New Ru(II) complexes containing oxazoline ligands as epoxidation catalysts. Influence of the substituents on the catalytic performance

Article abstract of DOI:10.1021/ic200053f

The synthesis of a family of new Ru complexes containing the facial tridentate ligand with general formula [Ru^II(T)(D)(X)]ⁿ⁺ (T = trispyrazolylmethane (tpm); D = ((4S,4′S)-(-)-4,4′,5,5′- tetrahydro-4,4′-bis(1-methylethyl)-2,2′-bioxazole) (iPr-box-C) or N-(1-hydroxy-3-methylbutan-(2S)-(-)-2-yl)-(4S)-(-)-4-isopropyl-4, 5-dihydrooxazole-2-carbimidate (iPr-box-O); X = Cl, H₂O) has been described. All complexes have been spectroscopically characterized in solution through ¹H NMR and UV-vis techniques, and the redox properties of complexes have also been studied by means of cyclic voltammetry (CV). Furthermore, the chloro complexes presented here have been characterized in the solid state through monocrystal X-ray diffraction analysis. The oxazolinic iPr-box-C ligand undergoes a Ru-assisted hydrolysis reaction generating the corresponding amidate anionic ligand iPr-box-O, that keeps coordinated to the Ru metal center and that produces a strong σ-donation effect over it. The reactivity of the Ru-OH₂ complexes described in this paper together with other similar ones, previously synthesized by us, has been tested with regard to the epoxidation of different olefins. Complexes [Ru ^II(R-box-C)(tpm)OH₂](BF₄)₂, R = Bz, 3′c/iPr, 3c, show high stereoselectivity in the epoxidation of cis-β-methylstyrene, with the exclusive formation of the cis-epoxide. However, there is a significant difference in regioselectivity between the two catalysts in the epoxidation of 4-vinylcyclohexene; complex 3′c leads to the regioselective oxidation at the ring alkene position, whereas complex 3c leads to the oxidation at the terminal position. Computational calculations indicate only small energy differences between the two possible products of 4-vinylcyclohexene epoxidation, but the energy barriers for the interaction of the catalytic systems with the alkene groups of 4-vinylcyclohexene agree with the reactivity differences found for the two catalysts having isopropyl or benzyl as substituent of the oxazole ligand. Computed local Fukui functions help to explain the observed reactivity trends.

Full text of DOI:10.1021/ic200053f

ARTICLE
pubs.acs.org/IC
New Ru(II) Complexes Containing Oxazoline Ligands As Epoxidation
Catalysts. Influence of the Substituents on the Catalytic Performance
†
†
‡
§
†
† ꢀ
Isabel Serrano, M. Isabel Lꢀopez, Ingrid Ferrer, Albert Poater, Teodor Parella, Xavier Fontrodona,
Miquel Solꢁa,† Antoni Llobet,§, Montserrat Rodríguez,*,† and Isabel Romero*,†
||
†Departament de Química, Serveis Tꢁecnics de Recerca and Institut de Química Computacional, Universitat de Girona,
Campus de Montilivi, E-17071 Girona, Spain
‡Catalan Institute for Water Research (ICRA), H2O Building, Scientific and Technological Park of the University of Girona,
Emili Grahit 101, E-17003 Girona, Spain
§Departament de Química and Servei de RMN, Universitat Autꢁonoma de Barcelona, E-08193 Bellaterra, Barcelona, Spain
Institute of Chemical Research of Catalonia (ICIQ), Av. Països Catalans 16, E-43007 Tarragona, Spain
S Supporting Information
b
ABSTRACT: The synthesis of a family of new Ru complexes containing the facial
tridentate ligand with general formula [RuII(T)(D)(X)]nþ (T = trispyrazolyl-
methane (tpm); D = ((4S,40S)-(-)-4,40,5,50-tetrahydro-4,40-bis(1-methylethyl)-
2,20-bioxazole) (iPr-box-C) or N-(1-hydroxy-3-methylbutan-(2S)-(-)-2-
yl)-(4S)-(-)-4-isopropyl-4,5-dihydrooxazole-2-carbimidate (iPr-box-O); X = Cl,
H2O) has been described. All complexes have been spectroscopically character-
1
ized in solution through H NMR and UVꢀvis techniques, and the redox
properties of complexes have also been studied by means of cyclic voltammetry (CV). Furthermore, the chloro complexes presented
here have been characterized in the solid state through monocrystal X-ray diffraction analysis. The oxazolinic iPr-box-C ligand
undergoes a Ru-assisted hydrolysis reaction generating the corresponding amidate anionic ligand iPr-box-O, that keeps coordinated
to the Ru metal center and that produces a strong σ-donation effect over it. The reactivity of the RuꢀOH2 complexes described in
this paper together with other similar ones, previously synthesized by us, has been tested with regard to the epoxidation of different
olefins. Complexes [RuII(R-box-C)(tpm)OH2](BF4)2, R = Bz, 30c/iPr, 3c, show high stereoselectivity in the epoxidation of cis-β-
methylstyrene, with the exclusive formation of the cis-epoxide. However, there is a significant difference in regioselectivity between
the two catalysts in the epoxidation of 4-vinylcyclohexene; complex 30c leads to the regioselective oxidation at the ring alkene
position, whereas complex 3c leads to the oxidation at the terminal position. Computational calculations indicate only small energy
differences between the two possible products of 4-vinylcyclohexene epoxidation, but the energy barriers for the interaction of the
catalytic systems with the alkene groups of 4-vinylcyclohexene agree with the reactivity differences found for the two catalysts having
isopropyl or benzyl as substituent of the oxazole ligand. Computed local Fukui functions help to explain the observed reactivity
trends.
’ INTRODUCTION
A particularly interesting family of redox catalysts is the so-
called RuꢀOH2 type of complexes. These Ru-aquo complexes
can easily lose protons and electrons and reach higher oxidation
states as exemplified below, where L5 represents polypyridylic
type of ligands,
Metal-catalyzed oxidation is one of the most important atom
transfer reactions in chemistry and biology.1 Olefin epoxidation
has received considerable interest from both academics and in-
dustry; specifically, enantiomerically pure epoxides play an
eminent role as intermediates and buildings blocks in organic
synthesis and materials science.2 Although numerous procedures
have been developed, the need for understanding the mechan-
isms of metal-mediated oxygen processes demands the synthesis
of new, stable, and easily available catalysts.
In the field of redox catalysis, the relationship between per-
formance and structure of a catalyst is further complicated by the
presence of multiple redox state species involved in the catalytic
cycle. Therefore, the thermodynamic and kinetic characteriza-
tion of the reactions that undergo the different oxidation state
species of a particular catalyst is of paramount importance to
understand and optimize its performance.
ꢀHþ ꢀ eꢀ
h
ꢀHþ ꢀ eꢀ
h
L5RuIVdO
L5RuIIꢀOH2
L5RuIIIꢀOH
þHþ þ eꢀ
þHþ þ eꢀ
The sequential loss of protons and electrons allows to easily
reach reactive RuIVdO species and as a consequence these
Ru complexes have been widely used as redox catalysts for
the oxidation of both organic and inorganic substrates. A large
amount of literature has emerged during the past two decades
related to this system, mainly because of the rich oxidative
Received: January 9, 2011
Published: June 08, 2011
r
2011 American Chemical Society
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|

Inorganic Chemistry
ARTICLE
Chart 1. Drawing of Ligands Used in This Work
properties of the RuIVdO species. In addition, the reaction
mechanisms for the oxidation of several substrates by RuIVdO
have been thoroughly described together with the establishment
and optimization of catalytic processes.3
The redox potentials and, therefore, the performance of a
RuꢀOH2 catalyst can be fine-tuned through the addition of
electron donating or withdrawing groups to the ligands. These
are expected to respectively decrease and increase the Ru(IV)/
Ru(III) and Ru(III)/Ru(II) redox potentials and thus allow to
generate a family of catalysts with a controlled reactivity.4 In
addition to the electronic effects, examples also exist where
sterically hindered ligands strongly influence the reactivity of a
RudO system with regard to substrate oxidation.5
Oxazoline-containing ligands have been shown to be a con-
venient choice because of their easy synthetic accessibility, mod-
ular nature, and applicability in a wide range of metal-catalyzed
transformations.6ꢀ8 Recently, we have reported a family of new
Ru complexes containing tridentate and oxazolinic ligands to-
gether with the preliminary studies of their epoxidation catalytic
activity.9 Our results have shown that the reactivity of the Ruꢀ
OH2 complexes is practically independent of the redox potentials
of the catalyst but that it is strongly dependent on the geometry
of the tridentate ligands (meridional, trpy, or facial, tpm). Given
these results, we have now focused our attention on the
oxazolinic ligand with the aim of studying the influence of its
substituents on the catalytic performance of the complexes.
