Highly soluble dichloro, dibromo and dimethyl dioxomolybdenum(VI)- bipyridine complexes as catalysts for the epoxidation of olefins

Article abstract of DOI:10.1016/j.molcata.2010.08.014

The reaction of solvent substituted MoO₂X₂(S) ₂ (X = Cl, S = THF; X = Br, S = DMF) complexes with one equivalent of bidentate nitrogen donor ligands at room temperature leads within a few minutes to the quantitative formation of complexes of the type [MoO₂X ₂L₂] (L = 4,4′-bis-methoxycarbonyl-2,2′- bipyridine, 5,5′-bis-methoxycarbonyl-2,2′-bipyridine, 4,4′-bis-ethoxycarbonyl-2,2′-bipyridine, 5,5′-bis- ethoxycarbonyl-2,2′-bipyridine). Treatment of the complexes [MoO ₂Cl₂L₂] with Grignard reagents at low temperatures yields dimethylated complexes of the formula [MoO ₂(CH₃)₂L₂]. [MoO₂Br ₂(4,4′-bis-ethoxycarbonyl-2,2′-bipyridine)], [MoO ₂Br₂(5,5′-bis-methoxycarbonyl-2,2′-bipyridine) ] and [MoO₂Br₂(5,5′-bis-ethoxycarbonyl-2,2′- bipyridine)] have been exemplary examined by single crystal X-ray analysis. The complexes were applied as homogenous catalysts for the epoxidation of cyclooctene with tert-butyl hydroperoxide (TBHP) as oxidising agent under solvent-free conditions. The complexes containing L = Cl have been additionally investigated with room temperature ionic liquids (RTILs) as solvents. The catalytic activity of the [MoO₂X₂L₂] complexes in olefin epoxidation with tert-butyl hydroperoxide is on average very good. The main advantage of the synthesised complexes in comparison to previously reported complexes is their high solubility. This good solubility is apparently the reason that the catalytic potential of the compounds can unfold. The turnover frequencies (TOFs) in RTILs are even higher, showing the performance of the catalysts under optimised conditions.

Full text of DOI:10.1016/j.molcata.2010.08.014

Journal of Molecular Catalysis A: Chemical 331 (2010) 117–124
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Highly soluble dichloro, dibromo and dimethyl dioxomolybdenum(VI)-bipyridine
complexes as catalysts for the epoxidation of olefins
Alev Günyara, Daniel Betzb, Markus Dreesb, Eberhardt Herdtweckb, Fritz E. Kühna,b,∗
a Molecular Catalysis, Catalysis Research Center, Technische Universität München, Lichtenbergstrasse 4, D-85747 Garching bei München, Germany
b Chair of Inorganic Chemistry, Catalysis Research Center, Technische Universität München, Lichtenbergstrasse 4, D-85747 Garching bei München, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 May 2010
Received in revised form 29 July 2010
Accepted 5 August 2010
Available online 14 August 2010
The reaction of solvent substituted MoO2X2(S)2 (X = Cl, S = THF; X = Br, S = DMF) complexes with one
equivalent of bidentate nitrogen donor ligands at room temperature leads within a few minutes
to the quantitative formation of complexes of the type [MoO2X2L2] (L = 4,4ꢀ-bis-methoxycarbonyl-
2,2ꢀ-bipyridine, 5,5ꢀ-bis-methoxycarbonyl-2,2ꢀ-bipyridine, 4,4ꢀ-bis-ethoxycarbonyl-2,2ꢀ-bipyridine,
5,5ꢀ-bis-ethoxycarbonyl-2,2ꢀ-bipyridine). Treatment of the complexes [MoO2Cl2L2] with Grignard
reagents at low temperatures yields dimethylated complexes of the formula [MoO2(CH3)2L2].
Keywords:
Homogeneous catalysis
Molybdenum
Bipyridine complexes
Epoxidation
Ionic liquid
[MoO2Br2(4,4ꢀ-bis-ethoxycarbonyl-2,2ꢀ-bipyridine)],
[MoO2Br2(5,5ꢀ-bis-methoxycarbonyl-2,2ꢀ-
bipyridine)] and [MoO2Br2(5,5ꢀ-bis-ethoxycarbonyl-2,2ꢀ-bipyridine)] have been exemplary examined by
single crystal X-ray analysis. The complexes were applied as homogenous catalysts for the epoxidation
of cyclooctene with tert-butyl hydroperoxide (TBHP) as oxidising agent under solvent-free conditions.
The complexes containing L = Cl have been additionally investigated with room temperature ionic
liquids (RTILs) as solvents. The catalytic activity of the [MoO2X2L2] complexes in olefin epoxidation with
tert-butyl hydroperoxide is on average very good. The main advantage of the synthesised complexes in
comparison to previously reported complexes is their high solubility. This good solubility is apparently
the reason that the catalytic potential of the compounds can unfold. The turnover frequencies (TOFs) in
RTILs are even higher, showing the performance of the catalysts under optimised conditions.
© 2010 Published by Elsevier B.V.
1. Introduction
with Lewis bases (L or L2), such as pyridine, 2,2-bipyridine, 1,10-
phenanthroline and 4,4ꢀ-tert-butyl-2,2ꢀ-bipyridine, and with donor
Molybdenum(VI) complexes are applied as homogeneous cata-
lysts in industrial processes for the epoxidation of propylene with
alkyl hydroperoxides as oxygen source since many years [1]. Epox-
ides are important organic intermediates undergoing ring-opening
reactions with a variety of reagents to yield mono- or bifunctional
organic products [2,3]. In general, epoxides can be prepared by
the reaction of olefins with hydrogen peroxide or alkyl hydroper-
oxides, catalyzed by transition metal complexes [1b,1c,4]. Among
the variety of efficient catalysts known today for olefin epoxida-
tion are several organometallic and inorganic oxides containing
a metal in its highest oxidation state [5]. It is well known that
complexes of the type [MoO2X2(L)n] (X = Cl, Br, CH3; L = mono- or
bidentate neutral N-ligand) are versatile catalyst precursors for the
epoxidation of olefins in the presence of tert-butyl hydroperoxide
(TBHP) [6–8]. Treatment of [MoO2X2] species (X = halide, OR, OSiR3)
solvents such as acetonitrile and THF, leads to adducts of com-
position [MoO2X2L2] [5,9–11]. In the presence of TBHP as oxygen
source some of these complexes have shown quite good catalytic
activities. TBHP is one of the industrially preferred oxygen sources,
partly because it is a mild and selective oxidant, not particularly cor-
rosive or hazardous, and the byproduct of the reaction, tert-butyl
alcohol, can be separated and recycled for use in other industrial
processes [12]. Important properties, such as the solubility of the
complex and the Lewis acidity of the metal centre can be fine-tuned
by variation of both ligands X and L of the [MoO2X2L2] complexes.
