7
CH2Cl2
(ScSc,RaSa)-3b
phosphate buffer pH 7
CH2Cl2
(ScSc,RaSa)-4b
(RcRc,SaRa)-5a
(ScSc,RaSa)-5b
(ScSc,RaSa)-6b
CH2Cl2
CH2Cl2
dπǞπ*
πǞπ*
dπǞπ*
CH2Cl2
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M. Rodríguez, A. Llobet et al.
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transitions generally take place at much lower energies than lower electron-donating capability of the py ligand with re-
those of analogous complexes having aromatic bidentate li- gard to the chlorido ligand. Pyridine ligands could, in prin-
gands,[20] thereby manifesting the increased electron-donor ciple, produce a π-accepting effect involving the empty anti-
capacity of the oxazoline entities.
bonding orbitals of the aromatic system, although this ap-
Acid-base titration was performed on complex pears to be negligible for this family of complexes as in-
(ScSc,RaSa)-3b by slow addition of NaOH to an acidic ferred, as described above, from the NMR and UV/Vis
aqueous solution of the complex [Equation (1)].
spectra. The π-acceptor nature of the monodentate acetoni-
trile ligand is significantly manifested in (ScSc,RaSa)-4b,
however, which presents a redox potential of 1.22 V.
[Ru(terpy)(phbox-iPr)(OH2)]2+ Ǟ [Ru(terpy)(phbox-iPr)(OH)]+ + H+
(1)
The 2-OH-py complex (ScSc,RaSa)-6b presents a surpris-
ingly low E1/2 value of 0.755 V in dichloromethane. Con-
sidering the relatively long Ru–N2-OH-py bond length in-
ferred from DFT calculations[17] (around 2.23 Å) one would
expect a redox potential somewhat higher than those found
for pyridine complexes 5a,b due to the decreased σ do-
nation. The OH group in the ring must therefore play a
determinant role in the redox behavior of the complex by
stabilizing the RuIII oxidation state through extra σ do-
nation, probably involving deprotonation of the ligand (the
pKa value for 2-OH-py is 0.75) and effective coordination
of the corresponding anionic, 2-pyridonyl tautomeric form.
The cyclic voltammograms of the aqua complexes
(RcRc,SaRa)-3a and (ScSc,RaSa)-3b were recorded in both
dichloromethane and aqueous phosphate buffer solution.
Only one redox process, corresponding to the RuIV/RuIII
redox couple, appears in CH2Cl2 at E1/2 = 0.98 V for both
compounds. The intensity decrease or even the disappear-
ance of the RuIII/RuII wave for this type of complex in
dichloromethane is assigned to kinetic factors and has been
described previously for similar complexes.[21] Two revers-
ible waves, corresponding to the RuIV/RuIII and RuIII/RuII
redox couples, can be observed in aqueous solution (pH 6)
at E1/2 = 0.64 and 0.46 V, respectively, for complex
(RcRc,SaRa)-3a and at E1/2 = 0.65 and 0.46 V, respectively,
for (ScSc,RaSa)-3b (the cyclic voltammogram recorded for
the latter is provided as Supporting Information).
The difference in the potential values of these two oxi-
dation processes in a specific Ru-aquo complex seems to be
directly related to the σ-donor and π-acceptor capacity of
the ancillary ligands, as has been described by Meyer and
co-workers[22a] on the basis of some empirical parameters
previously calculated by Lever.[22b] As shown by the au-
thors, two pseudo-linear relationships correlate the differ-
ence between the E1/2 value for the two oxidation processes,
∆E1/2 (IV/III – III/II), with an empirical coefficient ob-
tained from the ligands coordinated to the metal center
(Figure 4). Two groups of compounds can be clearly distin-
guished depending on the σ-donor or π-acceptor properties
of the ligands, denoted with squares and diamonds, respec-
The initial and final UV/Vis spectra, together with sev-
eral intermediate spectra (inset), are displayed in Figure 3.
