Structural studies of mono- and dimetallic MoVI complexes - A new mechanistic contribution in catalytic olefin epoxidation provided by oxazoline ligands

Article abstract of DOI:10.1002/ejic.200400331

New seven-coordinate oxo(peroxo)molybdenum(VI) complexes with bidentate chiral oxazolines, anionic κ²-N,O (V) and neutral κ²-N,N (VI), have been prepared and fully characterised by NMR spectroscopy and single-crystal X-ray diffraction studies. Dimetallic dioxo(μ-oxo)molybdenum(VI) compounds containing oxazolinylpyridine ligands II-IV have also been synthesised. NMR studies showed the presence of several isomers (up to six species) due to the Mo-O-Mo bridge angle and the unsymmetrical nature of the N,N ligand. The roles of mono-(I, V and VI) and dimetallic (II) oxomolybdenum(VI) complexes in the catalytic epoxidation of cyclooctene, norbornene and (R)-limonene have been studied. The high activity of the oxomono(peroxo) precursor V containing the hemilabile oxazolinylphenolate, in contrast to the unexpected low activity for the oxobis(peroxo) species VI, could be justified by the inertness observed for the oxazolinylpyridine ligand. In addition, the dimetallic system II afforded high selectivity towards limonene epoxide formation (trans/cis-10 ratio up to 9:1). Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Full text of DOI:10.1002/ejic.200400331

FULL PAPER
VI
Structural Studies of Mono- and Dimetallic Mo Complexes ؊
A New Mechanistic Contribution in Catalytic Olefin Epoxidation Provided by
Oxazoline Ligands
[
a]
[a]
[a]
[b]
Jos e´ A. Brito, Montserrat G o´ mez,* Guillermo Muller, Helena Teruel,*
[c]
[c]
[d]
Jean-Claude Clinet, Elisabet Du n˜ ach, and Miguel A. Maestro
Keywords: Molybdenum / Chiral oxazolines / Selective epoxidation / Oxo peroxo complexes / Catalysis
New seven-coordinate oxo(peroxo)molybdenum(VI
)
com-
ation of cyclooctene, norbornene and (R)-limonene have
2
plexes with bidentate chiral oxazolines, anionic κ -N,O (V) been studied. The high activity of the oxomono(peroxo) pre-
2
and neutral κ -N,N (VI), have been prepared and fully char-
cursor V containing the hemilabile oxazolinylphenolate, in
contrast to the unexpected low activity for the oxobis(peroxo)
species VI, could be justified by the inertness observed for
the oxazolinylpyridine ligand. In addition, the dimetallic sys-
tem II afforded high selectivity towards limonene epoxide
formation (trans/cis-10 ratio up to 9:1).
acterised by NMR spectroscopy and single-crystal X-ray dif-
fraction studies. Dimetallic dioxo(µ-oxo)molybdenum(VI
)
compounds containing oxazolinylpyridine ligands II−IV
have also been synthesised. NMR studies showed the pres-
ence of several isomers (up to six species) due to the
Mo−O−Mo bridge angle and the unsymmetrical nature of the
N,N ligand. The roles of mono- (I, V and VI) and dimetallic
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
(
II) oxomolybdenum(VI) complexes in the catalytic epoxid-
Germany, 2004)
Introduction
on the peroxo oxygen atom (with a three-membered ring
intermediate). In this case, the coordinated peroxo group
[
5]
High-valent oxometal species have demonstrated the is thought to have a sufficiently increased electrophilic
ability to catalyse the oxidation of a variety of organic sub- character by decreasing the negative charge on the peroxo
[
1,2]
strates by homogeneous as well as heterogeneous routes.
oxygen atoms. A ‘‘hybrid’’ mechanism was proposed by
In particular, the Mo-catalysed alkene-to-epoxide conver- Jørgensen and Hoffman in which intramolecular slipping
sion has received most attention due to its industrial inter- of the olefin after coordination to the metal atom, which
est. However, the oxygen transfer mechanism is still a con- depends on their relative orientation, was believed to form
[
6,7]
troversial subject and three main hypotheses have domi- a three-membered metallacycle intermediate.
nated the discussion. Mimoun suggested that the olefin co-
ordinates to the metal atom before subsequent insertion
In terms of the spectator part of the catalyst molecule, S-,
N- and O-donor ligands have been mainly used because of
their stability towards oxidation processes. However, under
2
into the η -MoϪO bond to form a five-membered alkyl
2
peroxide metallacycle.[ Coordination of the olefin to the
metal centre makes the olefin electrophilic and thus ready
for further reaction with a peroxo group acting as a nucle-
ophile. On the other hand, Sharpless proposed a concerted
reaction mechanism involving a direct attack by the olefin
3,4]
catalytic conditions, they frequently suffer partial decoordi-
[8,9]
nation.