Dioxomolybdenum(VI) halides and related compounds are use-
ful precursors for the synthesis of other molybdenum complexes
via oxygen or halogen/ligand abstraction or substitution [13–16].
The usual route to bromo complexes consists of reacting anhy-
drous [MoO2Br2] with the appropriate ligand in aprotic solvents
[17]. However, [MoO2Br2] is not readily available [18,19] and has
accordingly limited the coordination chemistry of this dibromodi-
oxomolybdenum(VI) family. Another synthetic procedure is based
on the reaction of aqueous solutions of molybdates in hydrobromic
acid with neutral ligands [20]. However, the procedure has not been
extended to the synthesis of related species. Thus, the number of
∗
Corresponding author at: Molecular Catalysis, Catalysis Research Center, Tech-
nische Universität München, Lichtenbergstrasse 4, D-85747 Garching bei München,
Germany.
E-mail address: fritz.kuehn@ch.tum.de (F.E. Kühn).
1381-1169/$ – see front matter © 2010 Published by Elsevier B.V.
doi:10.1016/j.molcata.2010.08.014

118
A. Günyar et al. / Journal of Molecular Catalysis A: Chemical 331 (2010) 117–124
dioxomolybdenum(VI) bromide adducts that has been reported is
low in comparison to those of [MoO2Cl2] [17].
4,4ꢀ-Bis-methoxycarbonyl-2,2ꢀ-bipyridine:
Yield
0.76 g
(2.8 mmol, 62%). 1H NMR (400 MHz, CDCl3, 20 ◦C, ppm): ı = 8.94
ꢀ
ꢀ
3
In this work the synthesis of well soluble compounds is
described, since most of the [MoO2X2(bipy)] compounds reported
so far are only fairly soluble in most organic solvents. This low
to moderate solubility appears to be responsible for the fact that
[MoO2X2L2]-type catalysts are usually regarded as inferior to other,
related homogeneous epoxidation catalysts, such as CH3ReO3
(MTO) or [CpMoO2R] derivatives [5], although the [MoO2X2L2]
compounds would be both cheaper and more easily accessible than
the former compounds. Furthermore, the electronic contribution of
the different functional groups in different positions at the bipyri-
dine rings and the implications of these differences on the catalyst
activity are examined.
(s, 2H, py-H6,6 ), 8.84 (d, JH,H = 5.1 Hz, 2H, py-H3,3 ), 7.88 (dd,
ꢀ
3JH,H = 5.1 Hz, 2H, py-H5,5 ), 3.98 (s, 6H, CH3).
4,4ꢀ-Bis-ethoxycarbonyl-2,2ꢀ-bipyridine:
Yield
0.95 g
(3.2 mmol, 71%). 1H NMR (400 MHz, CDCl3, 20 ◦C, ppm): ı = 8.93
ꢀ
ꢀ
(s, 2H, py-H6,6 ), 8.84 (d, JH,H = 4.9 Hz, 2H, py-H3,3 ), 7.89 (dd,
3
ꢀ
3JH,H = 4.9 Hz, 2H, py-H5,5 ), 4.44 (q, JH,H = 7.2 Hz, 4H, CH2), 1.42
3
(t, 3JH,H= 7.8 Hz, 6H, CH3).
5,5ꢀ-Bis-methoxycarbonyl-2,2ꢀ-bipyridine:
Yield
0.72 g
(2.6 mmol, 58%). 1H NMR (ꢀ400 MHz, CDCl3, 20 ◦C, ppm): ı = 9.28 (d,
ꢀ
3JH,H = 1.66 Hz, 2H, py-H3,3 ), 8.57 (dꢀd, 3JH,H = 8.35 Hz, 2H, py-H6,6 ),
8.43 (dd, 3JH,H = 8.26 Hz, 2H, py-H4,4 ), 3.98 (s, 6H, CH3).
5,5ꢀ-Bis-ethoxycarbonyl-2,2ꢀ-bipyridine: Yield 0.90 g (3 mmol,
67%). 1H NMR (400 MHz, CDCl3, 20 ◦C, ppm): ı = 9.28 (s, 2H, py-
ꢀ
ꢀ
H3,3 ), 8.56 ꢀ(d, JH,H = 8.2 Hz, 2H, py-H6,6 ), 8.42 (dd, JH,H = 8.6 Hz,
2H, py-H4,4 ), 4.43 (q, 3JH,H = 7.3 Hz, 4H, CH2), 1.42 (t, 3JH,H = 7.7 Hz,
6H, CH3).
3
3
2. Experimental
General: All preparations and manipulations were carried out
under an oxygen- and water-free argon atmosphere with standard
Schlenk techniques. [MoO2Cl2], TBHP (5.5 M solution in decane,
stored over molecular sieve) was purchased from Aldrich, 4,4ꢀ-
dicarboxy-2,2ꢀ-bipyridine and 5,5ꢀ-dicarboxy-2,2ꢀ-bipyridine were
purchased from HetCat and used as received. Solvents were dried
by standard procedures, distilled, and kept under argon over molec-
ular sieves (hexane, THF, and diethyl ether over Na/benzophenone
ketyl, dichloromethane, chloroform over CaH2, methanol and
ethanol over Mg/I2). [MoO2Br2(DMF)2] was prepared according
to published procedures [21]. Elemental analyses were performed
with a Flash EA 1112 series elemental analyzer. 1H, 13C and
95Mo NMR spectra were measured in CDCl3 with a 400 MHz
Bruker Avance DPX-400 spectrometer. IR spectra were recorded
with a PerkinElmer FT-IR spectrometer using KBr pellets as the
IR matrix. Catalytic runs were monitored by GC methods on a
Varian CP-3800 instrument equipped with a FID and a VF-5ms
column. Thermogravimetric analyses were performed using a Net-
zsch TG209 system at a heating rate of 10 K min−1under argon.
X-ray structure determination [22] was carried out on an area
detecting system (APEX II, ꢀ-CCD) at the window of a rotating
anode (Bruker AXS, FR591) and graphite monochromated Mo K␣
2.2. Complex synthesis
Synthesis of complexes 1–4: The complex [MoO2Cl2(THF)2]
was dissolved in CH2Cl2 (5 ml) and treated with 1 equiv. of ligands
that were also dissolved in CH2Cl2 (10 ml). The resulting solutions
were each stirred for 1 h. The solvent was removed in vacuo, and
the product washed with diethyl ether (2 × 5 ml) and dried under
vacuum.
[MoO2Cl2(4,4ꢀ-bis-methoxycarbonyl-2,2ꢀ-bipyridine)](1)
[MoO2Cl2(THF)2] (0.96 mmol). Yield: 0.45 g (95%). Color: white.