In the present paper we report the synthesis, structure, spec-
troscopy, and redox properties of new Ru complexes containing
neutral and anionic oxazolinic ligands, iPr-box-C and iPr-box-O,
together with the facial tridentate tpm ligand, Chart 1. The
catalytic performance of this family of complexes has been tested
with regard to the epoxidation of alkenes and is reported here
together with the activity of other oxazolinic complexes pre-
viously synthesized in our group for purposes of comparison.
The predictions based on computed energy barriers agree with
the distinctive catalytic results experimentally found for two
catalysts that bear different substituents (isopropyl or benzyl)
at the oxazole ring.
Figure 1. ORTEP view (ellipsoids are drawn at 50% probability level)
of the molecular structure of cation 2c (a) and neutral complex 2o (b)
including the atom numbering scheme.
Mili-Q water purification system. RuCl3 2H2O, was supplied by John-
3
son and Matthey Ltd. and was used as received.
Preparations. Ligands (see Chart 1) iPr-box-C ((4S,40S)-(-)-
4,40,5,50-tetrahydro-4,40-bis(1-methylethyl)-2,20-bioxazole)10 and tpm
(tris-pyrazolylmethane)11 as well as the complexes 112 and 30c/30o9
were prepared as described in the literature. All synthetic manipulations
were routinely performed under nitrogen atmosphere using Schlenk
tubes and vacuum line techniques. Electrochemical experiments were
performed under either N2 or Ar atmosphere with degassed solvents.
[RuIICl(iPr-box-C)(tpm)](BF4) 1.7(CH3CH2)2O, 2c 1.7-
3
3
(CH3CH2)2O, and [RuIICl(iPr-box-O)(tpm)] 2.5CH3OH, 2o
3
3
2.5CH3OH. A sample of 1 (100 mg, 0.24 mmol) was added to a
100 mL round bottomed flask containing a degassed solution of LiCl (20
mg, 0.47 mmol) in EtOH:H2O 3:1 (40 mL), under magnetic stirring.
Then, NEt3 (0.07 mL) was added, and the reaction mixture was stirred at
room temperature (RT) for 30 min. At this point, iPr-box-C (53 mg,
0.24 mmol) was added, and the mixture was then heated at reflux for 5 h.
The hot solution was filtered off in a frit, and the volume was reduced to
dryness in a rotary evaporator. The solid obtained was then dissolved in
CH2Cl2 and washed several times with water. The organic phase was
then dried over MgSO4, and the mixture again reduced to dryness. The
solid obtained in this manner was a mixture of chloro complexes 2c and
2o that were separated by column chromatography (alumina; CH2Cl2/
MeOH 95:5). Complex 2c was dissolved in MeOH (10 mL), and a
saturated aqueous solution of NaBF4 (1.5 mL) was added. The red solid
obtained was filtered off in a frit and recrystallized from a hot mixture of
’ EXPERIMENTAL SECTION
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 deion-
ized water was obtained by passing distilled water through a nanopure
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Inorganic Chemistry
ARTICLE
CH2Cl2/ether (1:1). Chloro complex 2o was recrystallized in methanol-
diethylether solution. For 2c, yield: 45 mg (24%). Anal. Found (Calcd.)
Table 1. Crystal Data for Complexes 2c and 2o
2c
2o
for C22H30N8O2ClRuBF4 1.7C4H10O: C, 43.91(43.91), N, 13.88
3
(14.22), H, 5.62 (6.01). IR: ν = 1503 cmꢀ1 (CN). 1H NMR (CDCl3,
500 MHz, 298 K): δ 9.64 (H4, s), 8.63 (H5, d, 2.4 Hz), 8.51 (H3, d, 2.4
Hz), 8.47 (H8, d, 2.4 Hz), 8.22 (H1, d, 1.9 Hz), 8.20 (H10, d, 1.9 Hz),
7.13 (H7, d, 1.7 Hz), 6.50 (H2, H9, m), 6.46 (H6, t, 2.5 Hz), 4.98 (H12b,
t, 9.5 Hz), 4.89 (H18, d, 8.6 Hz), 4.86 (H12a, dd, 6.2 and 9.5 Hz), 4.53
(H13, dddd, 2.7, 6.2, and 9.5 Hz), 4.21 (H19, dt, 9.0 and 3.4 Hz), 2.45
(H20, m), 1.23 (H14, m), 1.17 (H21, d, 6.6 Hz), 0.94 (H22, d, 7.1 Hz),
0.73 (H15, d, 7.6 Hz), 0.46 (H16, d, 6.9 Hz). 13C NMR (CDCl3, 500
MHz, 298 K): δ 158.6 ppm (C11,C17), 148.4 (C1), 146.8 (C10), 143.9
(C7), 135.4 (C5), 134.6 (C3, C8), 108.9 (C2, C9), 108.5 (C6), 75.4
(C4), 72.9 (C12), 72.2 (C18), 71.1 (C19), 69.9 (C13), 28.6 (C20), 19.0
(C14), 18.9 (C22), 18.7 (C15), 14.5 (C21), 14.4 (C16). E1/2 (CH2Cl2)
= 0.79 V vs SSCE. UVꢀvis (CH2Cl2): λmax, nm (ε, Mꢀ1 cmꢀ1) 2922
(89216), 326 (sh 6505), 464 (5204). For 2o, yield: 43 mg (31%). Anal.
empirical formula
Fw
C
22H30N8O2ClRuBF4
C25H39N8O5ClRu
668.16
661.87
monoclinic
P21
cryst system
space group
a, Å
monoclinic
P21
7.497(2)
16.765(5)
11.495(4)
90
7.7404(18)
17.076(4)
11.628(3)
90
b, Å
c, Å
R, deg
β, deg
90.351(5)
90
97.103(4)
90
γ, deg
V, Å3
1444.6(8)
2
1525.2(6)
2
formula units/cell
temp, K
λ(MoꢀKR), Å
300(2)
0.71073
1.522
100(2)
Found (Calcd.) for C22H31N8O3ClRu 2.5 CH3OH: C, 43.78 (43.83),
3
N, 16.67 (16.65), H, 6.15 (6.44). IR: ν = 1568 cmꢀ1 (CN). E1/2
0.71073
1.455
(CH2Cl2) = 0.22 V vsSSCE. UVꢀvis (CH2Cl2): λmax, nm (ε, Mꢀ1 cmꢀ1
)
F
calc., g cmꢀ3
a
285 (8400), 312 (7350), 413 (sh, 4500).
R1
0.0287
0.0719
0.0374
For the NMR assignment we used the same numbering scheme as
that for the X-ray structures displayed in Figure 1.
b
wR2
0.0741
a R1 = ∑||Fo| ꢀ |Fc||/∑|Fo|. b wR2 = [∑w(Fo ꢀ Fc2)2/∑w(Fo ) ]
,
2
2 2 1/2
[RuII(iPr-box-C)(tpm)OH2](PF6)2 0.8 H2O, 3c 0.8H2O. Asam-
2
2
where w = 1/[σ2(Fo )þ(0.0377P)2þ1.65P] and P = (Fo þ 2Fc2)/3.
3
3
ple of AgPF6 (16.9 mg, 0.067 mmol) was added to a solution of acetone:
H2O 1:2 (20 mL) containing 2c (40 mg, 0.051 mmol), and the mixture
was heated at reflux for 1.5 h. The precipitate of AgCl formed was filtered
off through a frit containing Celite, and the volume reduced in a rotary
evaporator under reduced pressure until a brown precipitate appeared.
The solid obtained was filtered through a frit, washed with cold water, and
dried with ether. Yield: 41.6 mg (95%). Anal. Found (Calcd.) for
ICH-660 potentiostat using a three electrode cell. Glassy carbon disk
electrodes (3 mm diameter) from BAS were used as working electrodes,
platinum wire as auxiliary, and SSCE as the reference electrode. All cyclic
voltammograms presented in this work were recorded at a 100 mV/s
scan rate under nitrogen atmosphere. The complexes were dissolved in
previously degassed solvents containing the necessary amount of
supporting electrolyte to yield a 0.1 M ionic strength solution. In
dichloromethane, (n-Bu4N)(PF6), TBAH, was used as supporting
electrolyte. In aqueous solutions the pH was adjusted from 0 to 2 with
HCl, and potassium chloride was added to keep a minimum ionic
strength of 0.1 M. From pH 2ꢀ10, 0.1 M phosphate buffers were used,
and from pH 10ꢀ12 diluted, CO2 free, NaOH. 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 þ Ep,c)/2. Unless
explicitly mentioned the concentration of the complexes was approx-
imately 1 mM.
