Ruthenium(II) complexes with 2,6-pyridyl-diimine ligands: Synthesis, characterization and catalytic activity in epoxidation reactions

Article abstract of DOI:10.1016/S1381-1169(98)00285-4

Reaction of [RuCl₂ (p-cymene)]₂ with the tridentate N-N'-N ligands, 2,6-pyridyl-diimines, led to substitution of p-cymene. The resulting complexes, believed to be coordinatively unsaturated, exhibit efficient activity for the epoxidation of cyclohexene in the presence of iodosobenzene (PhIO): the complexes formed initially take up donor molecules such as acetonitrile to achieve hexacoordination. The molecular structure for one of these, (acetonitrile){2,6-bis[1-(4- methoxyphenylimino)ethyl]pyridine}dichlororuthenium(II), 2, has been determined by X-ray diffraction. The immediate coordination sphere is a distorted octahedron with trans chloride atoms and a short Ru-N(py) (1.906 A) bond.

Full text of DOI:10.1016/S1381-1169(98)00285-4

Ž
.
Journal of Molecular Catalysis A: Chemical 142 1999 101–112
ž /
Ruthenium II complexes with 2,6-pyridyl-diimine ligands:
synthesis, characterization and catalytic activity in epoxidation
reactions
b
c
c,1
Bekir C¸etinkaya a,), Engin C¸etinkaya , Maurice Brookhart , Peter S. White
a
Department of Chemistry, Ege UniÕersity, Izmir, Turkey
Department of Chemistry, Inonu UniÕersity, Malatya, Turkey
Department of Chemistry, The UniÕersity of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3290, USA
b
¨ ¨
c
Received 6 May 1998; accepted 4 August 1998
Abstract
with the tridentate N-NX-N ligands, 2,6-pyridyl-diimines, led to substitution of
w
Ž
.x
Reaction of RuCl2 p-cymene
2
p-cymene. The resulting complexes, believed to be coordinatively unsaturated, exhibit efficient activity for the epoxidation
Ž
.
of cyclohexene in the presence of iodosobenzene PhIO : the complexes formed initially take up donor molecules such as
Ž
.Ä
w Ž
acetonitrile to achieve hexacoordination. The molecular structure for one of these, acetonitrile 2,6-bis 1- 4-
Ž .
.
x
4
methoxyphenylimino ethyl pyridine dichlororuthenium II , 2, has been determined by X-ray diffraction. The immediate
˚
.
Ž
. Ž
coordination sphere is a distorted octahedron with trans chloride atoms and a short Ru–N py 1.906 A bond. q 1999
Elsevier Science B.V. All rights reserved.
Ž .
Ž .
Keywords: Pyridine-2,6-diimines; Ruthenium II complexes; Epoxidation catalysts; Epoxidation of cyclohexene; X-ray structure of Ru II
1. Introduction
There is great current interest in pyridine-based tridentate ligands. This is primarily due to their
stability under a variety of oxidative and reductive conditions and the fact that pyridine ligands are on
the borderline between hard and soft Lewis bases. Examples of such ligands include the well-known
X
X
Y
Ž
.
w
x
w
x
w x
2,2 :6 ,2 -terpyridine terpy 1–4 , pyridylbisamines 5,6 , pyridylbisamides 7 , pyridylbisoxazolines
Ž
.
w
x
w
x
w x
pybox 8–12 , pyridylbispyrazolines 13 and pyridylbispyrimidines 14 . In this context, 2,6-
pyridiyl-diimines pydim 15,16 have emerged as an alternative to terpy. In virtually all of the
complexes prepared to date e.g., five-coordinate MX2 pydim MsMn, Fe, Co, Ni, Cu, Zn, Cd;
XsCl, Br and six-coordinate M pydim X2 XsBF4, ClO4; not all combinations the pydim
Ž
.
w
x
Ž
w
Ž
.x Ž
.
w
Ž
.
x
Ž
..
2
)
Corresponding author. E-mail: cetikaya@fenfak2.ege.edu.tr
1 To whom correspondence concerning crystallographic studies should be addressed. E-mail: pwhite@pyrite.chem.unc.edu.
1381-1169r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
Ž
.
PII: S1381-1169 98 00285-4

(
)
102
B. C¸etinkaya et al.rJournal of Molecular Catalysis A: Chemical 142 1999 101–112
w
x
behaves as a tridentate ligand 17–25 . Pydim has been observed to function as a bidentate chelating
w
x
ligand in certain carbonyl derivatives 26,27 .
Pyridine ligands, substituted in the 2,6-positions with a variety of chiral groups bearing coordinat-
Ž .
ing nitrogen atoms, capable of forming chelates with ruthenium II have been prepared and investi-
w
x
gated in catalytic applications 9–12 . We are not aware of any previous reports of reactions of
Ru II –pydim complexes. Thus, it was the goal of this work to prepare well-defined Ru II catalyst
precursors incorporating pydim ligands for alkene epoxidation.
Ž .
Ž .
2. Results and discussion
2.1. Synthesis and characterization of ligands and ruthenium complexes
X
Ž
.
Ž .
The tridentate N-N -N pydim ligands L were prepared via condensation of commercially
available 2,6-diacetylpyridine with two molar equivalents of the corresponding aniline. Ligands L1
and L have been reported previously 17 , ligands L –L were prepared by the same procedure, but
with minor modification and identified via microanalytical and NMR data.
2
3
7
w
x
w
Ž
.x
2
Complexation of pydims with 0.5 equivalent of RuCl2 p-cymene in boiling dichloromethane
Ž
.
resulted in deep-purple solutions Scheme 1 . The choice of reaction solvent proved to be quite
4
6
Ž
.
important. Thus, in the case of ligands bearing ortho-substituted aryl groups L –L the complexa-
tion reactions were not complete, even in boiling 1,2-dichloroethane DCE . However, ethanol was
found to be a satisfactory solvent for these syntheses. The ‘ RuCl2 L ’ products obtained from these
mixtures gave complex and often variable H NMR spectra: this property was attributed to the
Ž
.
w
x
1
solvated molecules such as CH2Cl2 or C2 H5OH which could partly be removed upon recrystalliza-
tion or washing with pentane. Therefore, no attempt was made to fully characterize these products.
Ž
.
w
x
However, they were used as epoxidation catalysts vide infra . The RuCl2 L stoichiometry of these
complexes was unequivocally established by trapping with two-electron donor ligands such as
w
Ž
.x
acetonitrile to achieve well-defined hexacoordinate complexes RuCl2 L NCCH3 . In this manner the
1
compounds 1–7 were fully characterized via elemental analysis and NMR spectroscopy. H NMR
details for the ligands and complexes are presented in Table 1. 13C NMR and physical data are given
in the Section 3.
The NMR spectra of the complexes were diagnostic. Only a single set of signals were observed for
1
the two pendant groups of pydim complexes in both the H and 13C NMR spectra which indicate that