Selected IR (KBr): ꢂ(cm−1) = 1726(vs), 1563(s), 1435(s), 1020(w),
951(vs), (Mo Osym), 918(vs) (Mo Oasym). 1H NMR (400 MHz,
ꢀ
CDCl3, 20 ◦C, ppm): ı = 9.72 (d, 3JH,H = 5.5 Hz, 2H, py-H6,6 ), 8.90 (s,
ꢀ
ꢀ
2H, py-H3,3 ), 8.28 (d, 3JH,H = 5.5 Hz, 2H, py-H5,5 ), 4.10 (s, 6H, CH3).
13C NMR (100 MHz, CDCl3, 20 ◦C, ppm): ı = 163.7 (C O), 153.2
ꢀ
ꢀ
ꢀ
ꢀ
(py-C4,4ꢀ ), 150.3 (py-C2,2 ), 142.1 (py-C6,6 ), 126.8 (py-C3,3 ), 122.7
(py-C5,5 ), 54.0 (CH3). Anal. Calc. For C14H12Cl2MoN2O6 (471.10):
C 35.69, H 2.57, N 5.95. Found C 34.88, H 2.65, N 5.70.
[MoO2Cl2(5,5ꢀ-bis-methoxycarbonyl-2,2ꢀ-bipyridine)](2)
[MoO2Cl2(THF)2] (1.75 mmol). Yield: 0.77 g (94%). Color:
white. Selected IR (KBr): ꢂ(cm−1) = 1730(vs), 1608(s), 1431(vs),
˚
radiation (ꢁ = 0.71073 A). Raw data were corrected for Lorentz
1045(s), 942(vs), (Mo Osym), 910(vs) (Mo Oasym). 1H NMR
polarization, and, arising from the scaling procedure, for latent
decay and absorption effects. The structures were solved by a com-
bination of direct methods and difference Fourier syntheses. All
non-hydrogen atoms were refined with anisotropic displacement
ꢀ
(400 MHz, CDCl3, 20 ◦C, ppm): ı = 10.14 (d, 2H, py-H3,3 ), 8.79 (dd,
ꢀ
ꢀ
3JH,H = 8.2 Hz, 2H, py-H6,6 ), 8.41 (d, JH,H = 8.4 Hz, 2H, py-H4,4 ),
3
4.05(s, 6H, CH3). 13C ꢀNMR (100 MHz,ꢀCDCl3, 20 ◦C, ppm): ı = 163.3
ꢀ
(C O), 153.5 (py-C2,2 ), 151.7 (py-C6,6 ), 142.0 (py-C4,4 ), 130.0 (py-
parameters. Full-matrix least-squares refinements were carried out
ꢀ
ꢀ
C5,5 ), 123.2 (py-C3,3 ), 53.4 (CH3). 95Mo NMR (26 MHz, CDCl3, 20 ◦C,
ppm): ı = 201 ppm. Anal. Calc. For C14H12Cl2MoN2O6 (471.10): C
35.69, H 2.57, N 5.95. Found C 36.16, H 2.88, N 5.67.
ꢀ
by minimising
w(Fo2 − Fc2)2 with SHELXL-97 weighting scheme.
The final residual electron density maps showed no remarkable
features. Neutral atom scattering factors for all atoms and anoma-
lous dispersion corrections for the non-hydrogen atoms were taken
from the International Tables for Crystallography. Detailed infor-
mation on the crystallographic data of compounds 6, 7, and 8 are
provided in the supporting information.
[MoO2Cl2(4,4ꢀ-bis-ethoxycarbonyl-2,2ꢀ-bipyridine)](3)
[MoO2Cl2(THF)2] (1.01 mmol). Yield: 0.53 g (98%). Color: white.
Selected IR (KBr): ꢂ(cm−1) = 1730(vs), 1616(w), 1487(w), 1093(w),
950(vs) (Mo Osym), 914(vs) (Mo Oasym). 1H NMR (400 MHz,
ꢀ
CDCl3, 20 ◦C, ppm): ı = 9.67 (d, JH,H = 5.5 Hz, 2H, py-H6,6 ), 8.88
3
ꢀ
ꢀ
Warning: TBHP (in decane) is toxic, possibly mutagen and a
strong oxidiser. It is a combustible liquid and is readily absorbed
through the skin and must be stored below 8 ◦C.
(s, 2H, py-H3,3 ), 8.27 (d, JH,H = 5.5 Hz, 2H, py-H5,5 ), 4.55 (q,
3
3JH,H = 7.1 Hz, 4H, CH2), 1.48 (t, JH,H = 7.8 Hz, 6H, CH3). 13C NMR
3
ꢀ
(100 MHz, CDCl3, 20 ◦C, ppm): ı = 163.2 (C O), 153.1 (py-C4,4 ),
ꢀ
ꢀ
ꢀ
150.3 (py-C2,2 ), 142.5 (py-C6,6 ), 126.8 (py-C3,3 ), 122.7 (py-
ꢀ
2.1. Ligand synthesis
C5,5 ), 63.4 (CH2), 14.4 (CH3). 95Mo NMR (26 MHz, CDCl3, 20 ◦C):
ı = 202 ppm. Anal. Calc. For C16H16Cl2MoN2O6 (499.15): C 38.50, H
3.23, N 5.61. Found C 37.64, H 3.17, N 5.41.
An appropriate amount of 2,2ꢀ-bipyridine ligand precursors
(1.0 g, 4.5 mmol) in methanol (17 ml) was added to concentrated
sulfuric acid (2 ml), which is placed into an ice bath. After refluxing
overnight, the solution was poured into water (42 ml) forming a
white slurry. The pH of the slurry was adjusted to 8 with 25% (w/v)
ammonia solution. The product was then extracted with chloro-
form, dried over magnesium sulfate and evaporated to dryness.
[MoO2Cl2(5,5ꢀ-bis-ethoxycarbonyl-2,2ꢀ-bipyridine)](4)
[MoO2Cl2(THF)2] (1.00 mmol). Yield: 0.5 g (94%). Color: white.
Selected IR (KBr): ꢂ(cm−1) = 1730(vs), 1607(s), 1471(w), 1059(w),
945(vs) (Mo Osym), 911(vs) (Mo Oasym). 1H NMR (400 MHz,
ꢀ
CD3NO2, 20 ◦C, ppm): ı = 9.96 (s, 2H, py-H3,3 ), 8.88 (dd,
ꢀ
ꢀ
3
3JH,H = 8.6 Hz, 2H, py-H6,6 ), 8.73 (d, JH,H = 8.1 Hz, 2H, py-H4,4 ),

A. Günyar et al. / Journal of Molecular Catalysis A: Chemical 331 (2010) 117–124
119
3
3
was extracted with dichloromethane and the organic phase was
dried over anhydrous MgSO4. The solvent removed in vacuo and
the residue was recrystallised from CH2Cl2/Et2O/hexane.
4.51 (q, JH,H = 7.3 Hz, 4H, CH2), 1.44 (t, JH,H = 7.8 Hz, 6H, CH3).