This fact discourages their use as selective cata-
lysts in certain organic processes.
Oxazolines have been used as stabilising agents for many
transition metals based on their robustness under a variety
[
10]
[
a]
of reaction conditions when coordinated.
Consequently,
Departament de Qu ´ı mica Inorg a` nica, Universitat de Barcelona,
Mart ´ı I Franqu e` s 1Ϫ11, 08028 Barcelona, Spain
Fax: (internat.) ϩ 34-934907725
they have been frequently employed in asymmetric catalytic
[11,12]
organic transformations.
E-mail: montserrat.gomez@qi.ub.es
[
[
b]
c]
Departamento de Qu ´ı mica, Universidad Sim o´ n Bol ´ı var,
Baruta 89000, Caracas 1081, Venezuela
Laboratoire de Chimie Bioinorganique UMR 6001,
Universit e´ de Nice-Sophia Antipolis,
We have been carrying out research with dioxomolybden-
um() compounds (type-I complexes, Figure 1) and we her-
ein describe the first examples of oxo(peroxo) and dioxodi-
metallic molybdenum complexes containing chiral biden-
tate oxazoline ligands (Figure 1). Both types of systems
have been explored as catalytic precursors in olefin epoxid-
0
6108 Nice C e´ dex 2, France
[
d]
Departamento de Qu ´ı mica Fundamental,
Universidade da Coru n˜ a,
Campus da Zapateira, s/n, 15071 A Coru n˜ a, Spain
4278
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejic.200400331
Eur. J. Inorg. Chem. 2004, 4278Ϫ4285

Structural Studies of Mono- and Dimetallic MoVI Complexes
FULL PAPER
Ϫ1
two strong bands at 906 and 941 cm for MoϭO as well
Ϫ1
as 772 cm for the µ-oxo moiety (MoϪOϪMo). In the
positive FAB-MS spectra, despite the absence of the parent
ϩ
ion, fragments such as [Mo O (NCS)L ] , [Mo O -
2
5
2
2
4
ϩ
ϩ
(
NCS)L] and [Mo O(NCS)L] indicate the dimetallic nat-
2
ure of the complexes.
Efforts to grow single crystals were unsuccessful, prob-
ably due to the existence of some isomers (see below).
Studies in solution showed the presence of several species
and the 2D NOESY spectra revealed exchange signals
among them all. In the case of II, the signals were well
enough separated to allow us to suggest the presence of at
least six species (relative ratios 8.0:5.6:3.3:2.5:1.6:1) in
CDCl . The most abundant isomer (ca. 40%) shows two
3
Figure 1. Oxazoline chiral ligands 1Ϫ4 and oxomolybdenum com-
plexes IϪVI
equivalent oxazolinylpyridine fragments. The other isomers,
however, show signals indicating that the two bidentate li-
gands are not chemically equivalent. In the case of sym-
metrical ligands, as in the 4,4Ј-disubstituted bipyridine
compound (µ-O)[MoO (NCS)(4,4Ј-tert-butyl-2,2Ј-bpy)] ,
ation processes. Related structural studies, especially in
solution, afforded new insight into the nature of the cata-
lytic species involved.
2
2
two isomers were formed depending on the MoϪOϪMo
bridge angle (180° and ca. 156°, respectively, relative ratios
Results and Discussion
[13]
44:56).
The formation of more than two species would
be expected for nonsymmetrical oxazolinylpyridines 2Ϫ4
and the major isomer exhibiting the symmetrical structure
must correspond to the conformer with an MoϪOϪMo an-
gle of 180°. In Figure 2, the arrangements of the three non-
equivalent isomers are depicted with an MoϪOϪMo bridge
angle of 180° and where the isocyanate ligand is opposite to
the bridging oxygen atom. In addition, the corresponding
isomers with an MoϪOϪMo angle less than 180°, could
also be expected.