(
)
B. C¸etinkaya et al.rJournal of Molecular Catalysis A: Chemical 142 1999 101–112
103
Scheme 1.
Ž
the two ‘arms’ of the ligands are magnetically equivalent in solution and the ligands are tridentate for
.
atom labeling scheme see Fig. 1 .
1
The effect of complexation on H chemical shifts of the ligand is also informative. Attachment of
Ž
.
Ž
the metal shifts the resonances of the pyridine hydrogens AB2 system towards high field Dda
1
.
Ž
0.44–0.60 while all the H resonances of the imine substituents move towards lower field Ddc
.
Ž .
0.24–0.32 . The upfield shift of the pyridine resonances suggest significant backbonding from Ru II
to the pU orbitals of pyridine. The coordinated CH3CN ligands show 1H methyl resonances at higher
frequencies relative to the free ligand. In general, the nitrile ligands are labile in all ruthenium
w
x
complexes although complete substitution of these ligands does not occur readily 28–30 .
The solid-state structure for the methoxy derivative was determined by X-ray diffraction. Details of
the data collection are given in Table 2 and a list of selected bond lengths and bond angles are in
Table 3. An ORTEP view of 2 is shown in Fig. 2. The immediate Ru coordination sphere is a
Ž .
Ž
.
Ž .
distorted octahedron, with the major distortion arising via the N 4 –Ru–N 15 angle, at 158.7 3 8.

(
)
104
B. C¸etinkaya et al.rJournal of Molecular Catalysis A: Chemical 142 1999 101–112
Table 1
1
H NMR dataa,b for the ligands L and the complexes 1–8
d
Ž .c
Ž
.
X
Compound number
L1
a
b
c
1
1
2
2X
3
Others
7.84t
8.31d
2.39s
7.17d
7.17d
6.74d
6.74d
2.35s
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
7.8
8.0
8.2
8.2
8.2
8.2
1
7.59t
7.83d
2.73s
2.41s
2.74s
2.41s
2.76s
2.43s
2.63s
2.34s
2.62s
2.27s
2.59s
2.40s
2.72s
2.38s
7.23d
7.23d
7.19d
7.19d
2.38s
3.82s
3.83s
1.33s
1.40s
7.20dt
2.07se
Ž
8.4
.
Ž
8.0
.
Ž
8.7
.
Ž
8.7
.
Ž
8.4
.
Ž
8.4
.
f
L2
2
7.83t
8.30d
6.92d
6.92d
6.80d
6.80d
Ž
8.0
.
Ž
7.2
.
Ž
7.7
.
Ž
7.7
.
Ž
7.7
.
Ž
7.7
.
7.60t
7.82d
7.26d
7.26d
6.94d
6.94d
2.14e
Ž
7.6
.
Ž
7.7
.
Ž
9.1
.
Ž
9.1
.
Ž
9.1
.
Ž
9.1
.
L3
3
7.83t
8.31d
7.37d
7.37d
6.78d
6.78d
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
7.8
7.53t
7.9
7.76d
8.6
7.36d
8.6
7.36d
8.5
7.15d
8.5
7.15d
2.05se
3.76sg
Ž
8.0
.
Ž
7.8
.
Ž
8.4
.
Ž
8.4
.
Ž
8.2
.
Ž
8.2
.
L4
4
7.93t
8.42d
1.38s
1.34s
2.33s
2.32s
2.08s
2.23s
6.47s
6.93s
6.90mi
7.45dd
6.56dd
7.08dt
Ž
7.8
.
Ž
8.0
.
Ž
.
.
Ž
.
Ž
.
Ž
.
1.4, 7.4
1.4, 7.4
1.4, 7.4
1.4, 7.4
7.65t
7.78d
7.55mh
7.55mh
7.20mh
7.20mh
2.10s
2.16s
6.97t
1.99se
Ž
8.0
.
Ž
7.6
.
L5
5
7.88t
8.40d
7.00d
7.08s
6.80d
Ž
.
Ž
.
Ž
.
Ž
.
7.9
7.61t
8.0
7.84d
8.4
7.38dd
8.4
7.01d
7.05s
2.04se
5.32h
Ž
8.1
.
Ž
7.8
.
Ž
Ž
7.8
.
2.1, 7.8
L6
6
7.92t
8.51d
2.08s
2.23s
6.47s
6.93s
6.90mi
7.10d
7.10d
Ž
7.8
.
Ž
8.0
.
Ž
.
Ž
.
Ž
.
8.5
7.6
7.6
7.65t
7.91d
7.09s
2.33s
2.34s
6.90mi
7.09s
2.33s
2.34s
6.90mi
7.09s
6.76s
6.93s
3.84s
2.28se
5.32h
Ž
7.4
.
Ž
8.4
.
L7
7
7.84t
8.31d
Ž
.
Ž
7.8
.
7.8
7.59t
7.82d
2.10se
5.32h
Ž
.
Ž
8.1
.
7.8
8
7.35mi
j
6.7mi,k
7.1mi,k
a For labeling pydim protons see Fig. 1: the numbers represent arene or alkyl protons.
b
Ž
.
Ž
.
Chemical shifts ppm relative to residual CDCl3 or CD2Cl2 , coupling constants JrHz given in parentheses.
c Measured in CDCl3.
d Measured in CD2Cl2.
e Due to coordinated acetonitrile.
f
w
x
Taken from Ref. 16 .
Due to solvated 1,2-dichloroethane DCE .
g
Ž
.
h Due to solvated dichloromethane.
i Tentative assignment, center of multiplet.
jMultiplet overlaps with the Ha resonances.
k Due to P–C6 H5.
This angle is considerably smaller than the ideal angle of 1808 and there is no steric barrier to
coordination of a fourth ligand in the equatorial plane trans to the pyridine moiety.
Ž .
Ž
.
The angle N 1 –Ru–N 12 involving the acetonitrile and the pyridine nitrogen, is normal at
Ž .
Ž .
Ž .
Ž
.
179.1 3 8. Ru–N 1 , Ru–N 4 and Ru–N 15 bond lengths are comparable to the reported values for
RuCl2 pybox-dihydro C2 H4 12 ; however, the Ru-py Ru–N 12 bond length, 1.906 7 A, is
short and even shorter than the Ru–acetonitrile Ru–N 1 bond length. The phenyl rings are twisted;
the angles between these two rings and the plane defined by the Ru and the four donor N-atoms are
54.788 and 68.588 for the ‘a’ and ‘b’ rings, respectively.
˚
Ž .
w
Ž
.Ž
.
x w
x
Ž
Ž
..
Ž
Ž ..
Ligands considerable bulkier than acetonitrile can be incorporated into the sixth coordination site.
w
x
w
Ž
.x
Reaction of RuCl2 L2 with PPh3 in refluxing 1,2-dichloroethane gave RuCl2 L2 PPh3 , 8, as a