13C NMR (100 MHz, CD3NO2, 20 ◦C, ppm): ı = 164.9 (C O), 154.7
ꢀ
ꢀ
ꢀ
ꢀ
(py-C2,2ꢀ ), 153.3 (py-C6,6 ), 143.8 (py-C4,4 ), 132.2 (py-C5,5 ), 126.1
(py-C3,3 ), 69.2 (CH2), 14.6 (CH3). 95Mo NMR (26 MHz, CD3NO2,
20 ◦C): ı = 191 ppm. C16H16Cl2MoN2O6 (499.15): calc. C 38.50, H
3.20, N 5.61; found C 38.65, H 3.27, N 5.18.
[MoO2(CH3)2(4,4ꢀ-bis-methoxycarbonyl-2,2ꢀ-bipyridine)] (9)
[MoO2Cl2(THF)2] (0.53 mmol). Yield: 0.18 g (79%). Color: yellow.
Selected IR (KBr): ꢂ(cm−1) = 1731(vs), 1612(w), 1441(w), 1064(w),
934(vs), (Mo Osym), 900(vs) (Mo Oasym). 1H NMR (400 MHz,
Synthesis of complexes 5–8: The complex [MoO2Br2(DMF)2]
was dissolved in CH2Cl2 (5 ml) and treated with 1 equiv. of ligands
that were also dissolved in CH2Cl2 (10 ml). The resulting solutions
were each stirred for 1 h. The solvent was removed in vacuo, and
the product washed with diethyl ether (2 × 5 ml) and dried under
vacuum.
ꢀ
CDCl3, 20 ◦C): ı = 9.76 (d, 3JH,H = 5.5 Hz, 2H, py-H6,6 ), 8.90 (s, 2H, py-
ꢀ
ꢀ
H3,3 ), 8.11 (dd, 3JH,H = 5.6 Hz, 2H, py-H5,5 ), 4.08 (s, 6H, O-CH3), 0.57
(s, 6H, Mo-CH3). 13C NMR (100 MHz, CDCl3, 20 ◦C, ppm): ꢀı = 163.7
ꢀ
ꢀ
(C O), 153.5 (py-C4,4 ), ꢀ 150.5 (py-C2,2 ), 142.1 (py-C6,6 ), 126.8
ꢀ
(py-C3,3 ), 122.9 (py-C5,5 ), 54.0 (CH3), 22.4 (Mo-CH3). 95Mo NMR
(26 MHz, CDCl3, 20 ◦C): ı = 449 ppm. Anal. Calc. For C16H18MoN2O6
(430.26): C 44.66, H 4.22, N 6.51. Found C 45.74, H 4.81, N 6.16.
[MoO2(CH3)2(4,4ꢀ-bis-ethoxycarbonyl-2,2ꢀ-bipyridine)] (10)
[MoO2Cl2(THF)2] (1.02 mmol). Yield: 0.14 g (32%). Color: yellow.
Selected IR (KBr): ꢂ(cm−1) = 1729(vs), 1613(w), 1467(w), 1099(w),
932(vs) (Mo Osym), 898(vs) (Mo Oasym). 1H NMR (400 MHz, CDCl3,
[MoO2Br2(4,4ꢀ-bis-methoxycarbonyl-2,2ꢀ-bipyridine)](5)
[MoO2Br2(DMF)2] (0.46 mmol). Yield: 0.25 g (98%). Color: yellow.
Selected IR (KBr): ꢂ(cm−1) = 1733(vs), 1619(w), 1486(w), 1073(w),
946(vs), (Mo Osym), 912(vs) (Mo Oasym). 1H NMR (400 MHz,
ꢀ
CDCl3, 20 ◦C, ppm): ı = 9.77 (d, JH,H = 6.2 Hz, 2H, py-H6,6 ), 8.90
3
ꢀ
ꢀ
(s, 2H, py-H3,3 ), 8.27 (d, JH,H = 4.7 Hz, 2H, py-H5,5 ), 4.10 (s, 6H,
3
ꢀ
ꢀ
20 ◦C): ı = 9.75 (d, 3JH,H = 6.0 Hz, 2H, py-H6,6 ), 8.90 (s, 2H, py-H3,3 ),
CH3). 13C NMR (100 MHz, CDCl3, 20 ◦C, ppm): ı = 163.7 (C O),
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
8.10 (d, 3
J
H,H = 5.8 Hz, 2H, py-H5,5 ), 4.53 (q, 3JH,H = 7.2 Hz, 4H, CH2),
153.5 (py-C4,4 ꢀ), 150.5 (py-C2,2 ), 142.1 (py-C6,6 ), 126.8 (py-C3,3 ),
122.9 (py-C5,5 ), 54.0 (CH3). 95Mo NMR (26 MHz, CDCl3, 20 ◦C):
ı = 249 ppm. Anal. Calc. For C14H12Br2MoN2O6 (560.00): C 30.03, H
2.16, N 5.00. Found C 29.83, H 2.30, N 5.05.
3
1.48 (t, JH,H = 7.9 Hz, 6H, CH3), 0.57 (s, 6H, Mo-CH3). 13C NMR
ꢀ
(100 MHz, CDCl3, 20 ◦C, ppm): ı = 163.7 (C O), 152.2 (py-C4,4ꢀ ),
ꢀ
ꢀ
ꢀ
149.9 (py-C2,2 ), 140.8 (py-C6,6 ), 125.1 (py-C3,3 ), 122.5 (py-C5,5 ),
63.2 (CH2), 14.5 (CH3), 22.3 (Mo-CH3). 95Mo NMR (26 MHz, CDCl3,
20 ◦C): ı = 449 ppm. Anal. Calc. For C18H22MoN2O6 (458.32): C
47.17, H 4.84, N 6.11. Found C 46.31, H 4.72, N 5.92.
[MoO2Br2(5,5ꢀ-bis-methoxycarbonyl-2,2ꢀ-bipyridine)](6)
[MoO2Br2(DMF)2] (0.60 mmol). Yield: 0.32 g (95%). Color: yel-
low. Selected IR (KBr): ꢂ(cm−1) = 1729(vs), 1607(s), 1429(s),
1058(w), 938(vs), (Mo Osym), 906(vs) (Mo Oasym)ꢀ. 1H NMR
(400 MHz, CDCl3, 20 ◦C, ppm): ı = 10.18 (d, 2H, py-H3,3 ), 8.80 (dd,
[MoO2(CH3)2(5,5ꢀ-bis-ethoxycarbonyl-2,2ꢀ-bipyridine)](11)
[MoO2Cl2(THF)2] (0.60 mmol). Yield: 0.14 g (51%). Color: yellow.