C22H32N8O3RuP2F12 0.8H2O: C, 31.02 (30.66), N, 12.79 (13.00), H,
3
4.22 (3.93). IR: ν = 1507 cmꢀ1 (CN). 1H- NMR (d6-acetone, 600 MHz,
298 K): δ 9.67 (H4, s), 8.67 (H5, d, 2.8 Hz), 8.58 (H3, d, 2.1 Hz), 8.51
(H8, d, 1.9 Hz), 8.44 (H1, d, 1.9 Hz), 8.14 (H10, d, 1.9 Hz), 6.81 (H7, t,
2.5 Hz), 6.78 (H2, H9, m), 6.63 (H6, t, 2.5 Hz), 5.50 (H2O), 5.20ꢀ5.12
(H12b, H18, H12a, m), 4.87 (H13, m), 4.3 (H19, m), 2.55 (H20, m),
1.20 (H14, m), 1.07 (H21, d, 6.7Hz), 0.99 (H22, d, 7.6 Hz), 0.73 (H15, d,
7.0 Hz), 0.53 (H16, d, 7.0Hz). E1/2 (phosphatebufferpH= 7) = 0.38V vs
SSCE. UVꢀvis (phosphate buffer pH = 7): λmax, nm (ε, Mꢀ1 cmꢀ1) 275
(1863), 308 (sh, 1517), 421 (1327). UVꢀvis (CH2Cl2): λmax, nm
(ε, Mꢀ1 cmꢀ1) 277 (9810), 311 (8049), 417 (7070).
[RuII(iPr-box-O)(tpm)OH2](PF6) 1.1H2O, 3o 1.1H2O. A sam-
NMR experiments were carried out on Bruker 500MHz and 600MHz
spectrometers. Samples were run in d6-acetone or CDCl3, and spectra
were calibrated using the residual solvent and/or tetramethysilane
signals. Elemental analyses were performed using a CHNS-O Elemental
Analyzer EA-1108 from Fisons.
3
3
ple of AgPF6 (23.8 mg, 0.094 mmol) was added to 15 mL of H2O
containing 2o (49 mg, 0.073 mmol) and a few crystals of ascorbic acid,
and the mixture was heated at reflux for 3 h under N2 atmosphere. AgCl
was filtered off through a frit containing Celite, and the volume reduced in
a rotary evaporator until the appearance of a yellow precipitate. The solid
obtained was filtered and washed with diethylether. Yield: 38.2 mg (71%).
Catalytic Oxidation Experiments. Catalytic essays have been
performed in dichloromethane dried over CaH2 at room temperature
(RT). In a typical run, ruthenium catalyst (0.002 mmol), alkene
(0.2 mmol), and PhI(OAc)2 (0.4 mmol) were stirred at RT in
dichloromethane (2.5 mL) for 24 h. The end of the reaction was
indicated by the disappearance of solid co-oxidant. After addition of an
internal standard, an aliquot was taken for GC analysis. The oxidized
products were analyzed in a Shimadzu GC-2010 gas chromato-
graphy apparatus equipped with an Astec CHIRALDEX G-TA Column
(10 m ꢁ 0.25 mm diameter) incorporating a FID detector. GC
conditions: initial temperature 80 °C for 10 min, ramp rate 10°/min,
final temperature 170 °C, injection temperature 220 °C, detector
temperature 250 °C. All catalytic oxidations were carried out under
nitrogen atmosphere.
Anal. Found (Calcd.) for C22H33N8O4Ru 1.1 H2O: C, 30.19 (29.88), N,
3
1
12.51 (12.67), H, 4.22 (4.01). IR: ν = 1590 cmꢀ1 (CN). H NMR
(d6-acetone, 600 MHz, 298 K): δ 9.62 (H4, s), 8.62 (H5, d, 2.8 Hz), 8.59
(H8, d, 2.8 Hz), 8.54 (H3, d, 2.3 Hz), 8.47 (H7, d, 2.3 Hz), 8.24 (H10, d,
2.8 Hz), 7.86 (H1, d, 2.3 Hz), 6.74 (H6, t, 2.5 Hz), 6.69 (H9, t, 2.5 Hz),
6.62 (H2, t, 2.5 Hz), 4.83 (H12, broad s), 4.64 (H13, ddd, 3.3, 7.6, and 8.8
Hz), 1.50 (H14, m), 0.73 (H15, d, 7.2 Hz), 0.60 (H16, d, 6.4 Hz). E1/2
(phosphate buffer pH = 7) = 0.22 V vs SSCE. UVꢀvis (phosphate buffer
pH = 7): λmax, nm (ε, Mꢀ1 cmꢀ1) 289 (1947), 323 (2033), 400 (sh).
Instrumentation and Measurements. IR spectra were re-
corded on a Mattson Satellite FT-IR using a MKII Golden Gate Single
Reflection ATR System. UVꢀvis spectroscopy was performed in a Cary
50 Scan (Varian) UVꢀvis spectrophotometer with 1 cm quartz cells.
pH measurements were done using a Micro-pH-2000 from Crison.
Cyclic voltammetry (CV) experiments were performed in a IJ-Cambria
X-ray Structure Determinations. Suitable crystals of 2c and 2o
were grown by slow diffusion of ether into a MeOH solution as dark red
needles or plates.
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Inorganic Chemistry
ARTICLE
Scheme 1. Synthetic Pathways and Labeling Scheme for Complexes Described in This Work
Data Collection. Intensity data for 2o and 2c were collected at 100 and
300 K respectively on a BRUKER SMART APEX CCD diffractometer
using graphite-monochromated Mo KR radiation from an X-ray Tube.
Full-sphere data collection was carried out with ω and j scans.
Structure Solution and Refinement. The structure was solved by
direct methods and refined by full-matrix least-squares methods on F2
using the Shelxs-97 and Shelxl-97 softwares, respectively. The non-
hydrogen atoms were refined anisotropically. The H-atoms were placed
in geometrically optimized positions and forced to ride on the atom to
which they were attached, except for the water molecule H atoms of 2o
which were placed on the difference electron density map and refined
with an antibumping restraint of 1.50(1) Å.
The crystallographic data as well as details of the structure solution
and refinement procedures are reported in Table 1. CCDC 827784 and
827785 contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from the Cambridge Crystal-
lographic Data Centre via www.ccdc.cam.ac.uk/conts/retrieving.html.
Computational Details. The density functional calculations were
performed on all the systems at the GGA level with the Gaussian09 set of
programs.13 The density functional theory (DFT) calculations were
carried out with the functional BP86 with the gradient corrections taken
from the work of Becke and Perdew.14 The electronic configuration of
the molecular systems was described by the standard SVP basis set,
that is, the split-valence basis set with polarization functions of Ahlrichs
and co-workers, for H, C, N, and O.15 For Ru we used the
small-core, quasi-relativistic Stuttgart/Dresden effective core potential
(standard SDD basis set in Gaussian 09) basis set, with an associated
(8s7p6d)/[6s5p3d] valence basis set contracted according to a (311111/
22111/411) scheme.16
The geometry optimizations were performed without symmetry
constraints, and the nature of the extrema was checked by analytical
frequency calculations. Furthermore, all the extrema were confirmed
by calculation of the intrinsic reaction paths. The energies reported in
this work are Gibbs energies computed at 298 K in gas phase includ-
ing solvent effects. These latter effects were estimated in single point
calculations on the gas-phase optimized structures using the pola-
rizable continuous solvation (PCM) method and considering H2O as
the solvent.17
Calculations of the local Fukui functions were also performed. The
Fukui function18 is a reactivity index that connects the concepts of the
Fukui frontier orbitals with DFT. It was defined by Yang and Parr as the
partial derivative of the electron density with respect to the total number
of electrons with constant external potential or as the derivative of the
chemical potential with respect to the external potential keeping
constant the total number of electrons of the system:
derivative of the eq 1 is not continuous with the number of electrons
and difficult to evaluate.20 So, Parr and Yang18 defined the Fukui
functions: fþ(r) and fꢀ(r) corresponding to the reactivity indexes that
describe the attack toward our system by a nucleophilic or electrophilic
species, where the superscripts þ and ꢀ refer to the derivatives of the
right and left of the central part, respectively.
By applying the finite-difference approximation it is easily shown that
the Fukui functions can be obtained from density differences. If these
functions are condensed to the specific atomic regions of the three-
dimensional (3D) molecular space we obtain the condensed Fukui
functions:
þ
fx ¼ qxðN þ 1Þ ꢀ qxðNÞ
ð2Þ
ꢀ
fx ¼ qxðNÞ ꢀ qxðN ꢀ 1Þ
ð3Þ
where the parameters qx are the charges of the atom X calculated in the
systems with N, N ꢀ 1, and N þ 1 electrons keeping the optimized
geometry of the molecule with N electrons. In this work, we have used
charges derived from Natural Population Analysis (NPA) to calculate
the condensed Fukui functions.21
’ RESULTS AND DISCUSSION
Synthesis and Solid State Structure. The synthetic strategy
followed for the preparation of the complexes described in the
present paper is outlined in Scheme 1.