(
)
B. C¸etinkaya et al.rJournal of Molecular Catalysis A: Chemical 142 1999 101–112
105
Fig. 1. Atom labeling scheme for 1H NMR assignments.
red-brown crystalline solid. The same compound was also obtained from the reaction of L2 with
w
Ž
.
x
Ž .
RuCl2 PPh3 3 in refluxing toluene, Eq. 1 . Thus, there is no problem accommodating ligands as
bulky as PPh3 in these complexes. However, here we do not wish to imply that PPh3 is trans to the
pyridine nitrogen; in fact NMR data would also be consistent with an axial PPh3.
L2
PPh3
RuCl2 L2 ™ RuCl L2 PPh
§ RuCl PPh
1
Ž .
Ž
.
2Ž
.
2
3
3
3
8
1
Elemental analysis and NMR data are consistent with the proposed structure for 8. In the H NMR
spectrum of 8 the methyl and the methoxy protons are observed as singlets at 2.32 and 3.85 ppm,
31
Ä
4
respectively, whereas the pyridine protons are complex multiplets at 7.29–7.42 ppm. The P H
NMR spectrum showed a singlet at 32.9 ppm.
2.2. Catalytic epoxidation of cyclohexene
Metal complexes able to catalyze the selective oxidation of organic substrates under mild
w
x
conditions are very attractive for many industrial processes 31 . In this context, the versatility of
Ž .
w
x
ruthenium II complexes via Ru5O species has been explored 32–40 . An important feature of
high-valent Ru5O complexes is their capability to oxidize C–H and C5C bonds. A number of
ruthenium complexes exhibit good selectivities for the conversion of norbornene, cyclooctene, and
linear alkenes to their respective epoxides, but in the case of cyclohexene, allylic attack is normally
w
x
dominant to yield 2-cyclohexene-1-ol, 2-cyclohexene-1-one, etc. 32–34 . However, the bisoxazoline
w
Ž
. x
complex, RuCl2 biox , has very recently been reported as a stereospecific and regiospecific
2
w
x
epoxidation catalyst 38 .
We have observed that some ruthenium II complexes of pydim, RuCl2 pydim and the aceto-
nitrile adduct, RuCl2 pydim NCCH3 , are effective catalysts for cyclohexene epoxidation by
iodosobenzene PhIO in 1,2-dichloroethane. The results are presented in Table 4.
When a mixture of 0.01 mmol of RuCl2 L or RuCl2 L NCMe and 0.5 mmol of PhIO was
allowed to react at 228C with a solution of cyclohexene 0.5 mmol in DCE a slow reaction ca. 4
Ž .
w
Ž
.x
w
Ž
Ž
.Ž
.x
.
w
x
w
Ž
.x
Ž
.
Ž
.
turnoverrh ensued to produce cyclohexene oxide in a modest yield. In addition, minor amounts of
byproducts were found, among which was 2-cyclohexene-1-one -5% of the products . The
Ž
.
reactions were monitored by GC and products identified by comparison to authentic samples. The
study showed that the ligands with one Me or one But substituent ortho to the imine nitrogens gave
t
Ž
.
lower yields of epoxidation entries 1 and 2 . Whereas, Me, OMe or Bu substituents in the p-position
Ž
.
of the phenyl ring do not inhibit the epoxidation and even enhance it entries 3, 4 and 5 . The