Selected IR (KBr): ꢂ(cm−1) = 1725(vs), 1604(w), 1470(w), 1057(w),
932(vs), (Mo Osym), 900(vs) (Mo Oasym). 1H NMR (400 MHz,
ꢀ
ꢀ
3JH,H = 8.3 Hz, 2H, py-H6,6 ), 8.42 (d, JH,H = 8.3 Hz, 2H, py-H4,4 ),
3
4.05 (s, 6H, CH3). 13C NMR (100 MHz, CDCl3, 20 ◦C, ppm): ꢀı = 163.3
ꢀ
ꢀ
ꢀ
CDCl3, 20 ◦C): ı = 10.16 (d, JH,H = 1.2 Hz, 2H, py-H3,3 ), 8.69 (dd,
3
(C O), 153.7 (py-C2,2 ),ꢀ 151.8 (py-C6,6 ), 142.1 (py-C4,4 ), 130.0
ꢀ
ꢀ
ꢀ
3JH,H = 8.4 Hz, 2 H, py-H6,6 ), 8.41 (d, JH,H = 8.7 Hz, 2H, py-H4,4 ),
4.50 (q, 3JH,H = 7.1 Hz, 4H, CH2), 1.45 (t, 3JH,H = 8.8 Hz, 6H, CH3), 0.61
(s, 6H, Mo-CH3). 13C NMR (100 MHz, CDCl3, 20 ◦C, ppm): ꢀı = 163.5
3
(py-C5,5 ), 123.4 (py-C3,3 ), 53.6 (CH3). 95Mo NMR (26 MHz, CDCl3,
20 ◦C): ı = 249 ppm. Anal. Calc. For C14H12Br2MoN2O6 (560.00): C
30.03, H 2.16, N 5.00. Found C 29.74, H 2.34, N 4.98.
ꢀ
ꢀ
(C O), 152.6 (py-C2,2 ), ꢀ 150.9 (py-C6,6 ), 140.0 (py-C4,4 ), 129.2
[MoO2Br2(4,4ꢀ-bis-ethoxycarbonyl-2,2ꢀ-bipyridine)](7)
[MoO2Br2(DMF)2] (0.40 mmol). Yield: 0.24 g (97%). Color: yellow.
Selected IR (KBr): ꢂ(cm−1) = 1729(vs), 1616(w), 1461(w), 1093(w),
948(vs) (Mo Osym), 911(vs) (Mo Oasym). 1H NMR (400 MHz,
ꢀ
(py-C5,5 ), 122.8 (py-C3,3 ), 62.6 (CH2), 14.5 (CH3), 21.7 (Mo-CH3).
95Mo NMR (26 MHz, CDCl3, 20 ◦C): ı = 461 ppm. C18H22MoN2O6
(458.32): calc. C 47.17, H 4.84, N 6.11; found C 46.01, H 4.95, N
5.35.
ꢀ
CDCl3, 20 ◦C, ppm): ı = 9.74 (d, JH,H = 5.7 Hz, 2H, py-H6,6 ), 8.88
3
ꢀ
ꢀ
(s, 2H, py-H3,3 ), 8.25 (d, JH,H = 5.8 Hz, 2H, py-H5,5 ), 4.55 (q,
3
3JH,H = 7.2 Hz, 4H, CH2), 1.49 (t, JH,H = 7.8 Hz, 6H, CH3). 13C NMR
3
2.3. Synthesis of the ionic liquids
ꢀ
(100 MHz, CDCl3, 20 ◦C, ppm): ı = 163.5, (C O), 153.8 (py-C4,4 ),
ꢀ
ꢀ
ꢀ
150.7 (py-C2,2 ), 142.8 (py-C6,6 ), 127.1 (py-C3,3 ), 123.2 (py-
The RTILs [BMIM]PF6, [C8MIM]PF6, [BMIM]NTf2 and [BMIM]BF4
were prepared and purified as described in the literature [23,27].
Their spectroscopic data are in accordance with the data reported
previously.
ꢀ
C5,5 ), 63.8 (CH2), 14.8 (CH3). 95Mo NMR (26 MHz, CDCl3, 20 ◦C):
ı = 253 ppm. Anal. Calc. For C16H16Br2MoN2O6 (588.06): C 32.68,
H 2.74, N 4.76. Found C 32.69, H 3.02, N 4.90.
[MoO2Br2(5,5ꢀ-bis-ethoxycarbonyl-2,2ꢀ-bipyridine)](8)
[MoO2Br2(DMF)2] (0.46 mmol). Yield: 0.25 g (91%). Color: yel-
low. Selected IR (KBr): ꢂ(cm−1) = 1723(vs), 1609(w), 1470(w),
1061(w), 943(vs), (Mo Osym), 911(vs) (Mo Oasym)ꢀ. 1H NMR
(400 MHz, CDCl3, 20 ◦C, ppm): ı = 10.16 (d, 2H, py-H3,3 ), 8.79 (dd,
2.4. Catalytic reactions
Without solvent: Cis-cyclooctene (7.3 mmol), mesitylene
(2.00 g, internal standard), and 1 mol% of compounds 1–11
(73 ␮mol, catalyst) were mixed in the reaction vessel under air
at 55 ◦C. With the addition of TBHP (11 mmol, 5.5 M in decane)
the reaction was started under vigorous stirring. The course of the
reaction was monitored by quantitative GC analysis. Samples were
taken in regular time intervals, diluted with CH2Cl2, and treated
with a catalytic amount of MgSO4 and MnO2 to remove water and to
destroy the excess of peroxide. The resulting slurry was filtered and
the filtrate injected into a GC column. The conversion of cyclooctene
and the formation of the according oxide were calculated from
calibration curves (r2 = 0.999) recorded prior to the reaction course.
With RTILs: Cis-cyclooctene (7.3 mmol), mesitylene (2.00 g,
internal standard), and TBHP (11 mmol, 5.5 M in decane) were
mixed in the reaction vessel under air at room temperature. The
reaction is started under vigorous stirring by adding compounds
1–4 (7.3 ␮mol, 0.1 mol%), dissolved in RTIL (0.5 ml). The course
ꢀ
ꢀ
3
3JH,H = 8.4 Hz, 2H, py-H6,6 ), 8.42 (d, JH,H = 8.1 Hz, 2H, py-H4,4 ),
3
3
4.52 (q, JH,H = 7.1 Hz, 4H, CH2), 1.47 (t, JH,H = 7.8 Hz, 6H, CH3).
13C NMR (100 MHz, CDCl3, 20 ◦C, ppm): ı = 163.2 (C O), 154.1
ꢀ
ꢀ
ꢀ
ꢀ
(py-C2,2ꢀ ), 152.3 (py-C6,6 ), 142.3 (py-C4,4 ), 130.9 (py-C5,5 ), 123.7
(py-C3,3 ), 63.4 (CH2), 14.8 (CH3). 95Mo NMR (26 MHz, CDCl3,
20 ◦C): ı = 246 ppm. C16H16Br2MoN2O6 (588.06): calc. C 32.68, H
2.74, N 4.76; found C 32.06, H 2.73, N 4.54.