The trichloro Ru complex [RuIIICl3(tpm)], 1, is used as
starting material, followed by reduction with NEt3 in the
presence of the bisoxazolinic ligand, iPr-box-C, to form the
corresponding monochloro Ru(II) complex 2c. However, the
reaction leads to a mixture of two complexes: the expected 2c and
a second chloro complex, 2o, where the oxazolinic ligand has
suffered a nucleophilic attack by an OHꢀ group, leading to ring
cleavage and thus generating the new anionic unsymmetric
oxazolinic-amidate ligand iPr-box-Oꢀ (see Chart 1). This ring-
opening is attributed to a Ru-assisted process once the oxazoline
ligand is coordinated, because of an enhancement of the iminic C
atom electrophilicity upon coordination as has previously been
described by our group.9 These RuꢀCl complexes are easily
separated by column chromatography since 2c is cationic
whereas 2o is a neutral complex. The corresponding RuꢀOH2
complexes 3 are easily obtained from the corresponding RuꢀCl
complexes in the presence of stoichiometric amounts of Agþ.
The X-ray diffraction structures for both chloro complexes have
been resolved, and the corresponding crystallographic data and
selected bond distances and angles are reported in Table 1 and
Supporting Information, Table S1. ORTEP views together with
their labeling schemes are depicted in Figure 1.
!
ꢀ
ꢁ
δμ
DFð r Þ
B
fð r Þ ¼
B
¼
ð1Þ
δνð r Þ
DN
B
N
νð
Þ
r
B
In all cases, the Ru metal center adopts an octahedrally dis-
torted type of coordination where the tpm ligand is bonded in a
facial manner and the iPr-box-C and the iPr-box-Oꢀ oxazoline
ligands act in a didentate fashion. The facial topology of the tpm
Over the past years, Fukui functions have been used to explain the
regioselectivity in chemical reactions.19 The Fukui function describes
the local changes in the electronic density of a system due to a perturba-
tion in the total number of electrons. For a molecule or an atom, the
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Table 2. UVꢀvis Spectroscopic Features in CH2Cl2 for the
RuꢀCl Complexes and in Aqueous Solution for the Ru-Aquo
Complexes
solution, none of the complexes described in this paper possesses
any symmetry element so all the resonances of the ligands are
expected to be different as confirmed experimentally. The
complete 1D and 2D NMR characterization of complexes 2c,
2o, 3c, and 3o is presented in the Supporting Information.
In the case of RuꢀH2O complex 3c, the 1H NMR spectrum
registered in d6-acetone (Supporting Information) shows a
splitting of the signals which is consistent with the presence of
two different species. A study on the evolution of the resonances
with time reveals the presence of a main species corresponding to
the aqua complex 3c together with a new complex, resulting from
the substitution of aqua by acetone coming from the solvent.
The UVꢀvis spectra of all complexes are displayed in the
Supporting Information whereas their main features are pre-
sented in Table 2 together with other similar complexes recently
described by our group.9
The complexes exhibit ligand based πꢀπ* bands below
350 nm and relatively intense bands above 350 nm assigned
mainly to dπꢀπ* MLCT transitions. For the RuꢀCl complexes
the MLCT bands are shifted to the red with regard to the
analogous RuꢀOH2 species because of the relative destabiliza-
tion of the dπ(Ru) levels provoked by the chloro ligand. In
particular, the replacement of the Cl ligand by aqua (3c vs 2c or
3o vs 2o) produces a blue shift of roughly 13ꢀ43 nm. The
replacement of benzyl by isopropyl groups in the oxazolinic
ligand does not reveal any important change in the bands of the
UVꢀvis spectra for the complexes having the closed ligands (2c
vs 20c or 3c vs 30c), but a slight bathochromic shift is observed for
complexes containing the iPr-box-O ligand (2o and 3o) with
regard to the analogous with Bz-box-O (20o and 30o), which is in
accordance with the higher electron-donor character of the iPr
substituent when compared to Bz. This indicates that the open-
ing of the oxazole ring seemingly allows a more effective
transmission of the electron density from the oxazoline substit-
uents to the metal center in Ru(II) species.
λmax, nm
compound
assignment
(ε, Mꢀ1 cmꢀ1
)
[RuIICl(Bz-box-C)(tpm)](BF4), 20ca
πfπ* 271 (9900), 326 (7200)
dπfπ* 468 (6400)
[RuIICl(Bz-box-O)(tpm)], 20oa
πfπ* 270 (6900), 335 (sh)
dπfπ* 388 (8600)
[RuIICl(iPr-box-C)(tpm)](BF4), 2c
πfπ* 292 (8916), 326
(sh, 6505)
dπfπ* 464 (5204)
[RuIICl(iPr-box-O)(tpm)], 2o
πfπ* 285 (8400), 312 (7350)
dπfπ* 413 (sh, 4500)
[RuII(Bz-box-C)(tpm)OH2](BF4)2, 30ca πfπ* 263 (9800), 310 (sh),
dπfπ* 423 (4150)
[RuII(Bz-box-O)(tpm)OH2](BF4) 30oa πfπ* 260 (6210), 330 (5250)
dπfπ* 387 (sh)
[RuII(iPr-box-C)(tpm)OH2](BF4) 3c
πfπ* 275 (1863), 308
(sh, 1517)
dπfπ* 421 (1327)
[RuII(iPr-box-O)(tpm)OH2](BF4) 3o πfπ* 289 (1947), 323 (2033)
dπfπ* 400 (sh)
a Reference 9.
ligand forces that in both cases the chloride ligand coordinates
cis to the oxazoline ligands. All bond distances and angles are
within the expected values for this type of complexes.22 The 3D
arrangement of molecules in complex 2c displays a network of
intermolecular H-bonding interactions (with interatomic dis-
tances below 2.75 Å, see Supporting Information) involving the
chloride ligand as well as the PF6ꢀ counterions of the structure.
The close contact of each Cl ligand with a tpm H atom of the
neighboring molecule allows the establishment of one-dimen-
sional (1D) chains in the a axis direction that are joined together
through the PF6ꢀ anions.
Redox Properties. The redox properties of the RuꢀCl and
Ru-aqua complexes described in the present work were investi-
gated by means of CV techniques. All cyclic voltammograms are
shown in the Supporting Information, Figure S7.
The X-ray structure of complex 2o presents C17ꢀO2 and
C17ꢀN8 bond distances (see Figure 1b) of 1.266 and 1.320 Å,
respectively, that are consistent with an intermediate situation
between the two resonant forms of the amidate ligand,9 therefore
pointing to a charge delocalization through the NCO backbone:
The cyclic voltammograms of chloro complexes 2c and 2o,
registered in CH2Cl2þ0.1 M TBAP, exhibit chemically reversible
waves assigned to the corresponding Ru(III/II) couples. For
complex 2c this wave appears at E1/2 = 0.79 V vs SSCE, whereas
for the complex 2o this wave is cathodically shifted to E1/2 = 0.22
V, as expected from the higher electron-donating capacity of the
anionic iPr-box-Oꢀ ligand with regard to the neutral iPr-box-C. If
we compare these redox potentials with those corresponding to
the analogous chloro complexes having the Bz-Box-C and Bz-
Box-O oxazolinic ligands, 20c (E1/2 = 0.83 V) and 20o (E1/2
=
0.24 V), we can see that complexes 2c and 2o display a slight
cathodic shift of 40 and 20 mV respectively because of the
stronger electron-withdrawing capacity of the benzyl groups with
regard to the iPr groups. It is interesting to note that the dis-
tinctive electronic effects of the iPr/Bz substituents were man-
ifested in the UVꢀvis spectra only for complexes containing the
open R-box-O ligands thus indicating that oxidation of the Ru
metal center may enhance the electronic influence exerted by the
substituents of the closed R-box-C ligands.
The redox potentials for the Ru-aqua complexes 3c and 3o are
summarized in Table 3 together with other relevant Ru-aqua
complexes previously described in the literature for purposes of
comparison.9,23,24
Spectroscopic Properties. The combination of 1D and 2D
NMR spectra, registered in d6-acetone or CDCl3, allows to
unambiguously identify the resonances of all the protons for
complexes 2c and 3c. The NMR assignment is described in the
Experimental Section and is consistent with the structures found
in the solid state as expected for a Ru(II) d6 type of ion. However,
for complexes 2o and 3o some of the expected resonances either
appear as wide signals or are missing because of dynamic effects
arising from the open oxazole ring in the iPr-box-O ligand. In
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Table 3. pKa and Electrochemical Data (pH = 7) for the
Aquocomplexes Described in This Work and Others for
Purposes of Comparison
complexa
E1/2
E1/2
b
b
entry (T)(D)RuꢀOH2 (III/II) (IV/III) ΔEc pKa(II) pKa(III) ref
1
2
3
4
5
6
7
8
9
tpm-bpy
0.40
0.71 310 10.8
>10
1.9
1.8
3.9
1.8
2.3
1.7
2
23a
9
tpm-Bz-box-C, 30c 0.38
tpm-Bz-box-O, 30o 0.17
0.37 200 11.0
>12
9
d
3c
0.38
0.16
0.49
0.21
0.38
0.19
d
3o
10.5
trpy-bpy
trans-trpy-pic
cis-trpy-pic
trpy-acac
0.62 130 9.7
0.45 240 10
0.56 180 10
0.56 370 11.2
23b
12
3.7
5.2
12
24a
a T stands for tridentate ligand whereas D for didentate; acac = acetylacetonate,
pic = picolinate. b Redox potentials in volts are reported with regard to
the SSCE reference electrode. c ΔE = E1/2(IV/III) ꢀ E1/2(III/II) in mV.
d This work.