The MLCT bands shift to longer wavelengths (520 and
518 nm) upon deprotonation of the aquo ligand, in agree-
ment with the relative destabilization of the dπ(Ru) levels
provoked by the anionic hydroxido ligand in a similar way
to that described above for the chlorido ligand.[20d]
Figure 3. UV/Vis spectra of the aquo and hydroxo forms of com-
plex (ScSc,RaSa)-3b at pH 3.33 and 12.23, respectively. The inset
shows a detail of three of the five isosbestic points found from acid-
base titration of the aquo complex with NaOH (pH values are 3.3,
8.81, 9.41, 9.60, 10.21, 10.48, 10.88, and 12.23).
The five isosbestic points at λ = 322, 335, 352, 424, and
556 nm suggest that the aquo and hydroxo species intercon-
vert in a net way, without generation of any side product.
Addition of p-toluenesulfonic acid at the end of the titration
regenerates the initial aquo complex. The pKa of 10.45 cal-
culated for the deprotonation of the aquo ligand from acid-
base titration is in good agreement with the value extracted
from its Pourbaix diagram (see below). The complex
(RcRc,SaRa)-3a behaves in an analogous manner, and the
UV/Vis spectra of its aquo and hydroxido forms are pre-
sented as Supporting Information.
Electrochemical and Catalytic Properties
The redox behavior of the complexes 2a,b–5a,b and 6b tively, in Figure 4.
was studied by cyclic voltammetry. The chlorido complexes
The relatively weak π-acceptor character of the oxazoline
(RcRc,SaRa)-2a and (ScSc,RaSa)-2b show a simple chemi- ligand (with an empirical Lever contribution of 0.21; see
cally and electrochemically reversible wave in acetonitrile, the Supporting Information for a detailed description of the
which corresponds to the RuIII/RuII couple, at E1/2 values calculations) in complexes (RcRc,SaRa)-3a and (ScSc,RaSa)-
of 0.70 and 0.71 V respectively. The analogous pyridine 3b places these complexes in the middle right half of the
complexes (RcRc,SaRa)-5a and (ScSc,RaSa)-5b undergo an graph, far away from the central zone for two-electron pro-
equivalent redox process at higher potential values (E1/2
=
cesses (the aquo complexes described here are represented
1.15 and 1.18 V, respectively), which is consistent with the by a dot in Figure 4).
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Olefin Epoxidation Catalysts
Ruthenium aquo complexes show a pH-dependence of
their redox potentials due to deprotonation of the aqua li-
gand bonded to the metal center [Equation (2)].
(2)
This dependence is usually reflected in so-called Pour-
baix diagrams, and the results obtained for complex
(ScSc,RaSa)-3b are displayed in Figure 6.
Figure 4. Meyer–Lever plot of ∆E1/2 vs. ΣEL. [where ∆E1/2 is the
difference between the E1/2(IV/III) and E1/2(III/II) redox potentials
for the family of complexes studied]. (1) [Ru(terpy)(acac)(OH2)]+;
(2) cis-[Ru(terpy)(pic)(OH2)]+ (pic = picolinate); (3) [Ru(terpy)-
(tmen)(OH2)]2+ (tmen
=
N,N,N,N-tetramethylethylenediamine);
(4) [Ru(CNC)(bpy)(OH2)]2+ (see ref.[27]); (5) [Ru(CNC)(nBuCN)-
(OH2)]2+ [27] (6) [Ru(terpy)(bpy)(OH2)]2+ (bpy = 2,2Ј-bipyridine);
(7) [Ru(bpy)2(OH2)(PPh3)]2+ (8) [Ru(terpy)(dppene)(OH2)]2+
;
;
[dppene = cis-1,2-bis(diphenylphosphanyl)ethylene].
Generation of the higher oxidation state species was fol-
lowed spectrophotometrically for the aquo complexes 3a,b.
Figure 5 shows the spectra obtained upon progressive ad-
dition of small amounts of CeIV to an aqueous solution
of complex (ScSc,RaSa)-3b at pH 1 until addition of one
equivalent. The five isosbestic points observed at λ = 270,
283, 297, 333, and 420 nm again suggest a net generation
of the RuIII species.