Synthesis of complexes 9–11. A solution of [MoO2Cl2(THF)2]
in THF (15 ml) was treated with one equiv. of ligands that were
also dissolved in THF (10 ml). The colour of the solution changed
immediately to yellow and the reaction mixture was stirred for fur-
ther 30 min. To this solution, 2.1 equiv. CH3MgBr were slowly added
at −20 ◦C. The reaction mixture was allowed to warm-up to room
temperature and was stirred for 2 h. The dark brown suspension
was taken to dryness and distilled water was added. The product

120
A. Günyar et al. / Journal of Molecular Catalysis A: Chemical 331 (2010) 117–124
Scheme 1. Synthesis route for Mo(VI) complexes.
of the reaction was monitored by quantitative GC analysis. Two
phases could be easily recognised and the samples were taken from
the upper organic phase in regular time intervals. After diluting the
samples with CH2Cl2 they were treated with a catalytic amount of
MgSO4 and MnO2 to remove water and to destroy the excess of
peroxide. The resulting slurry was filtered and the filtrate injected
into a GC column. The conversion of cyclooctene and the forma-
tion of the according oxide were calculated from calibration curves
(r2 = 0.999) recorded prior to the reaction course.
of bipyridine derived ligands, which should lead to good complex
solubility has been synthesised and applied. Scheme 1 shows the
synthesis routes for the Mo(VI) complexes.
3.1. Synthesis and spectroscopic characterisation of complexes
1–11
The dichlorodioxo molybdenum(VI) complexes 1–4 (Scheme 2)
were obtained as white microcrystalline powders in yields between
94 and 98% by addition of 1 equiv. of the bidentate Lewis-base
ligands to a solution of the adduct [MoO2Cl2(THF)2] in CH2Cl2
at room temperature. The bromides are readily prepared from
[MoO2Br2(DMF)2] and the corresponding Lewis-base ligands for
compounds 5–8 (Scheme 2) according to described procedures
[25]. When synthesizing the complexes 9–11 [6a], the dichlorides
do not need to be isolated prior to Grignard reagent addition. In
some cases this isolation of the intermediates has negative effects
on the overall yield of the final dialkyl products. Therefore, the
whole process consists of dissolving [MoO2Cl2(THF)2] in CH2Cl2,
3. Results and discussion
The synthesis of a series of dioxodichloromolybdenum(VI)
bipyridine derived complexes has been described previously. In
these cases the ligands had different functional groups at differ-
ent positions on the bipyridine rings [24]. Among the complexes
described previously is [MoO2Cl2(4,4ꢀ-bis-ethoxycarbonyl-2,2ꢀ-
bipyridine)], which is also included in this paper as complex 4 for
sake of comparison. For the present work the reaction of a variety
Scheme 2. Structures of complexes 1–11.

A. Günyar et al. / Journal of Molecular Catalysis A: Chemical 331 (2010) 117–124
121
Table 1
Selected FT-IR (KBr), calculated force constants f (Mo O) for MoO2Cl2L2 and 95Mo NMR spectroscopic data for complexes 1–11.
−1
ꢃ(Mo O) [cm−1
]
f (Mo O)mdyn A
Decomp. temp.◦C
ı (95Mo) ppm
Deuterated solvent
˚
Complex
ꢃas
ꢃs
ꢃaverage
1
2
3
4
5
6
7
8
918
910
914
911
912
906
911
911
900
898
900
951
942
950
945
946
938
948
943
934
932
932
935
926
932
928
929
922
930
927
917
915
916
7.00
6.88
6.97
6.91
6.92
6.82
6.93
6.89
6.74
6.71
6.73
275
290
250
275
250
275
250
275
210
200
225
Not determined
Not determined
CDCl3
CDCl3
CD3NO2
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
201
202
191
249
249
253
246
449
449
461
9
10
11
CDCl3
the composition [MoO2Cl2L2] generally display their 95Mo NMR
signals between 160 and 220 ppm, whereas the bromine deriva-
tives show their 95Mo signals between 170 and 280 ppm. The
methyl complexes of formula [MoO2(CH3)2L2] described in the lit-
erature display 95Mo NMR shifts between 370 and 520 ppm. Methyl
complexes with N-bidentate heterocyclic aromatic ligands display
their signals in the low-field region of this range between 370
and 450 ppm [6c]. The obtained 95Mo NMR spectra for the com-
plexes synthesised are summarised in Table 1; the [MoO2Cl2L2]
complexes show their signals around 200 ppm, the [MoO2Br2L2]
complexes exhibit their signals between 246 and 253 ppm and
the [MoO2(CH3)2L2] complexes display their signals in the range
between 449 and 461 ppm, which is in consistence with the litera-
ture reported results summarised above [6c].
Thermogravimetric analyses (TGA) were performed for all
compounds (see Table 1). The onset of the first decomposition tem-
perature for the compounds range between 200 and 290 ◦C. The
synthesised dimethyldioxomolybdenum(VI) complexes are less
stable than the dihalogenodioxomolybdenum(VI) complexes. In
the case of methoxycarbonyl groups placed in the 4,4ꢀ-positions
on the bipy-ring the decomposition temperature increases in the
order 9 < 5 < 1. Related to ethoyxcarbonyls – regardless of the posi-
tion of the functional groups located on the bipy-ring – both
bromo and chloro molybdenum(VI) complexes have similar sta-
bility whereas in the case of complexes with methoxycarbonyl
groups, dichlorodioxomolybdenum(VI) complexes are more stable
than dibromodioxomolybdenum (VI) complexes. From the mass
loss of the first decomposition step it can be concluded that in all
cases one ligand X is lost first, triggering the follow-up decompo-
sition of the whole molecule.
treatment with the corresponding bidentate ligand and reaction of
the in situ formed complex with the required amount of CH3MgBr at
low temperature. After warming to room temperature, the result-
ing solution is evaporated to dryness and the residue is treated with
water under aerobic conditions. The resulting solution is extracted
with dichloromethane. The dichloromethane phase is dried and the
residue recrystallised to give the alkyl complexes 9–11 (Scheme 2)
in yields between 32 and 79%. All compounds are stable under
laboratory atmosphere and can be handled in air.
FT-IR spectroscopic data for complexes 1–11 are in consistence
with other Lewis-base adducts of bis(halogeno)dioxomolybdenum
described before [25]. The symmetric and asymmetric IR stretch-
ing vibrations for the cis-dioxo unit of the complexes are in the
expected range between 900 and 950 cm−1 (Table 1). The corre-
sponding force constants of the Mo O bonds can be derived from
the ꢃ(Mo O) values. The M O vibrations of the complexes are
always shifted to higher wave numbers compared to their pre-
cursor, [MoO2X2]. This is an indication for comparatively strong
metal–Lewis-base ligand interactions. Additionally, this trend is
reflected in the calculated force constants (see Table 1). A com-
parison of the observed IR-values for the complexes 9–11 (Table 1)
indicates that the replacement of a halide by a methyl group weak-
ens the equatorial metal–ligand bonds. The observed stretching
vibrations as well as the calculated force constants (Table 1) show
for both complex series, 1, 5, 9; 2, 6, 10; 3, 7, 11, that there is a
slight increase of Mo O bond strength in the order –CH3 < Br < Cl,
in accordance with the inductive effects of these three ligands. The
differences appear to be significant (the tendency is same in all
three series of compounds), but not particularly pronounced.