Dimetallic Dioxomolybdenum Complexes II؊IV
Dimetallic bis(oxazolinylpyridine)dioxo(µ-oxo)dimolyb-
denum() complexes IIϪIV with oxazolinylpyridines of the
2
general formula (µ-O)[MoO (NCS)(κ -N,N-L)] (L ϭ 2Ϫ4)
2
2
were prepared according to the methodology previously de-
scribed (Scheme 1, a).[ A dichloromethane solution of the
ligand 2Ϫ4 was added to an acidic aqueous solution of
13]
Na MoO ·2H O and KSCN under phase-transfer con-
2
4
2
ditions, affording the corresponding complexes in good
yields (ca. 70%). Using the same conditions, the analogous
complexes containing bis(oxazoline) ligands could not be
isolated due to the oxazoline moiety decomposing to give
amide derivatives. Modifications of the experimental con-
Monometallic Oxo(peroxo)molybdenum Complexes V, VI
New seven-coordinate monometallic oxo(peroxo)molyb-
ditions (higher pH values, different solution concentrations) denum() complexes V, VI containing chiral oxazoline li-
were also unsuccessful.[
14]
gands were prepared by previously reported methods.
[15]
The IR spectra of IIϪIV show the characteristic func- For the anion of 1, an oxo(peroxo) compound with two
2
tional group absorptions: a very strong band for NCS at bidentate N,O ligands, [MoO(O )(κ -N,O-L) ] (V), was iso-
2
2
Ϫ1
Ϫ1
ca. 2050 cm , 1658 cm for the oxazoline imine bond, lated while for the oxazolinylpyridine 2, the oxobis(peroxo)
Scheme 1. Synthesis of dioxo(µ-oxo) dimetallic (IIϪIV) (a) and oxo(peroxo) monometallic (V and VI) (b) molybdenum() complexes
www.eurjic.org 4279
Eur. J. Inorg. Chem. 2004, 4278Ϫ4285
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

M. G o´ mez, H. Teruel et al.
FULL PAPER
O(2)ϪN(1)ϪO(6)ϪN(2) 6.11°. The other bond lengths and
angles are similar to those in the only related published
[
16]
complex [MoO(O )(8-quinolate) ].
In our case, the two
nitrogen atoms are located almost opposite to each other
N(1)ϪMo(1)ϪN(2) ϭ 158.96°], while in the example re-
2
2
[
[
16]
ported in ref. , the two nitrogen atoms are vicinal
(NϪMoϪN ϭ 81.91°).
In structure VI, the molybdenum atom is also heptacoor-
dinated by an oxo group, two peroxo moieties and one bi-
dentate oxazolinylpyridine 2 and the geometry around the
metal atom can also be described as pseudo-pentagonal bi-
pyramidal, with the axial sites occupied by the oxo group
O(1) and the nitrogen atom from the pyridine moiety of the
oxazoline ligand N(1). The remaining coordinating atoms
are located in the equatorial plane in a nearly coplanar dis-
tribution, as indicated by the dihedral angles
O(3)ϪO(2)ϪO(4)ϪO(5) 0.991°, N(2)ϪO(5)ϪO(4)ϪO(2)
Figure 2. Isomers of dimetallic complex II containing the pure chi-
ral oxazolinylpyridine ligand 2, with an MoϪOϪMo angle of ca.
1.596°, O(2)ϪO(3)ϪN(2)ϪO(5) 3.143° and O(3)ϪN(2)Ϫ
180°
O(5)ϪO(4) 2.460°. These angles are similar to other hepta-
coordinate oxobis(peroxo)molybdenum complexes contain-
[
17,18]
[19Ϫ21]
ing bipyridine
and pyrazolylpyridine
derivatives.
2
complex [MoO(O ) (κ -N,N-2)] (VI) was produced In the X-ray diffraction studies of the N,N-unsymmetrical
2
2
(Scheme 1, b).
ligands, the pyridine fragment is in an equatorial position
MoO was dissolved in H O (30%) to prepare an aque- while in VI it is axial. Intramolecular non-bonding O···H
3
2
2
˚
ous solution of [Mo(O)(O ) (H O) ] (pH ϭ 4). A methanol distances are longer than 3 A.