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)
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B. C¸etinkaya et al.rJournal of Molecular Catalysis A: Chemical 142 1999 101–112
Table 2
Summary of crystallographic data, collection parameters and refinement parameters for compound 2
Empirical formula
Formula weight
C27 H26 N4Cl2O2 Ru
586.48
Ž
.
Crystal dimensions mm
Crystal system
0.35=0.30=0.20
monoclinic
Space group
P21 rn
˚
Ž
.
Ž .
12.0587 5
a A
˚
Ž
.
Ž .
14.4898 6
b A
˚
Ž
.
Ž .
14.7997 7
c A
Ž .
Ž .
b
8
92.896 1
3
˚
Ž
.
.
Ž
.
Cell volume A
2575.64 19
Z
4
Ž
.
F 000
1188.70
1.512
3.00-2u -45.00
3
Ž
D calc Mgrm
2u range deg
Ž
.
.
y1
Ž
m mm
0.85
Number of reflections measured
Number of unique reflections
Number of reflections with 1 net
10538
3357
2940
Ž
.
3.0 sigma 1 net
Merging R-value on intensities
0.060
Ž
.
2,6-dimethylaniline-derived ligand was found to be the best of the pydim ligands entries 6 and 7 .
Ž
.
PPh3 and MeCN complexes are generally less effective, see entries 8 and 9 , but in the case of the
2,6-ortho-dimethyl substituted system the acetonitrile complex gives higher yields of epoxide entry
7 . Under the same conditions isomeric complexes, i.e., 5 or 7, are less effective than 6 entries 7, 10
and 11 . Since 5 and 6 are electronically very similar, steric factors seem to play an important role.
Ž
.
Ž
.
Thus, an appropriate choice of substituents around the aryl rings may lead to a more favorable
Ž .
Ž
epoxidation catalyst. To our surprise, the Ru II complexes of related ligands e.g., pybox or
.
Ž
.
pybmbim gave no epoxidation of cyclohexene entries 12 and 13 .
All catalytic reactions were run at room temperature 228C ; at higher temperature 65–708C the
epoxide yield was lower entry 14 . It is worth noting that the mole ratio of PhIO:cyclohexene has a
Ž
.
Ž
.
Ž
.
significant effect on the epoxide yield. Thus, the yield increased as the ratio increased from 1:1 to 2:1
Ž
.
compare entries 15, 16 with 5, 7 , but did not improve significantly upon further increase to 3:1.
Ž
.
Increasing the substrate concentration exhibited the opposite effect entry 17 . This is in sharp contrast
to that of RuCl diphos PF6rPhIO system for which a reversible catalyst-substrate intermediate has
been proposed 35–37 .
w
Ž
. x
2
w
x
We have observed that the amount of PhI released during the catalytic reaction does not reflect the
amount of the oxygen transferred. Consequently, for epoxidation reactions by PhIO, an excess of
w
x
oxidant is required 35–38 . It is found that even in the absence of cyclohexene, the Ru–pydim
complexes react with PhIO to yield PhI and liberate significant amount of oxygen as PhIO becomes
w
x
solubilized. The possible fate of the ‘lost oxygen’ has been discussed by Yang et al. 41 and they
concluded that only a small part of the ‘lost oxygen’ could be attributed to solvent oxidation.
Replacement of PhIO by other oxidants such as H2O2, O2, O2rPhCHO or N-oxides and the
influence of the solvent and additives were also studied. The use of oxidants H2O2, O2 or
Ž
.
O2rPhCHO failed to give any reaction, whereas N-methylmorpholine N-oxide MNO and pyridine
Ž
.
N-oxide were converted into their corresponding heterocycles entries 18–22 . The oxidation reac-
tions were carried out in polar and non-polar solvents with RuCl2 L as catalyst. The yield of
3
w
x

(
)
B. C¸etinkaya et al.rJournal of Molecular Catalysis A: Chemical 142 1999 101–112
107
Table 3
Selected bond lengths A and angles 8 with estimated standard deviations in parentheses for RuCl2 L NCCH3 , 2
2Ž
.x
˚
Ž
.
Ž .
w
Bond Length
Ž
.
Ž .
Ž
.
Ru 1 –Cl 1 2.392 3
Ž . Ž . Ž .
Ru 1 –Cl 2 2.371 3
Ž . Ž .
Ž .
Ž .
Ru 1 –N 1 2.093 9
Ž . Ž .
Ru 1 –N 4 2.068 8
Ž .
Ž
Ž
.
.
Ž .
Ru 1 –N 12 1.906 7
Ž .
Ž .
Ru 1 –N 15
2.038 9
Ž . Ž .
Ž . Ž .
C 2 –C 3
Ž
.
.
.
.
.
.
.
.
N 1 –C 2
1.121 15
Ž
1.447 16
Ž . Ž .
Ž
N 4 –C 5
1.313 12
Ž .
Ž
.
.
Ž
N 4 –C 16
1.446 14
Ž . Ž .
Ž
C 5 –C 6
1.503 15
Ž . Ž .
C 5 –C 7
Ž
1.460 15
Ž .
Ž
Ž
C 7 –N 12
1.376 13
Ž
.
Ž
.
Ž
C 19 –O 22
1.407 14
Bond
Angle
Ž .
Ž .
Ž . Ž .
Ž .
Ž .
176.91 9
Cl 1 –Ru 1 –Cl 2
Ž .
Ž
.
.
.
.
.
.
.
.
Cl 1 –Ru 1 –N 1
Ž . Ž . Ž .
87.54 23
Ž
90.85 23
Cl 1 –Ru 1 –N 4
Ž . Ž .
Ž
Ž
.
Ž
CL 1 –Ru 1 –N 12
93.20 24
Ž .
Ž .
Ž . Ž .
.
Ž
Cl 1 –Ru 1 –N 15
92.37 24
Ž .
Ž
Cl 2 –Ru 1 –N 1
Ž . Ž . Ž .
89.47 23
Ž
90.39 23
CL 2 –Ru 1 –N 4
Ž . Ž .
Ž
Ž
.
.
Ž
Cl 2 –Ru 1 –N 12
Ž . Ž .
89.80 24
Ž
87.50 24
Cl 2 –Ru 1 –N 15
Ž . Ž . Ž .
Ž .
99.4 3
Ž .
179.1 3
N 1 –Ru 1 –N 4
Ž . Ž .
Ž
Ž
Ž
Ž
.
N 1 –Ru 1 –N 12
Ž . Ž .
.
.
.
Ž .
N 1 –Ru 1 –N 15
Ž . Ž .
101.8 3
Ž .
80.0 3
Ž .
158.7 3
N 4 –Ru 1 –N 12
Ž . Ž .
N 4 –Ru 1 –N 15
Ž
.
Ž .
Ž
.
Ž .
N 12 –Ru 1 –N 15
78.8 3
Ž .
173.3 9
Ž . Ž . Ž .
Ru 1 –N 1 –C 2
Ž . Ž . Ž .
Ž
.
N 1 –C 2 –C 3
178.8 14
Ž .
Ž . Ž . Ž .
Ru 1 –N 4 –C 5
113.0 7
Ž . Ž .
C 5 –N 4 –C 16
Ž
.
Ž .
124.1 6
Ž .
122.9 8
Ru 1 –N 4 –C 16
Ž . Ž .
Ž
.
Ž . Ž . Ž .
Ž .
N 4 –C 5 –C 6
125.0 9
Ž . Ž . Ž .
Ž .
N 4 –C 5 –C 7
Ž . Ž . Ž .
116.5 9
Ž .
C 6 –C 5 –C 7
Ž . Ž . Ž .
118.5 8
Ž .
128.3 9
C 5 –C 7 –C 8
Ž
.
epoxide decreases in the order ClCH2CH2Cl)CH2Cl2 )C6H5CH3 entries 5, 23, and 24 . The
Ž
reaction is inhibited by strongly coordinating solvents such as acetone or acetonitrile entries 25 and
26 . The catalytic activities of the acetonitrile adducts, i.e., RuCl2 L NCMe , were less than
corresponding RuCl2 L species entries 5 and 8 . However, RuCl2 L NCMe is an exception entry
7 . Additives such as L or pyridine decreased the epoxidation yield entries 27 and 28 . In the
.
w
Ž
.x
w
x
Ž
.
w
6Ž
.
x
Ž
3
.
Ž
.
3
Ž
.
presence of 0.02 mmol pydim e.g., L or pyridine N-oxide which in situ is readily converted into
pyridine itself, the epoxidation is almost completely inhibited. As mentioned above, an excess of
w
x
cyclohexene also has a retarding effect. Tai et al. 42 have proposed that metal-catalyzed oxidation
by PhIO proceeds via a M–O–I–Ph reactive intermediate. Our attempts to isolate or characterize the