The aquocomplexes' redox potentials are pH dependent be-
cause of the capacity of the mentioned aqua ligand to lose pro-
tons. This ability to release Hþ is also responsible for the easy
accessibility of Ru higher oxidation states as shown in the follow-
ing equation as an example,
2þ
½RuIIðiPr-box-CÞðtpmÞðOH2Þꢂ
2þ
a ½RuIIIðiPr-box-CÞðtpmÞðOHÞꢂ þ 1eꢀ þ 1Hþ
Figure 2. E1/2 vs pH or Pourbaix diagram of complexes 3c (a) and 3o
(b). The pH-potential regions of stability for the various oxidation states
and their dominant proton compositions are indicated.
The complete thermodynamic information regarding the Ru-
aqua type of complex can be extracted from the Pourbaix
diagrams, exhibited in Figure 2 for 3c and 3o.
that the higher the oxidation state, the stronger the σ donation
capability provided by the anionic ligand.
For complex 3c the absence of information in the Pourbaix
diagram above pH = 12 is presumably because at this high pH the
hydrolysis of the oxazolinic iPr-box-C ligand is taking place.9 The
pKa values for the RuIIꢀOH2 and RuIIIꢀOH2 species, gathered
in Table 3, can be calculated from the slope breaks of the RuIII/
RuII couple.
Catalytic Epoxidations. The catalytic activity of the ruthe-
nium complexes 3c and 3o together with other aquocomplexes
previously synthesized in our group, 30c and 30o, was investigated
in the epoxidation of different alkenes given the interest of the
epoxidation reaction in both bulk and fine chemistry that use
epoxides as starting materials for a variety of reactions.25
A careful study of Table 3 allows extracting the following
conclusions: (a) as shown in entries 1, 2, and 4, the replacement
of a bpy by a Bz-box-C or iPr-box-C ligand produces a decrease of
20 mV of the Ru(III/II) redox couple because of the lower
electron-withdrawing capacity of the oxazolinic ligand vs the bpy
ligand. A similar effect is observed when comparing similar
complexes but replacing trpy by tpm as for instance in entries
1 and 6; (b) entries 4 and 5 show the cathodic shift of 220 mV
produced by the replacement of a neutral oxazolinic ligand with
the corresponding hydrolyzed anionic ligand because of stabili-
zation of Ru(III), as was the case for the previously described
complexes 30c and 30o (entries 2 and 3). This behavior is also
found when comparing Ru-trpy-bpy (entry 6) with the analogous
picolinate and acetylacetonate complexes (entries 7ꢀ9); (c) the
replacement of a benzyl substituent by iPr does not produce any
change in the Ru(III/II) couple of complexes 3c and 30c and only
a slight variation of 10 mV in the case of complexes 3o and 30o,
then suggesting that for aquocomplexes the redox behavior is
predominantly governed by the protonation/deprotonation pro-
cesses at the aquo ligand; (d) In all cases, the exchange of an
anionic ligand by a neutral ligand slightly affects pKa,II but
strongly influences pKa,III. This can be interpreted in the sense
Initial assays were performed with trans-stilbene as test sub-
strate using the following conditions: CH2Cl2 as solvent; PhI-
(OAc)2 as oxidant; catalyst:substrate:oxidant molar ratio of
1:100:200. The results obtained are gathered in Table 4 together
with other Ru-oxazoline complexes for purposes of comparison.9
No epoxidation occurred in the absence of catalyst in any case.
A first glance at Table 4 shows that in all the cases the main
product formed is trans-stilbene oxide (TSO) with minor
amounts of benzaldehyde (BzA) and that the catalysts bearing
the closed oxazolinic rings are in general more active than those
containing the open rings. Also, catalysts bearing the facial tpm
ligand (entries 1ꢀ5) are markedly more active than those
containing the meridional trpy (entries 6 and 7). The difference
between pairs of analogous complexes with open/closed ligands
is specially pronounced when comparing complexes 30c and 30o
(entries 1 and 2), and this effect has already been explained9 as a
consequence of π-stacking effects between the substrate and the
CH2Ph substituents of the Bz-box-C ligand. This kind of inter-
actions can only take place in the case of catalyst 30c given the
high mobility of the benzyl arm in the hydrolyzed ring of 30o that
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leads to a clearly better performance for the former. However, for
the case of the analogous complexes 3c and 3o, where π-stacking
cannot occur, a rather small difference in reactivity is observed
(entries 3 and 5). Finally we can perceive that, under dark con-
ditions (entry 4), catalyst 3c is slightly more active in the
epoxidation of trans-stilbene leading also to the best epoxide/
benzaldehyde ratio. To the best of our knowledge our Ru-oxazo-
line complexes are among the most selective ruthenium catalysts
described in the literature for the oxidation of trans-stilbene,26,27
even though the experimental conditions are not identical to
those reported previously.
With the aim to investigate the influence of the different sub-
stituents at the oxazolinic ligands in the reactivity of ruthenium
complexes, we decided to test 30c and 3c in the epoxidation of cis-
β-methylstyrene and 4-vinylcyclohexene under the same experi-
mental conditions. The results are gathered in Table 5, where the
main product formed is the corresponding epoxide with minor
amounts of other oxidation products such as benzaldehyde.
Both complexes display a good performance and selectivity
except for the epoxidation of 4-vinylcyclohexene with complex
3c, where a moderate activity is obtained. The selectivity for
the epoxide in the oxidation of cis-β-methylstyrene is moderate
in both cases, but it is remarkable that only the cis epoxide is
produced, with no traces for the isomerization toward the trans
diastereoisomer. This is in agreement with either a potential
O-atom transfer concerted mechanism or a stepwise mechanism
with a short life intermediate having a half-time smaller than the
time required for the rotation of the attacked CꢀC bond.28
Similar results had been observed with several ruthenium com-
plexes with nitrogen-donor ligands.27b,29
For 4-vinylcyclohexene, complex 30c leads to good conver-
sions and a very high regioselectivity for oxidation of the
substrate at the alkene ring position whereas complex 3c has
been shown to be regiospecific for the oxidation of the terminal
vinyl position, with a moderate conversion. To the best of our
knowledge, the selective epoxidation at the terminal position of
this substrate has never been reported with ruthenium aqua
complexes, although regioselective catalysts for alkene car-
bonꢀcarbon bond cleavage to form aldehydes with significant
preference for primary alkenes have been described.30 The
different behavior shown by the two catalysts cannot be ex-
plained in terms of steric hindrance since complex 30c, having a
benzyl substituent at the box-C ligand, should in advance exhibit
a certain preference for the less hindered terminal alkene.
Thermodynamic effects arising from the redox behavior of the
catalyst can neither be invoked here because, as has been
described previously, the substituent (iPr or benzyl) of the
oxazoline ligand has no apparent effect on the redox potentials.
The significant influence of this substituent on the reactivity of
the catalysts can then be explained by distinctive electronic
effects that may tune the electrophilic/nucleophilic character of
the intermediate species involved in the catalytic cycle, thus
governing the preference for a less electron-rich substrate in the
case of catalyst 3c. This unusual reactivity difference depending
on the substituents of the oxazolinic ligand led us to carry out a
theoretical analysis of the reaction mechanisms for the different
possible attacks to rationalize the reactivity differences observed.
Computational Results. We have initially performed DFT
calculations on the geometry of 2c, and the optimized structure
obtained is overall in excellent agreement with the X-ray struc-
ture (rmsd 0.024 Å on distances and 0.8° on angles).31 As in
previous similar studies,32 the DFT approach that we chose is
able to offer reliable geometries of reactants, intermediates, and
products. Starting from the Ru-oxo species obtained from the
oxidation of the RuꢀOH2 complexes 3c and 30c we analyzed
the epoxidation at the different alkene sites of the 4-vinylcyclo-
hexene substrate. Different epoxidized Ru-coordinated species
Table 4. Catalytic Oxidation of trans-Stilbene by Ru-Aqua
Complexes Using PhI(OAc)2 as Oxidanta
conversion, selectivity,
ratio epoxide/
benzaldehydec
b
entry
cat
%
%
1
2
3
4
5
6
7
30c
30o
3c
68.9
34.5
55.2
62.5
51.2
16.3
15.5
89
96
85
87
81
82
81
3.9
12.3
19.6
22.6
17.7
2.4
3cd
3o
trpy-Bz-box-Ce
trpy-Bz-box-Oe
2.1
a Reaction conditions: catalytic oxidations were performed by dissolving
the Ru-aqua complex (0.002 mmol) in degassed dichloromethane
(2.5 mL) containing trans-stilbene (0.2 mmol) and PhI(OAc)2 (0.4
mmol). The reaction mixture was stirred at RT for 24 h. After addition of
an internal standard, an aliquot was taken for GC analysis. b Selectivity
for epoxide, [yield/conversion] ꢁ 100. c Assuming that two molecules of
benzaldehyde come from the scission of a single stilbene molecule. d In
the dark. e Ref 9.