solution of the oxazoline 1Ϫ2 was then added and after
2
2
2
2
1
The H NMR spectrum of a freshly prepared solution of
addition of diethyl ether, V and VI were isolated as yellow V in CDCl revealed that the two anionic N,O-bidentate
3
solids, both stable in the solid state and in solution. No moieties are nonequivalent. A second set of signals ap-
corresponding amide derivatives of the oxazolines were ob- peared after approximately 12 h resulting in a relative ratio
served under these conditions. Attempts to isolate the ox- between the two molybdenum species of ca. 10:1. In deuter-
o(peroxo) species V by perhydrolysis of I were unsuccessful. ated acetone, the formation of the second species is fav-
In the solid state, the IR spectra of V and VI show two oured (relative ratio 4:1) and no changes were observed
characteristic strong absorption bands at 962 (MoϭO) and even after several days. This solvent dependence turned out
Ϫ1
8
1
65Ϫ863 (OϪO) cm , as well as a very strong band at to be similar to that observed for the analogous dioxomo-
Ϫ1
636Ϫ1630 (CϭN) cm corresponding to the oxazoline lybdenum complex I where partial decoordination of the
oxazoline ligand takes place in coordinating solvents.[
9]
imine bond.
Single crystals of V and VI were obtained from dichloro-
For the major compound, important differences in the
methane/diethyl ether solutions (Figure 3). In Table 1, selec- chemical shifts were observed. Compared with the free ox-
ted bond lengths and angles are collected. In V, the molyb- azoline [δ(H) ϭ 4.10 ppm] the stereo-centre proton signal
denum atom is heptacoordinated by an oxo, a peroxo and shifted remarkably downfield [δ(H) ϭ 5.06 and 5.73 ppm].
two bidentate anionic phenolate oxazoline ligands. The ge- This deshielding can be attributed to interaction with the
ometry around the metal atom can be described as pseudo- oxo and peroxo groups, as shown by the average in-
pentagonal bipyramidal with the axial sites occupied by the teratomic distances H(8A)···OϭMo and H(19A)···O ϪMo
2
˚
oxo group O(1) and one of the phenolate oxygen atoms from the crystallographic data, ca. 2.85 and 2.25 A, respec-
O(4). Thermal disorder is present around the MoϪoxo unit tively.
with high anisotropic parameters. The disorder was mod-
NMR studies of VI in solution clearly indicated only one
elled with the two positions O(1A) and O(1B) having partial compound in coordinating solvents. The chemical shifts of
occupancies of 0.46(3) and 0.54(3), respectively. The peroxo the oxazoline protons moved downfield (∆δ
ϭ
˚
bond O(2)ϪO(3) (1.3 A) is shorter than usual (ca. 1.4Ϫ1.5 0.2Ϫ0.7 ppm) relative to those of the free oxazoline 2. The
˚
A). This fact can be attributed to the relatively high thermal coordination of the oxazolinylpyridine ligand is limited to
parameter for O(2) (anisotropic displacement parameter the pyridine nitrogen atom located in an axial position op-
0.148) compared with those of the phenolate and oxazolinyl posite to the strong trans-directing oxo group in agreement
oxygen atoms (0.05Ϫ0.07). Consequently, the distance is in- with the X-ray study, although the thermodynamically fav-
accurate. The remaining coordinating atoms are located in oured isomer should bear the poorer σ-donor moiety trans
the equatorial plane in an almost coplanar arrangement, as to the oxo group. In addition, the stereochemical behaviour
indicated by the dihedral angles: N(1)ϪO(6)ϪN(2)ϪO(3) is similar to that observed for oxobis(peroxo)molybdenum
[
22]
4
.60°, O(6)ϪN(2)ϪO(3)ϪO(2) 0.93°, N(2)ϪO(3)ϪO(2)Ϫ complexes with N,N-pyrazolylpyridines,
but oxazolinyl-
N(1) 5.43°, O(3)ϪO(2)ϪN(1)ϪO(6) 8.16° and pyridine dissociation was not observed.