The 1H NMR spectra of complexes 1–11 were measured at
room temperature using CDCl3 as solvent. In general, the spectro-
scopic data for these complexes are in good agreement with those
obtained for previously described MoO2X2–Lewis-base adducts
[25]. Except for compounds 9–11 the protons are solely situated in
the Lewis base. The chemical shift of the ligand protons is reported
in the supporting information and compared to the chemical shifts
observed for the protons of the free ligand. As stated above, the
important feature of the most of the dioxomolybdenum(VI) com-
plexes studied previously is that they are of low to moderate
solubility in most common organic solvents [24]. However, the
complexes synthesised in this work are generally of better solu-
bility. The chemical shifts of the –CH3 ligands of compounds 9–11
appear in the range between 0.57 and 0.61 ppm in 1H NMR spec-
troscopy, in agreement with the values as previously reported for
similar complexes [6c].
3.2. Crystal structures of complexes 6–8
Single crystals of compounds 6–8 were obtained and examined
by X-ray crystallography (see Fig. 1, S1, S2, and S3).
All obtained structures exhibit the central molybdenum atom,
situated in a distorted octahedron formed by two trans-located
bromo, two cis-oriented oxo ligands, and two nitrogen atoms of
the chelating bipyridine ligand. The distortion of the octahedron
results from the displacement of the molybdenum atom away
from the centre towards the two oxo ligands. The Mo–O bond
˚
lengths in these complexes range from 1.688(2) to 1.704(2) A,
the lengths of the Mo–Br bonds are found within the range of
˚
2.5155(5)–2.5306(5) A, and the Mo–N bond lengths vary between
˚
2.321(2) and 2.350(2) A. All these values are expected for the
respective Mo–E bond lengths [26].
The differences in the 95Mo NMR shifts [25] between the dif-
ferent bipyridine ligated complexes is comparatively small, only
a few ppm in each case with the same ligand X (X = Cl, Br, –CH3).
The difference in the chemical shift between Cl and Br is ≈50 ppm,
between Br and –CH3 ligands it is nearly 200 ppm. Complexes of
3.3. DFT calculation of formation energies
Density functional calculations were carried out to get more
information of the thermodynamic behaviour of the complexes.

122
A. Günyar et al. / Journal of Molecular Catalysis A: Chemical 331 (2010) 117–124
Fig. 1. ORTEP style plot of compound 7 in the solid state. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected
◦
˚
bond lengths [A] and bond angles [ ]: Mo1–Br1 2.5269(4), Mo1–Br2 2.5197(4), Mo1–O1 1.700(2), Mo1–O2 1.696(2), Mo1–N1 2.328(2), Mo1–N2 2.321(2); Br1–Mo1–Br2
159.22(1), Br1–Mo1–O1 96.80(7), Br1–Mo1–O2 95.75(6), Br1–Mo1–N1 81.14(5), Br1–Mo1–N2 80.13(5), Br2–Mo1–O1 95.82(7), Br2–Mo1–O2 96.38(6), Br2–Mo1–N1 81.74(5),
Br2–Mo1–N2 82.73(5), O1–Mo1–O2 106.65(10), O1–Mo1–N1 161.93(9), O1–Mo1–N2 93.27(9), O2–Mo1–N1 91.42(8), O2–Mo1–N2 160.03(8), N1–Mo1–N2 68.67(7).
They show overall similar thermodynamic values. Reaction
enthalpies for the formation of the complexes out of solvent-
free [MoO2X2] range between −14 and −17 kcal/mol for X = –CH3,
between −20 and −23 kcal/mol for X = Cl and between −23 and
−26 for X = Br. Entropic considerations come more strongly into
account when looking at the complete ligand exchange of the
[MoO2X2(THF)2] complexes with the relevant bidentate bipy lig-
and. The free energy of these substitutions vary from −10 kcal/mol
to −16 kcal/mol for all substituents X (–CH3, Cl, Br) of the Mo cen-
tre. It can also be seen that substituents in the 4,4ꢀ-position lead
to complexes that are more stable (ca. 2 kcal/mol) in comparison
to the corresponding 5,5ꢀ-substituted compounds. The difference
between –OMe and –OEt as bipy functionalised group is negligi-
ble. All the thermodynamic data are summarised in the Supporting
Information.
Fig. 2. Time-dependent yield of cyclooctene oxide in the presence compounds 1, 5
and 6 as catalysts at 55 ◦C with 1 mol% catalyst concentration between 0 and 30 min.
The curve for compound 1 is typical for the remaining compounds 2–4, 7–11.
3.4. Catalytic epoxidation of cis-cyclooctene
The epoxidation of cyclooctene using TBHP as oxidant in the
presence of complexes 1–11 at 55 ◦C yields cyclooctene oxide as
the only product. Two series of experiments with different molar
ratio of catalyst:substrate:oxidant (1:100:150 and 1:1000:1500)
were undertaken in order to compare the catalytic potential of the
systems. The details of the catalytic reactions are given in the exper-
imental section. Control experiments confirm that epoxidation
does not take place in the absence of catalyst. The time-dependent
curves obtained for compounds 1–11 are typical for [MoO2X2L2]-
type complexes used as epoxidation catalysts with TBHP [6,25].
Initially the reaction is fast, indicating that the active oxidising
species are formed rapidly after addition of the peroxide to the reac-
tion medium. Progressively, the reaction rate decreases, when the
reaction is nearly complete, due to an increasing lack of substrate
(Fig. 2). All examined compounds show high catalytic activity.
Only complexes 5 and 6 show an induction period. Compounds
5 and 6 dissolve very slowly after addition of TBHP leading to
a slow reaction in the beginning, whereas the other complexes
dissolve immediately. After the catalyst has totally dissolved, the
reaction is fast in all cases. The activities of the examined cata-
lysts are in the range of 1600–2000 h−1 (Table 2) and yields 95%
within 30 min. These results differ from literature data where the
halide complexes, particularly the –Cl compounds, were found to
be significantly more active (for cyclooctene epoxidation) than the
methyl derivatives under reaction conditions identical to those
used in the present work [6c,25,26]. In the light of the results pre-
sented here it can be assumed that these lower activities of the
Table 2
The TOF values for [MoX2O2L2] type complexes 1–11 using
cyclooctene as substrate. The TOFs of all complexes have
been calculated at the time interval of highest conversion.