Table 5. Catalytic Performance of 30c and 3c for the Epoxidation of cis-β-Methylstyrene and 4-Vinylcyclohexene Using PhI(OAc)2
as Oxidanta
a Reaction conditions: catalytic oxidations were performed by dissolving the Ru-aqua complex (0.002 mmol) in degassed dichloromethane (2.5 mL)
containing the alkene (0.2 mmol) and PhI(OAc)2 (0.4 mmol). The reaction mixture was stirred at RT for 24 h. After addition of an internal standard, an
aliquot was taken for GC analysis. All the experiments have been done in the dark. b Selectivity for epoxide, [yield/conversion] ꢁ 100. c For both catalysts
100% of cis-epoxide is obtained. d Ratio [ring epoxide/vinyl epoxide].
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Figure 3. Relative energies (in kcal/mol) of the epoxidized Ru-coordinated species formed in the epoxidation of 4-vinylcyclohexene through either the
terminal alkene or the ring by [RuIVO(tpm)(R-box-C)]2þ complexes, R = Bz, 40c (a) and R = iPr, 4c (b).
Figure 4. Transition states of the rate determining step with the main geometric parameters and energy barriers (in kcal/mol) for the epoxidation of
4-vinylcyclohexene through either the terminal alkene or the ring by [RuIVO(tpm)(R-box-C)]2þ complexes, R = Bz, 40c (a) and R = iPr, 4c (b).
corresponding to the two possible attacks and different con-
formations were obtained from which the most stable ones are
collected in Figure 3. The BP86 calculated energy differences
between the two possible products are so small (less than 0.3
kcal/mol) that they cannot be used to provide an explanation of
the different behavior observed.
The formation of the epoxide at the terminal alkene is exergon-
ic by 7.6 and 5.9 kcal/mol for 40c (R = Bz) and 4c (R = iPr),
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geometry of these TS shows that the singlet state is the ground
state at this geometry and that it is stabilized by more than 25
kcal/mol with respect to the triplet state. Geometry optimiza-
tions of these transition states in the singlet state collapsed to the
product. This means that once the intermediate crosses from the
triplet to the singlet potential energy surface (a crossing that
should take place near the triplet intermediate with an energy
cost smaller than 6 kcal/mol and that should be possible through
spinꢀorbit coupling), it evolves directly to the final product in a
barrierless process. Therefore, the first step is the rate-determin-
ing step of the epoxidation process and is consequently the one
that governs the selectivity of the epoxidaton either on the ter-
minal or the ring alkene.
Figure 5. Condensed Fukui fþ values on the 4-vinylcyclohexene.
We have also performed calculations of Fukui functions at the
CdC double bonds of the substrate and at the oxo groups of
both complexes. Fukui functions are useful to predict the
reactivity of different atoms in a substrate toward a nucleophile
or an electrophile.18,19 In principle the catalyst should attack
nucleophilically, through the oxo group, one of the two CdC
bonds of the substrate, that is, the study involves the calculation
of the fþ for the four carbon atoms contained in both alkene
groups of the substrate. The fþvalues on the carbon atoms of the
CꢀC double bonds are displayed in Figure 5. In average the
terminal alkene displays higher regional Fukui functions toward a
nucleophilic attack. On the other hand, when looking at the other
agent, that is, the catalyst, we should pay attention to the fꢀ on
the oxygen atom. The values of fꢀ are 0.013 and 0.105 for the
catalysts containing benzyl and iPr groups, respectively. This
important difference reveals that the system with iPr groups is
significantly more nucleophilic.
From this static reactivity picture, one can conclude that the
iPr system should have a larger preference for the CdC bond
with the highest fþvalues from an electronic viewpoint. On the
other hand, the benzyl system should be less selective and have
more difficulties to distinguish, from an electronic point of view,
between the two CdC double bonds. In fact, this agrees with the
higher preference of the iPr system toward the terminal alkene.
respectively, with respect to the reactants, that is, the 4-vinylcy-
clohexene and the triplet oxo conformation of the corresponding
[RuIVO(tpm)(R-box-C)]2þ catalyst. The mechanism involves
two steps. First the formation of a CꢀO bond, that is, the oxo
group of the catalyst binds to one carbon atom of the attacked
CdC double bond. This step involves the transition states (TS)
displayed in Figure 4 that have in all cases a triplet ground state,
the lowest-lying singlet excited state being about at least 5 kcal/
mol higher in energy. The crossover from the singlet to the triplet
surface should be possible through spinꢀorbit coupling. It is
likely that the calculated barrier of about 25 kcal/mol for this step
is somewhat overestimated, because the entropic contribution
of the solute has been calculated in the gas-phase,33 and it is
well-known that ΔSrot and ΔStrans are larger in the gas-phase
than in solution where rotation and translation are much more
constrained.33b,34
Overall, the different reactivity of 40c and 4c can be discussed
through the relative energy stability characterized by this first
transition state (see Figure 4). For 40c the interaction of the oxo
species with the CdC double bond of the cyclohexene it is favored
by only 0.3 kcal/mol with respect to the CdC double bond of the
terminal alkene. The barriers are 24.4 and 24.7 kcal/mol for the
ring and terminal alkenes, respectively, calculated with respect to
separated reactants. This is in line with the observed preference of
40c for attacking the CdC double bond of the ring. The small
difference between energy barriers concurs with the fact that some
epoxidation occurs also on the terminal alkene (see Table 5). For
4c this energy difference increases to 1.5 kcal/mol (the barriers are
24.3and 25.8kcal/mol for the attack at the terminal alkeneand the
cyclohexene, respectively) and this agrees with the fact that 4c
epoxidizes only the terminal alkene.
The intermediate species that result from the formation of the
new CꢀO bond display a triplet ground state. For 40c, these
intermediates are placed 22.4 and 23.3 kcal/mol higher in energy
with respect to the final epoxidized species for the cyclic and
terminal alkenes, respectively, whereas for 4c the corresponding
values for ring and terminal epoxidation are in the order of 22.8
and 23.2 kcal/mol. Single-point energy calculations of these
intermediates in the singlet state indicate that the singlet state is
an excited state destabilized with respect to the triplet ground
state by about 6 kcal/mol. All attempts to perform a geometry
optimization of these intermediates with a singlet multiplicity led
to the final epoxidized products in their singlet ground state. On
the other hand, we have located the transition states that guide
the formation of the epoxide, that is, the formation of the second
CꢀO bond, in the triplet state surface. These TSs have energy
barriers of about 16 and 19 kcal/mol for 40c and 4c, respec-
tively. A single-point energy calculation of the singlet state at the
’ CONCLUSIONS
We have synthesized a new family of RuꢀOH2 complexes
where the alkylic iPr groups on the oxazolinic ligand do not have
any remarkable influence on the spectroscopic and redox proper-
ties when compared to the analogous complexes containing Bz
substituents but surprisingly determine a remarkable modification
in the reactivity of these complexes toward the epoxidation of
alkenes, with 3c being the first Ru catalyst capable to specifically
epoxidize the terminal alkene of 4-vinylcyclohexene. The BP86
computed energy barriers of the first (and rate-determining) step
corresponding to the interaction of the alkene groups with the
catalytic systems bearing benzyl or isopropyl groups are in line
with the different behavior observed in this regioselective epoxida-
tion. Furthermore, calculations based on condensed Fukui func-
tions reveal that the electronic factors play a key role and help to
provide an explanation for the unexpected reactivity differences
observed.
’ ASSOCIATED CONTENT
S
Supporting Information. Additional spectroscopic, elec-
b
trochemical data and full computational material, together with
crystallographic information files. CIF for 2c and 2o have CCDC
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Inorganic Chemistry
ARTICLE
nos. 827784 and 827785 respectively. This material is available
free of charge via the Internet at http://pubs.acs.org. The
supplementary crystallographic data can also be obtained free
of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or
from the Cambridge Crystallographic Data Centre, 12, Union
Road, Cambridge CB2 1EZ, U.K.; fax þ44 1223 336033 or E-mail
deposit@ccdc.cam.ac.uk).
Churchill, M. R.; Takeuchi, K. J. Inorg. Chem. 1995, 34, 4198–4205.
(e) Mishra, D.; Barbieri, A.; Sabatini, C.; Drew, M. G. B.; Figgie, H. M.;
Sheldrick, W. S.; Chattopadhyay, S. K. Inorg. Chim. Acta 2007,
360, 2231–2244. (f) Delaude, L.; Delfosse, S.; Richel, A.; Demonceau,
A.; Noels, A. F. Chem. Commun. 2003, 1526–1527.
(5) Huynh, M. H. V.; Witham, L. M.; Lasker, J. M.; Wetzler, M.;
Mort, B.; Jameson, D. L.; White, P. S.; Takeuchi, K. J. J. Am. Chem. Soc.