Figure 6. A plot of E1/2 vs. pH (Pourbaix diagram) for complex
(ScSc,RaSa)-3b. The pH/potential regions of stability for the various
oxidation states and their dominant proton compositions are indi-
cated by using abbreviations such as RuII–OH2, for example, for
[RuII(terpy)(phbox-iPr)(H2O)]2+. The pKa values are shown by the
vertical dashed lines in the various E/pH regions.
The two independent one-electron redox processes that
take place with simultaneous proton transfer at pH values
between 5 and 10.5 are assignable to RuIIǞRuIII and
RuIIIǞRuIV oxidations. The corresponding lines present a
slope of approximately 59 mV per pH unit, as expected for
a one-electron one-proton transfer; see Equation (2). At
lower pH, only the RuIII/II couple can be observed. The
diminishment or disappearance of the RuIV/III redox couple
in CV experiments is quite common for aquo complexes
and is assumed to be caused by slow heterogeneous elec-
tron-transfer kinetics from the solution to the electrode sur-
face.[23a,24] The stability regions for species having different
proton composition are indicated in the diagram along with
the pKa values for RuIII and RuII aquo complexes, which
are around 1.93 and 10.44. respectively. The latter value is
fully consistent with the value determined from acid-base
spectrophotometric titration, as described above.
Figure 5. UV/Vis spectra registered after addition of successive
amounts of CeIV (up to 1 equiv.) to a 0.14 m solution of complex
(ScSc,RaSa)-3b in 0.1  HClO4.
Addition of a second equivalent of CeIV produces a fur-
Complex (RcRc,SaRa)-3a behaves in a similar way to
ther oxidation of the complex that generates the corre- (ScSc,RaSa)-3b (the corresponding Pourbaix diagram is sup-
sponding RuIV-oxo species, which is almost featureless in plied as Supporting Information). In this case, the RuIV/III
the visible region, as has been described for analogous com- redox couple can be observed even at a pH lower than 4
plexes.[20d,23] This second redox process also takes place and the pKa values for the RuIII–OH2 and RuII–OH2 species
with the presence of isosbestic points, and the manifold of are 2.1 and 11.1, respectively.
spectra recorded is presented as Supporting Information to-
The redox catalytic properties of (ScSc,RaSa)-3b were
gether with the redox titration of complex (RcRc,SaRa)-3a, tested in the electrochemical oxidation of target substrates
which follows a similar trend.
such as benzyl alcohol and methyl p-tolyl sulfide in aqueous
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Table 3. Alkene epoxidation catalyzed by (ScSc,RaSa)-3b.[a]
Entry
Substrate
Oxidant
Substr./cat. % Conversion
% Epoxide yield % Bz[b] yield Catalytic cycles
1
2
3
4
styrene
trans-stilbene
styrene
PhI(OAc)2
PhI(OAc)2
PhIO
200:1
150:1
200:1
200:1
67.3
71
48.9
74.3
36.1
37.4
22.4
44
6.1
4.1
10.1
29.6
71.3
57.9
44.1
87.6
trans-stilbene
PhIO
[a] Reactions were performed at room temperature in 1,2-dichloroethane with an oxidant/substrate molar ratio of 2:1. [b] Bz stands for
benzaldehyde.
neutral media. The system “0.5 m 3b/50 m substrate/ and thoroughly characterized. The catalytic activity of the
pH 7 phosphate buffer” at an Eapp of 0.9 V generated the aquo complexes in epoxidation catalysis has been tested.
respective oxidation products (benzaldehyde and sulfoxide) These complexes show moderate activity and chemoselec-
with good selectivities but poor yields (50% and 13% tivity for the epoxide but with only low enantiomeric ex-
respectively), representing up to 50 and 13 catalytic cycles, cesses. We are now working towards the optimization of the
after seven hours. No ee was observed for the sulfoxide ob- reaction conditions by using clean oxidants such as H2O2,
tained from the prochiral sulfide substrate. The electrocata- and also towards a redesign of the catalysts’ structures in
lytic activity for the oxidation of benzyl alcohol is also vis- order to achieve higher enantioselectivities.
ible in the cyclic voltammogram, with an increase in the
anodic intensity at lower potential in the presence of the
Experimental Section
catalyst (see Figure S7 in the Supporting Information). The
electrocatalytic oxidation of benzyl alcohol was also tested
in a basic medium (0.1  NaOH; applied potential: 0.5 V);
conversions and selectivities were similar to those obtained
in a neutral medium.