(
)
108
B. C¸etinkaya et al.rJournal of Molecular Catalysis A: Chemical 142 1999 101–112
Ž
.
Ä
w
Ž
.
x
4
Ž .
Fig. 2. ORTEP diagram of acetonitrile dichloro- 2,6-bis 1- 4-methoxyphenylimino ethyl pyridine ruthenium II , 2.
active catalytic species in solution were unsuccessful. Therefore, we cannot unambiguously identify
the structure of the reactive complex that is responsible for the observed epoxidation reaction. Further
investigations of the mechanistic details are needed and are in progress.
3. Experimental section
1
13
1
Ä
4
All manipulations were performed under argon using standard Schlenk techniques. H, C H ,
31
1
19
1
Ä
4
Ä
4
P H and F H NMR spectra were obtained on a Varian Gemini 2000—300 MHz spectrometer
operating at 300, 75.4, 121.5 and 282.3 MHz. Purity of the substrate and analysis of the products were
determined on a Hewlett-Packard 5890A gas chromatograph using a Supelco Petrocol DH 50.2
Ž
.
capillary column 50 m, 0.20 mm i.d., 0.50 mm film thickness and a flame ionization detector.
Elemental analyses were performed by Atlantic Microlab.
Ž
.
Ž
.
RuCl3 P3H2O Johnson and Mathey , 2,6-diacetylpyridine and a-phellandrene Fluka , and substi-
x w x w
Ž
.
w
Ž
.
x
w
Ž
.
x
tuted anilines Aldrich were used as received. RuCl2 p-cymene
28 and RuCl2 PPh3 3 43
2
were synthesized according to published procedures. PhIO was prepared by hydrolysis of iodobenzene
Ž
.
w x
diacetate Aldrich and the samples were stored in a freezer 44 .
(
)
3.1. Ligands pydim
The general conditions for the preparation of pydim ligands were as follows. 2,6-Diacetylpyridine
Ž
.
Ž
.
Ž
1.00 g, 6.14 mmol , the required aniline 15 mmol , a catalytic amount of glacial acetic acid 2–3
.
Ž
.
drops and methanol 10 ml were heated under reflux for 1–4 h. Upon cooling to room temperature
yellow crystals were deposited. They were filtered off, washed with pentane 3=5 ml and dried. In
the case of L and L , longer refluxing periods 24 and 4 h, respectively and a large excess of the
Ž
.
6
7
Ž
.
1
Ž
.
aniline 20 mmol were required for completion of the condensation. H NMR data are presented in
Table 1. The yields, melting points, microanalytical and 13C NMR data are given below.
L3 1.80 g, 69% . Mp:199–2008C. Anal. calcd. for C29H35N3: C, 81.88; H, 8.24; N, 9.88. Found:
Ž
.
C, 81.7; H, 8.3; N, 9.8. 13C NMR: d 167.2, 155.8, 148.3, 146.2, 138.8, 125.8, 122.2, 119.1, 34.3,
16.2.