2003, 125, 308–309.
(6) (a) Nishiyama, H.; Itoh, Y.; Matsumoto, H.; Park, S.-B.; Itoh, K.
J. Am. Chem. Soc. 1994, 116, 2223–2224. (b) Jiang, Y.; Jiang, Q.; Zhang,
X. J. Am. Chem. Soc. 1998, 120, 3817–3818. (c) Braunstein, P.; Fryzuk,
M. D.; Naud, F.; Rettig, S. J. J. Chem. Soc., Dalton Trans. 1999, 589–594.
(d) Burguete, M. I.; Fraile, J. M.; García, J. I.; García-Verdugo, E.; Luis,
S. V.; Mayoral, J. A. Org. Lett. 2000, 2, 3905–3908. (e) Ostergaard,
N.; Jensen, J. F.; Tanner, D. Tetrahedron 2001, 57, 6083–6088.
(f) McManus, H. A.; Guiry, P. J. Chem. Rev. 2004, 104, 4151–4202.
(g) Le Maux, P.; Abrunhosa, I.; Berchel, M.; Simonneaux, G.; Gulea, M.;
Masson, S. Tetrahedron: Asymmetry 2004, 15, 2569–2573. (h) Tse,
M. K.; Klawonn, M.; Bhor, S.; D€obler, C.; Anilkumar, G.; Hugl, H.;
M€agerlein, W.; Beller, M. Org. Lett. 2005, 7, 987–990.
(7) (a) Hua, X.; Shang, M.; Lappin, A. G. Inorg. Chem. 1997,
36, 3735–3740. (b) Gꢀomez, M.; Muller, G.; Rocamora, M. Coord. Chem.
Rev. 1999, 193ꢀ195, 769–835. (c) Nishiyama, H.; Park, S.-B.; Haga,
M.-A.; Aoki, K.; Itoh, K. Chem. Lett. 1994, 1111–1114.
(8) Rechavi, D.; Lemaire, M. Chem. Rev. 2002, 102, 3467–3493.
(9) Serrano, I.; Sala, X.; Plantalech, E.; Rodríguez, M.; Romero,
I.; Jansat, S.; Gꢀomez, M.; Parella, T.; Stoeckli-Evans, H.; Solans, X.;
Font-Bardia, M.; Vidjayacoumar, B.; Llobet, A. Inorg. Chem. 2007, 46,
5381–5389.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: marisa.romero@udg.edu (I.R.), montse.rodriguez@
udg.edu (M.R.).
’ ACKNOWLEDGMENT
We acknowledge financial support from the MICINN of Spain
(CTQ2010-21532-C02-01 to I.R. and M.R., CTQ2009-08328 to
T.P., CTQ2010-21497 and Consolider Ingenio 2010 CSD2006-
0003 to A.L. and CTQ2008-03077/BQU to M.S.) and the DIUE
of Catalonia (2009SGR637). M.S. is also grateful to the DIUE of
the Generalitat de Catalunya for financial help through the
ICREA Academia 2009 prize for excellence in research. A.P.
thanks MICINN for a Ramꢀon y Cajal contract (ref RYC-2009-
04170). Johnson & Matthey LTD are acknowledged for a
RuCl3 nH2O loan. We also thank Centre de Supercomputaciꢀo
3
de Catalunya (CESCA) for partial funding of computer time.
(10) Onishi, M.; Isagawa, K. Inorg. Chim. Acta 1991, 179, 155–156.
(11) H€uckel, W.; Bretschneider, H. Ber. Chem. 1937, 9, 2024–2026.
(12) Llobet, A.; Doppelt, P.; Meyer, T. J. Inorg. Chem. 1998, 27,
514–520.
’ REFERENCES
(1) (a) Sheldon, R. A.; Kochi, J. K. Metal Catalyzed Oxidation of
Organic Compounds; Academic Press; New York, 1981. (b) Gamez, P.;
Aubel, P. G.; Driessen, W. L.; Reedjik, J. Chem. Soc. Rev. 2001, 30,
376–385.(c) Robert, A.; Meunier, B. In Biomimetic Oxidations Catalyzed
by Transition Metal Complexes; Meunier, B., Ed.; Imperial College Press:
London, U. K., 2000; Chapter 12. (d) Bruijnincx, P. C. A.; van Koten, G.;
Gebbink, R. J. M. K. Chem. Soc. Rev. 2009, 37, 2716–2744.
(2) (a) Ko, S. Y.; Lee, A. W. M.; Masamune, S.; Reed, L. A.;
Sharpless, K. B.; Walker, F. J. Science 1983, 220, 949–951. (b) Nicolaou,
K. C.; Winssinger, N.; Pastor, J.; Ninkovic, S.; Sarabia, F.; He, Y.;
Vourloumis, D.; Yang, Z.; Li, T.; Giannakakou, P.; Hamel, E. Nature
1997, 387, 268–272.(c) Gagnon, S. D. In Encyclopedia of Polymer Science
and Engineering, 2nd ed.; Mark, H. F., Bikales, N. M., Overberger, C. G.,
Menges, G., Kroschwitz, J. I., Eds.; John Wiley & Sons: New York, 1985;
Vol. 6, pp 273ꢀ307. (d) Darensbourg, D. J.; Mackiewicz, R. M.; Phelps,
A. L.; Billodeaux, D. R. Acc. Chem. Res. 2004, 37, 836–844.(e) Jacobsen,
E. N. Catalysis Asymmetric Synthesis; Ojima, I., Ed.; VCH: New York,
1993, 159.
(3) (a) Bryant, J. R.; Matsuo, T.; Mayer, J. M. Inorg. Chem. 2004,
43, 1587–1592. (b) Sala, X.; Poater, A.; Romero, I.; Rodríguez, M.;
Llobet, A.; Solans, X.; Parella, T.; Santos, T. M. Eur. J. Inorg. Chem.
2004, 612–618. (c) Yip, W.-P.; Yu, W.-Y.; Zhu, N.; Che, C.-M. J. Am.
Chem. Soc. 2005, 127, 14239–14249. (d) Zong, R.; Thummel, R. P.
J. Am. Chem. Soc. 2005, 127, 12802–12803. (e) Poater, A.; Mola, J.;
Saliner, A. G.; Romero, I.; Rodríguez, M.; Llobet, A.; Solꢁa, M. Chem.
Phys. Lett. 2008, 458, 200–204. (f) Mola, J.; Romero, I.; Rodríguez, M.;
Bozoglian, F.; Poater, A.; Solꢁa, M.; Parella, T.; Benet-Buchholz, J.;
Fontrodona, X.; Llobet, A. Inorg. Chem. 2007, 46, 10707–10716. (g)
Mola, J.; Rodríguez, M.; Romero, I.; Llobet, A.; Parella, T.; Poater, A.;
Duran, M.; Solꢁa, M.; Benet-Buchholz, J. Inorg. Chem. 2006,
45, 10520–10529.
(13) Frisch, M. J.; Trucks, G. W.;Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.;
Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.;
Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara,
M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.;Nakajima, T.; Honda,
Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, Jr., J. A.; Peralta, J. E.;
Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.;
Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell,
A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam,
N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo,
J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski,
V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels,
€
A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J.
Gaussian 09, Revision A.1; Gaussian, Inc.: Wallingford, CT, 2009.
(14) (a) Becke, A. Phys. Rev. A 1988, 38, 3098–3100. (b) Perdew,
J. P. Phys. Rev. B 1986, 33, 8822–8824. (c) Perdew, J. P. Phys. Rev. B
1986, 34, 7406–7406.
(15) Schaefer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992,
97, 2571–2577.
(16) (a) Haeusermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Mol. Phys.
1993, 78, 1211–1224. (b) Kuechle, W.; Dolg, M.; Stoll, H.; Preuss, H.
J. Chem. Phys. 1994, 100, 7535–7542. (c) Leininger, T.; Nicklass, A.;
Stoll, H.; Dolg, M.; Schwerdtfeger, P. J. Chem. Phys. 1996, 105,
1052–1059.
(17) (a) Barone, V.; Cossi, M. J. Phys. Chem. A 1998, 102,
1995–2001. (b) Tomasi, J.; Persico, M. Chem. Rev. 1994, 94, 2027–2094.
(18) Parr, R. G.; Yang, W. J. Am. Chem. Soc. 1984, 106, 4049–4050.
(19) (a) Yang, W.; Mortier, W. J. J. Am. Chem. Soc. 1986,
108, 5708–5711. (b) Poater, A.; Duran, M.; Jaque, P.; Toro-Labbꢀe, A.;
Solꢁa, M. J. Phys. Chem. B 2006, 110, 6526–6536. (c) Costas, M.; Ribas,
X.; Poater, A.; Valbuena, J. M. L.; Xifra, R.; Company, A.; Duran, M.;
Solꢁa, M.; Llobet, A.; Corbella, M.; Uson, M. A.; Mahia, J.; Solans, X.;
Shan, X. P.; Benet-Buchholz, J. Inorg. Chem. 2006, 45, 3569–3581.