Materials: All reagents used in the present work were obtained
from Aldrich Chemical Co and were used without further purifica-
tion. Reagent-grade organic solvents were obtained from SDS and
high purity deionized water was obtained by passing distilled water
through
a
nano-pure Mili-Q water purification system.
The catalytic activity of (ScSc,RaSa)-3b was also tested
with regard to its ability to epoxidize alkenes. Styrene and
trans-stilbene were chosen as substrates with PhIO and
PhI(OAc)2 as oxidants in 1,2-dichloroethane; the results,
which were quantified by GC chromatography, are reported
in Table 3. A first glance shows that the main products
formed are the corresponding epoxides (styrene oxide and
trans-stilbene oxide), along with minor amounts of benzal-
dehyde, in all cases. However, although good selectivity was
achieved for the desired products, the enantioselectivity val-
ues for these reactions were low (never higher than 10%
ee). The selectivity for the epoxide is slightly higher when
Ph(IOAc)2 is used as co-oxidant instead of PhIO (compare
for instance entries 2 and 4 in Table 3). trans-Stilbene also
provides better conversion and epoxide yields than styrene,
probably due to the tendency of the latter to undergo
double-bond cleavage and polymerization under oxidative
conditions.
A potential explanation for this low enantioselectivity
can be inferred from the complex structure. The approach
of a prochiral substrate to the active site of the catalysts
takes place in the region of the planar terpy ligand, there-
fore the stereogenic center in the oxazoline ligand only has
a very small effect on the orientation of the substrate (see
Figure 2 and the Supporting Information for different
structural views of the molecular catalyst). Thus, the cata-
lyst/substrate Interaction can occur through multiple orien-
tations, thereby leading to racemic mixtures of reaction
products.
RuCl3·2H2O, was supplied by Johnson–Matthey Ltd. and was used
as received.
Instrumentation and Measurements: UV/Vis spectra were recorded
with a Cary 50 Scan (Varian) UV/Vis spectrophotometer in 1-cm
quartz cells. pH measurements were recorded with a Micro-pH-
2000 from Crison.
Cyclic voltammetry (CV) experiments were performed in a PAR
263A EG&G potentiostat or an IJ-Cambria IH-660 potentiostat,
using a three-electrode cell. Glassy carbon disk electrodes (3 mm
diameter) from BAS were used as the working electrode, platinum
wire as the auxiliary electrode, and SSCE as the reference electrode.
Cyclic voltammograms were recorded at a scan rate of 100 mVs–1
under either N2 or Ar. The complexes were dissolved in previously
degassed solvents containing the necessary amount of supporting
electrolyte to yield a 0.1  ionic strength solution. (nBu4N)(PF6)
was used as supporting electrolyte when using acetonitrile and
dichloromethane as solvents. In aqueous solutions the pH was ad-
justed from 0 to 2 with HCl. Sodium chloride was added to keep
a minimum ionic strength of 0.1 . From pH 2 to pH 10, 0.1 
phosphate buffers were used, and from pH 10 to pH 12 dilute, CO2-
free NaOH was used. All E1/2 values reported in this work were
estimated from cyclic voltammetry as the average of the oxidative
and reductive peak potentials (Ep,a + Ep,c)/2. Unless explicitly men-
tioned, the concentration of the complexes was approximately
1 m.
Catalytic Experiments: Chemical catalysis was performed using a
catalyst:substrate molar ratio of between 1:150 to 1:200, with an
oxidant:substrate ration of 2:1, in 1,2-dichloroethane, with a typical
catalyst concentration of 1 m. Electrocatalytic studies at neutral
pH were performed in a phosphate buffer solution, at an applied
potential of 0.9 V vs. SSCE. At basic pH, a 0.1  NaOH solution
was used as solvent and the potential applied was 0.5 V vs. SSCE.