(
)
B. C¸etinkaya et al.rJournal of Molecular Catalysis A: Chemical 142 1999 101–112
109
Table 4
Ž
.
Ž
.
Ž
.
Ž
.
Effect of catalyst 0.01 mmol , solvent 5 ml , oxidant 0.5 mmol and additives on the epoxidation of cyclohexene 0.5 mmol at 228C for 8
h†
Entry
Metal complex
Solvent
Oxidant
Yielda
Tonb
5
w
w
w
w
w
w
w
w
w
w
w
w
w
w
w
w
w
w
w
w
w
w
w
w
w
w
w
w
x
x
x
x
x
x
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
RuCl2 L
RuCl2 L
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCM
DCE
DCE
DCE
DCE
DCM
PhMe
Me2CO
MeCN
DCE
DCE
PhIO
PhIO
PhIO
PhIO
PhIO
PhIO
PhIO
PhIO
PhIO
PhIO
PhIO
PhIO
PhIO
PhIO
PhIO
PhIO
PhIO
H2O2
O2
O2 rPhCHO
MNO
PyNO
PhIO
PhIO
PhIO
PhIO
PhIO
PhIO
5
9
26
25
27
31
42
10
2.5
4.5
13
12.5
13.5
15.5
21
5
5
6.5
9
–
–
6.5
18.5
32.5
9
–
–
–
–
–
9
4
1
RuCl2 L
2
RuCl2 L
RuCl2 L
3
6
RuCl2 L
6Ž
.x
RuCl2 L NCMe
2Ž
.x
RuCl2 L PPh3
3
Ž
.x
RuCl2 L NCMe
10
13
18
5Ž
.x
RuCl2 L NCMe
7Ž
.
RuCl2 L NCMe
Ž
Ž
.x
RuCl2 Pybox
-1
-1
13c
37d
65d
18e
-0.5
-0.5
-0.5
-0.5f
)0.5g
18
.x
RuCl2 Pybmbim
3
x
x
RuCl2 L
3
RuCl2 L
6Ž
.x
RuCl2 L NCMe
3
x
x
x
x
x
x
x
x
x
x
x
x
RuCl2 L
RuCl2 L
RuCl2 L
RuCl2 L
RuCl2 L
RuCl2 L
RuCl2 L
RuCl2 L
RuCl2 L
RuCl2 L
RuCl2 L
RuCl2 L
3
3
3
3
3
3
3
3
3
3
3
5
2.5
–
–
–
2
-1
-1
-1h
4i
†Abbreviations: DCE, 1,2-dichloroethane; DCM, dichloromethane; MNO, N-methymorpholine N-oxide; PhIO, iodosobenzene; Pybox,
Ž
.
Ž
.
2,6-bis oxazolin-2-yl pyridine; Pybmbim, 2,6-bis N-methylbenzimidazol-2-yl pyridine; PyNo, pridine N-oxide.
a Determined by GC, n-octane used as internal standard.
b
Ž
.
TON is amount mmol of epoxide formed per mmol of catalyst.
c Reaction carried out at 65-708C.
d1.0 mmol PhIO used.
e2.0 mmol olefin used, yield based on PhIO.
f N-oxide converted into N-methylmorpholine.
g N-oxide converted into pyridine.
h 0.02 mmol L3 added.
i 0.02 mmol pyridine N-oxide added.
L4 1.54 g, 58% . Mp: 132–1338C. Anal. calcd. for C29H35N3: C, 81.88; H, 8.24; N, 9.88. Found:
C, 81.7; H, 8.3; N, 9.8. 13C NMR: d 165.3, 155.6, 149.6, 139.7, 136.9, 126.5, 126.4, 123.8. 122.3,
119.8, 35.1, 29.6, 16.9.
Ž
.
L5 1.85 g. 81% . Mp: 118–1198C. Anal. calcd. For C25H27N3: C, 81.30; H. 7.32; N, 11.37.
Found: C, 81.1; H, 7.4; N, 11.5. 13C NMR: d 166.9, 155.5, 147.4, 136.7, 132.9, 131.1, 127.1, 126.9,
122.2, 118.1, 20.8, 17.7, 16.2.
Ž
.
L6 1.69 g, 61% . Mp: 172–1738C. Anal. calcd. for C25H27N3: C, 81.30; H, 7.32; N, 11.38. Found:
Ž
.
C, 80.7; H, 7.5; N, 11.3. 13C NMR: d 167.2, 155.2, 148.8, 136.9, 127.9, 125.4, 123.2, 122.3, 18.0,
16.5.

(
)
110
B. C¸etinkaya et al.rJournal of Molecular Catalysis A: Chemical 142 1999 101–112
7
Ž
.
L 1.18 g, 52% . Mp: 127–1298C. Anal. calcd. for C25H27N3: C, 81.30, H, 7.32; N. 11.38. Found:
C, 80.5; H, 7.4; N, 11.3. 13C NMR: d 167.0, 155.6, 151.3, 138.6, 136.8, 125.3, 125.2, 122.2, 116.9,
21.4, 16.2.
3.2. General procedure for the preparation of ruthenium complexes 1–7
w
Ž
.
x
Ž
.
Ž .
RuCl2 p-cymene
306 mg, 0.50 mmol and 1.06 equivalent of the corresponding pydim L
2
Ž
.
were dissolved in CH2Cl2 10 ml and the solution was heated under reflux for 10 h. The resulting
deep purple solution was cooled to room temperature and was concentrated to ca. 5 ml in vacuo. The
Ž
.
Ž
.
products were precipitated by addition of pentane 15 ml , filtered, washed with pentane 2=10 ml
4
6
Ž
.
and dried. However, o-substituted Schiff bases L –L were sluggish and gave incomplete substitu-
.x
tion reactions with RuCl2 p-cymene in CH2Cl2 or even in boiling ClCH2CH2Cl. Fortunately,
2
however, boiling ethyl alcohol 10 ml was found to induce complete displacement of p-cymene.
w
Ž
Ž
.
After the reaction the solvent was removed to dryness in vacuo. The residue was dissolved in CH2Cl2
Ž
.
Ž
.
5 ml and was precipitated by addition of pentane 15 ml . The microcrystalline precipitates,
containing one molecule of CH2Cl2 per molecule of the complex, were used as the epoxidation
catalysts.
For full characterization, each complex can be converted into the hexacoordinated acetonitrile
w
x
Ž
.
adducts by warming RuCl2 L in CH3CNrClCH2CH2Cl 0.5:5 ml or in CH3CNrCH2Cl2 and then
Ž
.
w
Ž
adding pentane 10 ml . Here, a more direct method was employed, i.e., a solution of RuCl2 p-cy-
.
x
Ž
. Ž . Ž .
mene
0.50 mmol and pydim 1.06 mmol was refluxed in EtOH or CH2Cl2 10 ml . After the
2
Ž
.
Ž
.
reaction, the alcohol was replaced by CH2Cl2 or ClCH2CH2Cl 5 ml , and acetonitrile 0.5 ml .
Pentane 10 ml was then added to precipitate the complexes 1–7 which are generally solvated with
CH2Cl2 or ClCH2CH2Cl. H NMR data are given in Table 1 together with data for the correspond-
Ž
.
1
w
Ž
.x
ing ligands. The reported yields are based on the RuCl2 p-cymene used.
2
Ž
.
1 0.45 g, 82% . Mp:)3008C. Anal. calcd. for C25H26N4Cl2Ru: C, 54.15; H, 4.69; N, 10.11.
Found: C, 53.7; H, 4.7; N, 10.0. 13C NMR: d 169.5, 162.8, 147.2, 135.7, 128.7, 127.1, 125.7, 121.1,
20.5, 17.0, 3.2.
Ž
.
2 0.45 g, 80% . Mp:)3008C. Anal. calcd. for C25H26N4Cl2O2Ru: C, 51.19; H, 4.44; N, 9.56.
Found: C, 49.7; H, 4.6; N, 9.2. 13C NMR: d 169.7, 163.1, 158.0, 143.2, 127.4, 126.1, 123.7, 122.3,
113.5, 55.4, 17.3, 1.7.
Ž
.
3 0.56 g, 76% . Mp:)3008C. Anal. calcd. for C31H38N4Cl2RuPClCH2CH2Cl: C, 53.73; H,
5.70; N, 7.60. Found: C, 53.9; H, 5.7; N, 7.7. 13C NMR: d 169.8, 163.2, 149.2, 147.5, 127.5, 125.9,
125.4, 122.5, 122.1, 44.0, 34.5, 31.2, 17.4, 3.5.
Ž
.
4 0.31 g, 58% . Mp:)3008C. Anal. calcd. for C31H38N4Cl2Ru: C, 58.30; H, 5.96; N, 8.78.
Found: C, 57.1; H, 5.8; N, 8.5. 13C NMR: d 173.7, 163.5, 148.9, 141.5, 128.8, 128.0, 126.6, 126.0,
125.7, 123.8, 36.3, 31.8, 19.8, 3.2.
Ž
.
5 0.39 g, 60% . Mp:)3008C. Anal. calcd. for C27H30N4Cl2RuPCH2Cl2: C, 50.38; H, 4.80; N,
8.41. Found: C, 51.7; H, 4.9; N, 8.4. 13C NMR: d 170.7, 162.8, 146.5, 135.3, 130.8, 129.2, 126.2,
126.1, 125.8, 122.7, 122.2, 20.4, 18.2, 17.0, 3.1.
Ž
.
6 0.49 g, 74% Mp:)3008C. Anal. calcd. for C27H30N4Cl2RuPCH2Cl2: C, 50.38; H, 4.80; N,
8.40. Found: C, 50.5; H, 4.9; N, 8.7. 13C NMR: d 172.9, 162.9, 147.9, 131.8, 128.7, 128.4, 125.3,
123.7, 20.4, 17.9, 3.1.
Ž
.
7 0.45 g, 68% Mp:)3008C. Anal. calcd. for C27H30N4Cl2RuPCH2Cl2: C, 55.67; H, 5.15; N,
9.62. Found: C, 54.5 H, 5.1; N, 9.4. 13C NMR: d 170.0, 163.4, 150.1, 138.4, 128.1, 128.0, 126.2,
122.6, 120.4, 120.3, 44.2, 21.4, 17.6, 3.5.