(d) Ayers, P. W.; Parr., R. G. J. Am. Chem. Soc. 2000, 122, 2010–2018.
(4) (a) Stagni, S.; Orselli, E.; Palazzi, A.; De Cola, L.; Zacchini, S.;
Femoni, C.; Marcaccio, M.; Paolucci, F.; Zanarini, S. Inorg. Chem. 2007,
46, 9126–9138. (b) Murali, M.; Mayilmurugan, R.; Palaniandavar, M.
Eur. J. Inorg. Chem. 2009, 3238–3249. (c) Llobet, A. Inorg. Chim. Acta
1994, 221, 125–131. (d) Szczepura, L. F.; Maricich, S. M.; See, R. F.;
6053
dx.doi.org/10.1021/ic200053f |Inorg. Chem. 2011, 50, 6044–6054

Inorganic Chemistry
ARTICLE
(20) (a) Kohn, W.; Becke, A. D.; Parr, R. G. J. Phys. Chem. 1996,
100, 12974–12980. (b) Perdew, J. P.; Parr, R. G.; Levy, M.; Balduz, J. L.
Phys. Rev. Lett. 1982, 49, 1691–1694.
Ragone, F.; Correa, A.; Szadkowska, A.; Barbasiewicz, M.; Grela, K.;
Cavallo, L. Chem.—Eur. J. 2010, 16, 14354–14364. (f) Poater, A.;
Cavallo, L. J. Mol. Catal. A 2010, 324, 75–79.
(21) (a) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988,
88, 899–926. (b) Yang, W.; Parr, R. G.; Pucci, R. J. Chem. Phys. 1984,
81, 2862–2863.
(33) (a) Ardura, D.; Lopez, R.; Sordo, T. L. J. Phys. Chem. B 2005,
109, 23618–23623. (b) Leung, B. O.; Reid, D. L.; Armstrong, D. A.;
Rauk, A. J. Phys. Chem. A 2004, 108, 2720–2725.
(22) (a) Sens, C.; Rodríguez, M.; Romero, I.; Parella, T.;
Benet-Buchholz, J.; Llobet, A. Inorg. Chem. 2003, 42, 8385–8394. (b)
Eskelinen, E.; Haukka, M.; Ven€al€ainen, T.; Pakkanen, T.; Wasberg, M.;
Chardon-Noblat, S.; Deronzier, A. Organometallics 2000, 19, 163–169.
(c) Velders, A. H.; Hotze, A. C. G.; Ugolozzi, F.; Biagini-Cingi, M.;
Manotti, A. M.; Haasnoot, J. G.; Reedijk, J. Inorg. Chem. 2000,
39, 3838–3844. (d) Serrano, I.; Rodríguez, M.; Romero, I.; Llobet, A.;
Parella, T.; Campelo, J. M.; Luna, D.; Marinas., J. M.; Benet-Buchholz, J.
Inorg. Chem. 2006, 45, 2644–2651. (e) Sala, X.; Poater, A.; von
Zelewsky, A.; Parella, T.; Fontrodona, X.; Romero, I.; Solꢁa, M.;
Rodríguez, M.; Llobet, A. Inorg. Chem. 2008, 47, 8016–8024. (f) Sala,
X.; Plantalech, E.; Romero, I.; Rodríguez, M.; Llobet, A.; Poater, A.;
Duran, M.; Solꢁa, M.; Jansat, S.; Gꢀomez, M.; Parella, T.; Stoeckli-Evans,
H.; Benet-Buchholz, J. Chem.—Eur. J. 2006, 12, 2798–2807.
(23) (a) Takeuchi, K. J.; Thompson, M. S.; Pipes, D. W.; Meyer, T. J.
J. Am. Chem. Soc. 1984, 23, 1845–1851. (b) Leising, R. A.; Takeuchi, K. J.
Inorg. Chem. 1987, 26, 4391–4393.
(34) (a) Sakaki, S.; Takayama, T.; Sumimoto, M.; Sugimoto, M.
J. Am. Chem. Soc. 2004, 126, 3332–3348. (b) Cooper, J.; Ziegler, T. Inorg.
Chem. 2002, 41, 6614–6622. (c) Bieniek, M.; Michrowska, A.; Usanov,
D. L.; Grela, K. Chem.—Eur. J. 2008, 14, 806–818.
(24) (a) Bessel, C. A.; Leising, R. A.; Takeuchi, K. J. J. Chem. Soc;
Chem. Commun. 1991, 883–835. (b) Dovletoglou, A.; Adeyemi, S. A.;
Meyer, T. J. Inorg. Chem. 1996, 35, 4120–4127.
(25) (a) Smith, J. G. Synthesis-Stuttgart 1984, 629–656. (b) Schneider,
C. Synthesis-Stuttgart 2006, 3919–3944.(c) Weissermel, K. Industrial
Organic Chemistry, 3rd ed.; VCH: Weinheim, Germany, 1997. (d) Xia,
Q.-H.; Ge, H.-Q.; Ye, C.-P.; Liu, Z.-M.; Su, K.-X. Chem. Rev. 2005,
105, 1603–1662. (e) Che, C.-M.; Huang, J.-S. Chem. Commun.
2009, 3996–4015. (f) Chatterjee, D. Coord. Chem. Rev. 2008, 252,
176–198. (g) Gupta, K. C.; Sutar, A. K.; Lin, C.-C. Coord. Chem. Rev.
2009, 253, 1926–1946.
(26) (a) Sala, X.; Santana., N.; Serrano, I.; Plantalech, E.; Romero, I.;
Rodríguez, M.; Llobet, A.; Jansat, S.; Gꢀomez, M.; Fontrodona, X. Eur.
J. Inorg. Chem. 2007, 5207–5214. (b) Tse, M. K.; Bhor, S.; Klawonn, M.;
Dꢀobler, C.; Beller, M. Tetrahedron Lett. 2003, 44, 7479–7483.
(27) (a) Tada, M.; Muratsugu, S.; Kinoshita, M.; Sasaki, T.; Iwasawa,
Y. J. Am. Chem. Soc. 2010, 132, 713–724. (b) Yu, S.-Q.; Huang, J.-S.; Yu,
W.-Y.; Che, C.-M. J. Am. Chem. Soc. 2000, 122, 5337–5342. (c) Stoop,
R. M.; Bachmann, S.; Valentini, M.; Mezzetti, A. Organometallics 2000,
19, 4117–4126.
(28) (a) Stultz, L. K.; Binstead, R. A.; Reynolds., M. S.; Meyer, T. J.
J. Am. Chem. Soc. 1995, 117, 2520–2532. (b) Fung, W.-H.; Yu, W.-Y.;
Che, C.-M. J. Org. Chem. 1998, 63, 7715–7726.
(29) (a) Masllorens, E.; Rodríguez, M.; Romero, I.; Roglans, A.;
Parella, T.; Benet-Buchholz, J.; Poyatos, M.; Llobet, A. J. Am. Chem. Soc.
2006, 128, 5306–5307. (b) Dakkach, M.; Lꢀopez, M. I.; Romero, I.;
Rodríguez, M.; Atlamsani, A.; Parella, T.; Fontrodona, X.; Llobet, A.
Inorg. Chem. 2010, 49, 7072–7079.
(30) Kogan, V.; Quintal, M.; Neumann, R. Org. Lett. 2005, 7,
5039–5042.
(31) Standard deviations for distances and angles: snꢀ1
=
[∑i=1N(CalcdVali ꢀ ExpVali)2/(N ꢀ 1)]1/2, where CalcdVal means
calculated value, ExpVal means experimental value (X-ray data), and N is
the number of distances or angles taken into account (distances and
angles used are given in the Supporting Information,Table S2 ). For
examples, see: (a) Poater, A.; Cavallo, L. Inorg. Chem. 2009, 48,
4062–4066. (b) Duran, J.; Polo, A.; Real, J.; Benet-Buchholz, J.; Poater,
A.; Solꢁa, M. Eur. J. Inorg. Chem. 2003, 414, 4147–4151. (c) Poater, A.
J. Phys. Chem. A 2009, 113, 9030–9040.
(32) (a) Poater, A.; Ragone, F.; Correa, A.; Cavallo, L. J. Am. Chem.
Soc. 2009, 131, 9000–9006. (b) Ragone, F.; Poater, A.; Cavallo, L. J. Am.
Chem. Soc. 2010, 132, 4249–4258. (c) Bosson, J.; Poater, A.; Cavallo, L.;
Nolan, S. P. J. Am. Chem. Soc. 2010, 132, 13146–13149. (d) Luan, X. J.;
Mariz, R.; Gatti, M.; Costabile, C.; Poater, A.; Cavallo, L.; Linden, A.;
Dorta, R. J. Am. Chem. Soc. 2008, 130, 6848–6858. (e) Poater, A.;
6054
dx.doi.org/10.1021/ic200053f |Inorg. Chem. 2011, 50, 6044–6054