The substrate:catalyst molar ratio was 100:1 in both cases. A car-
bon-felt electrode from SOFACEL was used as the working elec-
Conclusion
A new family of ruthenium complexes bearing terpy, ox-
azoline, and a monodentate ligand has been synthesized trode. Evolution of the oxidized products was followed with a Shi-
5212 Eur. J. Inorg. Chem. 2007, 5207–5214
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Olefin Epoxidation Catalysts
madzu GC-17A gas chromatography apparatus after chromatog-
raphy of an aliquot of the reaction mixture over alumina and fur-
ther re-dissolution in a 20 m biphenyl solution in diethyl ether,
which was used as an internal standard.
Acknowledgments
This research was financed by the Spanish Ministerio de Educación
y Ciencia (MEC) (projects BQU2003-02884, BQU2002-04112-C02-
02, and CTQ2004-01546/BQU). A. L. and M. G. are grateful to
the Consell Interdepartamental de Recerca i Innovació Tecnològica
(CIRIT), Generalitat de Catalunya, for financial support, and
A. L. for the aid SGR2001-UG-291. A. L., M. R., and I. R. also
thank Johnson Matthey for a loan of RuCl3·xH2O. E. P. and X. S.
are grateful for the award of doctoral grants from CIRIT and Uni-
versitat de Girona (UdG), respectively.
X-ray Structure Determination: Suitable crystals of (RcRc,SaRa)-2b
were grown by slow diffusion of diethyl ether into a dichlorometh-
ane solution of the complex at room temperature. They were ob-
tained as black plates. The crystal was mounted on a nylon loop
and intensity data were collected at low temperature [100(2) K]
with a Bruker Smart Apex CCD diffractometer using graphite-mo-
nochromated Mo-Kα radiation (λ = 0.71073 Å) in the θ range 2.12–
28.44°. Full-sphere data collection was carried out with ω and φ
scans. A total of 48274 reflections were collected, of which 7801
(Rint = 0.1281) were unique. Programs used: data collection: Smart
version 5.625 (Bruker AXS 1997-01); data reduction: Saint+ ver-
sion 6.36A (Bruker AXS 2001); absorption correction: SADABS
version 2.05 (Bruker AXS 2001). The structures were solved by
direct methods and refined by full-matrix least-squares methods on
F2 with SHELXTL Version 6.12 (Bruker AXS 2001). The non-
hydrogen atoms were refined anisotropically. All H-atoms were
placed in geometrically optimized positions and forced to ride on
the atom to which they are attached.
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2
CCDC-635355 contains the supplementary crystallographic data
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The nomenclature used for the absolute configuration of the com-
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lute configuration of the oxazoline ligand and the final two letters
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detailed and exemplified in Figure 1 for the chlorido complex 2b.
Thus, the complexes discussed here are (RcRc,SaRa)-[RuCl(terpy)-
(phbox-et)](BF4)
[(RcRc,SaRa)-2a],
(ScSc,RaSa)-[RuCl(terpy)-
(phbox-iPr)](BF4) [(ScSc,RaSa)-2b], (RcRc,SaRa)-[Ru(terpy)(phbox-
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iPr)(OH2)](BF4)2 [(ScSc,RaSa)-3b], (ScSc,RaSa)-[Ru(terpy)(phbox-
iPr)(MeCN)](PF6)2
[(ScSc,RaSa)-4b],
(ScSc,RaSa)-[Ru(terpy)-
(phbox-et)(py)](PF6)2 [(RcRc,SaRa)-5a], (ScSc,RaSa)-[Ru(terpy)-
(phbox-iPr)(py)](PF6)2 [(ScSc,RaSa)-5b], and (ScSc,RaSa)-[Ru-
(terpy)(phbox-iPr)(2-OH-py)](PF6)2 [(ScSc,RaSa)-6b].
Supporting Information (see also the footnote on the first page of
this article): Additional spectroscopic and electrochemical data for
the complexes described in this work.
Eur. J. Inorg. Chem. 2007, 5207–5214
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5213

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Received: April 4, 2007
Published Online: September 28, 2007
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© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 5207–5214