Compound
TOF [h−1
]
1
2
3
4
1950
1940
1880
1910
310
5
6
7
8
9
10
11
2010
1600
1970
1960
1900
1950

A. Günyar et al. / Journal of Molecular Catalysis A: Chemical 331 (2010) 117–124
123
Table 3
TOF values [h−1] for [MoO2Cl2L2] type complexes 1–4, using cyclooctene as substrate
and a RTILs as solvent. The TOFs of all complexes have been calculated for the time
interval of highest conversion (c = 0.1 mol%).
Compound
[BMIM]NTf2
[BMIM]BF4
[BMIM]PF6
[C8MIM]PF6
1
2
3
4
5390
8090
7910
3430
280
30
250
660
1360
320
2490
430
5390
7110
5240
2990
Fig. 3. Time-dependent yield of cyclooctene oxide with compound 8 as catalyst (ꢀ:
catalyst:substrate:oxidant ratio 1:100:150 at 55 ◦C; ꢁ: catalyst:substrate:oxidant
ratio 1:1000:1500 at 55 ◦C; ꢂ: 2nd run catalyst:substrate:oxidant ratio 1:100:150
at 55 ◦C; ♦: catalyst:substrate:oxidant ratio 1:100:150 at r.t.).
–CH3 complexes are mainly due to solubility problems. Further-
more, in some cases the TOFs have been calculated differently (not
at the steepest slope of the olefin epoxide formation curves), the
reported numbers can therefore be regarded as ‘lower limits’ not
as realistic turnover frequencies.
Fig. 4. Time-dependent yield of cyclooctene oxide in the presence of [MoO2Cl2L2]
and compounds 1–4 as catalysts at room temperature with 0.1 mol% catalyst con-
centration and [BMIM]NTf2 as solvent.
When changing the catalyst:substrate:oxidant ratio from
1:100:150 to 1:1000:1500, the activities lower except for complex
8. In the latter case the TOF increases slightly from 1970 to 2310 h−1
.
place and the product can be easily removed quantitatively via a
cannula. Additionally, oil pump vacuum allows the removal of t-
BuOH from the RTIL phase. Since both starting reagents and epoxide
are not present in the RTIL it can be concluded that the conver-
sion of cyclooctene is achieved by a biphasic reaction and not by a
homogeneous reaction. Addition of substrate and oxidising agent
restarts the reaction. This catalytic procedure was repeated three
times without any loss of activity and consequently without any
leaching effects. Fig. 4 shows the kinetics of the compounds 1–4 in
[BMIM]NTf2.
It appears that optimal activities for the system have been reached
when applying a 1:100:150 catalyst:substrate:oxidant ratio. Low-
ering the reaction temperature to 25 ◦C also lowers the activity. In
the case of compound 3, the TOF decreases from ca. 1884 h−1 (at
55 ◦C) to ca. 230 h−1 (at 25 ◦C) (Fig. 3).
A further set of experiments was performed to explore the
stability of the complexes under catalytic conditions. After 24 h
reaction time, when the product yield reaches ≈100%, more sub-
strate and TBHP were added to the reaction mixture. The catalyst
was still active and the reaction reaches more than 90% product
yield within 4 h, albeit somewhat slower in catalysing the reaction
(Fig. 3). This is most probably caused by the excess of tert-butyl
alcohol present in the reaction mixture hindering the reaction pro-
cess. This phenomenon is already known from related complexes
and has been ascribed to an adduct formation of the catalyst with
the byproduct t-BuOH, rather than a catalyst decomposition [6b].
4. Conclusion
The dioxomolybdenum(VI) complexes [MoO2X2L2] (X = Cl,
Br, –CH3; L = 4,4ꢀ-bis-methoxycarbonyl-2,2ꢀ-bipyridine, 5,5ꢀ-bis-
methoxycarbonyl-2,2-bipyridine,
4,4ꢀ-bis-ethoxycarbonyl-2,2ꢀ-
bipyridine, 5,5ꢀ-bis-ethoxycarbonyl-2,2ꢀ-bipyridine) are very
active and highly selective catalysts for the homogeneous epox-
idation of cyclooctene using tert-butyl hydroperoxide (TBHP).
Cyclooctene oxide is obtained quantitatively within 1 h and the
reaction solution containing the catalyst can be reused. Catalytic
results show that the reaction is selective to the desired epoxide
and no diol formation is observed. It is important to note that
almost all of the complexes described here are highly soluble in
organic solvents in contrast to many of the previously reported
related complexes. This shows that the ligand L is obviously more
important for the solubility than the ligand X and R.
3.5. Catalytic epoxidation of cis-cyclooctene with RTILs as
solvents
The chloride-containing complexes are investigated addi-
tionally in RTILs ([BMIM]PF6, [C8MIM]PF6, [BMIM]NTf2 and
[BMIM]BF4) (BMIM = 1-butyl-3-methylimidazolium, C8MIM = 1-
octyl-3-methylimidazolium) as solvents. In contrast to the catalysis
without an additional solvent this catalysis was performed at
room temperature and with a catalyst:substrate:oxidant ratio from
1:1000:1500. The catalyst was dissolved in the RTIL and this solu-
tion was added to the reaction solution. Since the catalytically
active Mo(VI) species are water-sensitive, the activity of the sys-
tem depends on the water-content of the ionic liquid. [BMIM]NTf2
has the lowest water-content of all the investigated RTILs [27], lead-
ing to the highest activity in this solvent. The TOFs in [BMIM]NTf2
are considerably higher than the TOF under solvent-free conditions
(see Table 3).
The major advantage of using RTILs as solvents is the formation
of a biphasic system, which can be easily separated. After product
removal the catalysts can be reused for additional cycles without
observable loss of activity. Furthermore, no leaching of the catalyst
can be observed. In [BMIM]NTf2 the reactions yields the best results,
correlating well with other reported Mo systems [28], because of
the low water-content of this particular RTIL.
A reduction of the concentration of compound 2 to 0.05 mol% in
[BMIM]NTf2 even leads to a TOF of 10,080 h−1. Another advantage
of the performance in RTILs is the possibility to reuse the catalyst.
After the reaction a phase separation ionic liquid/product takes
4.1. Additional information
Crystallographic data (excluding structure factors) for the
structures reported in this paper have been deposited with the

124
A. Günyar et al. / Journal of Molecular Catalysis A: Chemical 331 (2010) 117–124
Cambridge Crystallographic Data Centre as supplementary publi-
cation no. CCDC-776729 (6), CCDC-776730 (7), and CCDC-776731
(8). Copies of the data can be obtained free of charge on application
to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223 336
033; e-mail: deposit@ccdc.cam.ac.uk).
(c) F.E. Kühn, A.M. Santos, A.D. Lopes, I.S. Gonc¸ alves, J.E. Rodríguez-Borges, M.
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A.G. thanks the Elitenetzwerk Bayern NanoCat for a Ph.D.
grant and for financial support. D.B. is thankful to the Bayerische
Forschungsstiftung for a Ph.D. grant. The authors are acknowledged
to Dr. Mirza Cokoja and Dr. Bettina Bechlars for helpful discussions
and proofreading of the manuscript.
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