(
)
B. C¸etinkaya et al.rJournal of Molecular Catalysis A: Chemical 142 1999 101–112
111
2
[
(
)] ( )
3.3. Synthesis of RuCl2 L PPh3
8
Two different methods were used for the synthesis of 8.
( )
3.3.1. Method i
2
w
Ž
.
x
Ž
.
Ž
.
A solution of RuCl2 p-cymene 92 mg, 0.30 mmol and L 112 mg, 0.30 mmol in ClCH2CH2Cl
Ž
.
Ž
3 ml was refluxed for 10 min. The resulting deep-purple solution was cooled to 258C and PPh3 78
.
mg, 0.30 mmol was added and the solution refluxed for a further 10 min. while the color turned into
Ž
.
red-brown. The solution was cooled to 258C, Et2O 10 ml was added to obtain a crystalline product
which was filtered, washed with Et2O 2=5 ml and dried in vacuo. Yield: 0.22 g, 89%.
Ž
.
Mp:)3008C.
( )
3.3.2. Method ii
2
w
Ž
.
x
Ž
.
Ž
.
Ž
.
.
A mixture of RuCl2 PPh3 3 0.32 g, 0.33 mmol and L 0.12 g, 0.35 mmol in toluene 10 ml
Ž
was heated under reflux for 4 h. Volatiles were removed. The residue was dissolved in CH2Cl2 5 ml
Ž
.
and then Et2O 10 ml was added to precipitate the product which was filtered and washed with
Ž
.
hexane 2=10 ml and dried. Yield: 0.24 g, 96%. This material was identical in every respect to the
Ž .
product obtained by method i .
13C NMR: d 171.4, 163.0, 158.5, 141.1, 132.9, 132.3, 131.7, 129.0, 128.0, 127.2, 124.8, 112.4,
31
Ž .
55.6, 18.2. P NMR: d 32.9 s .
3.4. General procedure for reactions of cyclohexene with oxidants in the presence of metal catalysts
Ž
.
In a typical oxidation reaction, the catalysts 0.01 mmol and the olefin was dissolved in 5 ml
Ž
.
solvent. Co-oxidants 0.5 mmol, unless otherwise stated were added all at once to the solution under
argon with constant stirring. Aliquots were taken at timed intervals and analyzed by GLC; n-octane
was used as internal standard.
3.5. X-ray structure determinations
An X-ray quality crystal of 2 was grown by diffusion of pentane into a CH3CNrClCH2CH2Cl
solution at 228.
The intensity data were collected at y1008C on a Siemens SMART diffractometer, using the
omega scan mode and graphite monochromated MoKa radiation. The structure was solved using
direct methods. The last least squares cycle, calculated with 60 atoms, 308 parameters and 2719 out of
3357 reflections resulted in a final Rs0.076, Rw s0.104. Crystals diffracted very poorly so data
was cut off at 458. All computations were performed using the NRCVAX suite of programs.
Acknowledgements
We thank T.N. Sorrell for helpful discussion and suggestions. B.C. thanks the Fulbright Foundation
for a 5-month fellowship to initiate this work in Chapel Hill, NC. Partial support from the National
Ž
.
Institutes of Health GM28938 is acknowledged.

(
)
112
B. C¸etinkaya et al.rJournal of Molecular Catalysis A: Chemical 142 1999 101–112
References
w x
Ž .
J.C. Adrian Jr., L. Hassib, N. DeKimpe, M. Keppens, Tetrahedron 54 1998 2365.
1
w x
2
Ž .
E.C. Constable, T. Kulke, M. Neuburger, M. Zehnder, New J. Chem. 21 1997 1091.
Ž .
A.C. Benniston, Tetrahedron Lett. 38 1997 8279.
w x
3
w x
Ž .
4
E.C. Constable, P. Harverson, D.R. Smith, L. Whall, Polyhedron 16 1997 3615.
w x
Ž .
T.W. Welch, S.A. Ciftan, P.S. White, H.H. Thorp, Inorg. Chem. 36 1997 4812.
B.A. Markies, P. Wijkens, J. Boersma, H. Kooijman, N. Veldman, A.L. Spek, G. van Koten, Organometallics 13 1994 3244.
Ž .
S.M. Redmore, C.E.F. Rickard, S.J. Webb, J.L. Wright, Inorg. Chem. 36 1997 4743.
Ž .
R. Nesper, P. Pregosin, K. Puntener, M. Worle, A. Albinati, J. Organomet. Chem. 507 1996 85.
¨
Ž .
H. Nishiyama, M. Kondu, T. Nakaruma, K. Itoh, Organometallics 10 1991 500.
5
w x
Ž .
6
w x
7
w x
8
w x
9
w
w
w
w
w
w
w
w
w
w
w
w
w
w
w
w
w
w
w
w
w
w
x
Ž
.
10 H. Nishiyama, H. Sakaquchi, T. Nakamura, M. Horihata, M. Kondo, K. Itoh, Organometallics 8 1989 846.
x
Ž
.
11 H. Nishiyama, Y. Itoh, H. Matsumato, S. -Bong Park, K. Itoh, J. Am. Chem. Soc. 116 1994 2223.
x
Ž
.
12 H. Nishiyama, Y. Itoh, Y. Suguwara, H. Matsumoto, K. Aoki, K. Itoh, Bull. Chem. Soc. Jpn. 68 1995 1247.
x
Ž
.
13 D.L. Christenson, C.J. Tokar, W.B. Tolman, Organometallics 14 1995 2148.
x
Ž
.
14 J.H. Groen, P.W.N M. van Leeuwen, K. Vrieze, J. Chem. Soc., Dalton Trans. 1998 113.
x
Ž
.
15 P.R. Meehan, E.C. Alyea, G. Ferguson, Acta Crystallogr. C 53 1997 888.
x
Ž
.
16 H.F. Haarman, F.R. Bregman, J. Meine Ernsting, N. Veldman, A.L. Spek, K. Vrieze, Organometallics 16 1997 54.
x
Ž
.
17 E.C. Alyea, P.H. Merrell, Synth. React. Inorg. Metal-Org. Chem. 4 1974 535.
x
Ž
.
18 E.C. Alyea, Inorg. Chim. Acta 76 1983 L239.
x
Ž
.
19 P.H. Merrell, E.C. Alyea, L. Ecott, Inorg. Chim. Acta 59 1982 25.
x
Ž
.
20 E.C. Alyea, P.H. Merrell, Inorg. Chim. Acta 28 1978 91.
x
Ž
.
21 E.C. Alyea, G. Ferguson, R.J. Restivo, J. Chem. Soc. Chem. Commun. 1975 269.
x
Ž
.
22 E.C. Alyea, G. Ferguson, R.J. Restivo, Inorg. Chem. 14 1975 2491.
x
Ž
.
23 D.A. Edwards, M.F. Mahon, W.R. Martin, K.C. Molloy, P.E. Fanwick, R.A. Walton, J. Chem. Soc. Dalton Trans. 1990 3161.
x
Ž
.
24 A.J. Blake, A.J. Lavery, T.I. Hyde, M. Schroder, J. Chem. Soc. Dalton Trans. 1989 965.
¨
x
Ž
.
25 M.J. Blandamer, R.J. Burgess, R.I. Haines, F.M. Mekhail, P. Askalani, J. Chem. Soc., Dalton Trans. 1978 1001.
x
Ž
.
26 J.M. Albon, D.A. Edwards, P.J. Moore, Inorg. Chim. Acta 159 1989 19.
x
Ž
.
27 S. Lu, J. Selbin, Inorg. Chim. Acta 134 1987 229.
x
Ž
.
28 M.A. Bennett, A.K. Smith, J. Chem. Soc., Dalton Trans. 1974 233.
x
Ž
.
29 R.R. Schrock, B.F.G. Johnson, J. Lewis, J. Chem. Soc., Dalton Trans. 1974 951.
x
Ž
.
30 T.V. Ashworth, E.J. Singleton, J. Organomet. Chem. 77 1974 C31.
x
Ž
.
31 D. Riley, M. Stern, J. Ebner, in: D.H.R. Barton, A.E. Martell, D.T. Sawyer, Eds. , The Activation of Dioxygen and Homogeneous
Catalytic Oxidation, Plenum, New York, 1993, pp. 31–44.
w
w
w
w
w
w
w
w
x
Ž
.
32 G.A. Barf, R.A. Sheldon, J. Mol. Catal. 98 1995 143.
x
Ž
.
33 G.A. Barf, D. van den Hoek, R.A. Sheldon, Tetrahedron 52 1996 12971.
x
Ž
.
34 M. Bressan, A. Morvillo, Inorg. Chem. 28 1989 950.
x
Ž
.
Ž
35 A. Morvillo, M. Bressan, J. Mol. Catal. 125 1997 119.
x
.
36 A.S. Kanmani, S. Vancheesau, J. Mol. Catal. 125 1997 127.
x
Ž
Ž
.
.
37 M. Bressan, A. Morville, J. Chem. Soc., Chem. Commun. 1988 650.
x
38 V. Kesavan, S.J. Chandrasekaran, J. Chem. Soc., Perkin 1 1997 3115.
x
39 K. Mizushima, M. Nakaura, S. -Bong Park, H. Nishiyama, H. Mongjushiro, K. Harada, M. Haga, M. Aki, Inorg. Chim. Acta 261
Ž
.
1997 175.
w
w
w
w
w
x
Ž
.
40 R.I. Kureshy, N.H. Khan, S.H.R. Abdi, K.N. Bhatt, Tetrahedron: Asymmetry 4 1993 1693.
x
Ž
.
41 Y. Yang, F. Diederich, J.S. Valentine, J. Am. Chem. Soc. 113 1991 7195.
x
Ž
.
42 A.F. Tai, L.D. Margerum, J.S. Valentine, J. Am. Chem. Soc. 108 1986 5006.
x
Ž
.
43 S.F. Colson, S.D. Robinson, Inorg. Chim. Acta 149 1988 13.
x
Ž
.
44 H. Saltzman, J.G. Sharefkin, Org. Synth. 43 1963 60.