4280
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
Eur. J. Inorg. Chem. 2004, 4278Ϫ4285

Structural Studies of Mono- and Dimetallic MoVI Complexes
FULL PAPER
Figure 3. ORTEP diagrams of the molecular structures of V (a) and VI (b); hydrogen atoms are omitted for clarity
˚
perature (Scheme 2). When reactions were performed using
tBuOOH in decane, lower conversions were obtained. Cata-
lytic reactions were carried out with and without solvent
Table 1. Selected bond lengths [A] and bond angles [°] for V and
VI (with esds in parentheses)
V
VI
1.674(2)
(
toluene). Table 2 lists the catalytic results. In the case of I,
activities were higher in the presence of solvent (Entries 1,
, 5 and 6) but for II these differences were not significant
MoϪO(1)
MoϪO(2)
MoϪO(3)
MoϪO(4)
MoϪO(5)
MoϪO(6)
1.732(16)[a]
1.946(6)
1.979(7)
2.044(3)
Ϫ
2.029(3)
2.161(4)
2.167(3)
1.312(7)[
Ϫ
3
1.912(2)
1.940(2)
1.928(2)
1.9460(19)
Ϫ
(Entries 2, 4, 7 and 8). For both systems, cyclooctene oxide
8 and exo-norbornene oxide 9 were the only products ob-
tained and the monometallic system I was more active (up
to 57% conversion, Entry 3) than dimetallic II (Entries 1
and 3 versus 2 and 4). In the case of limonene oxidation, a
mixture of products was detected, trans/cis-limonene oxide
MoϪN(1)
MoϪN(2)
O(2)ϪO(3)
O(4)ϪO(5)
2.427(2)
2.1524(18)
1.459(3)
1.474(3)
105.66(12)
164.45(10)
94.17(10)
102.96(11)
101.78(12)
70.34(7)
Ϫ
44.73(8)
Ϫ
106.07(12)
44.52(10)
Ϫ
b]
(
(
10), the corresponding double epoxide (11) and alcohols
Scheme 2). Activity (substrate conversion) and chemoselec-
[
a]
O(1)ϪMoϪO(4)
O(1)ϪMoϪN(1)
O(1)ϪMoϪN(2)
O(1)ϪMoϪO(3)
O(1)ϪMoϪO(5)
N(1)ϪMoϪN(2)
O(4)ϪMoϪO(6)
O(4)ϪMoϪO(5)
O(6)ϪMoϪN(1)
O(2)ϪMoϪO(1)
O(2)ϪMoϪO(3)
N(1)ϪMoϪO(6)
O(6)ϪMoϪN(2)
N(2)ϪMoϪO(3)
160.35(6)
105.3(5)[
a]
a]
a]
[
tivity (ratio 10/11) are almost the same for both catalytic
systems (Entries 5Ϫ8). Ring epoxide opening to afford al-
cohols was lower for I than for II (alcohols/epoxide ϭ
94.7(6)
101.7(4)[
Ϫ
158.96(13)
81.16(12)
Ϫ
80.23(14)
100.9(4)[
39.1(2)
80.34(13)
83.34(13)
80.4(2)
0Ϫ0.1 for I and 0.2Ϫ0.5 for II) (Entries 5 and 6 versus
7
and 8). An outstanding diastereoisomeric induction was
observed for 10 when II was used (trans/cis-10 up to 9:1 for
77% of olefin conversion, Entry 8) but no discrimination
occurred with I (Entries 5 and 6).
a]
The absence of exocyclic oxide formation suggests that
the double epoxide 11 is probably produced by epoxidation
of 10. When trans/cis-10 (ratio ca. 1:1) was used as the sub-
strate and II as the catalyst, under the same experimental
conditions, a 45% conversion to 11 and alcohols (11/al-
cohols ഠ 4:1) was obtained (Scheme 3). The remaining 10,
enriched in one of the diastereoisomers (relative ratio trans/
cis-10 Ͼ 98:2), indicates that cis-10 reacts faster than trans-
Ϫ
86.97(9)
[
a]
Average bond length and angle. [b] See X-ray discussion in the
text.
Olefin Epoxidation Catalysed by Oxomolybdenum
Complexes
1
0. However, when monometallic complex I was used as a
The dioxo mono- and dimetallic complexes I and II,
respectively, were tested as catalytic precursors for the epox-
idation of cyclooctene (5), norbornene (6) and (R)-limonene
catalytic precursor, no kinetic resolution occurred.
Oxo(peroxo)molybdenum species are known key inter-
(7), using tBuOOH in water as the oxidant at room tem- mediates in epoxidation processes. The mono- and bis-
Eur. J. Inorg. Chem. 2004, 4278Ϫ4285
www.eurjic.org  2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 4281

M. G o´ mez, H. Teruel et al.
Table 2. Results of epoxidation of olefins 5Ϫ7 with tBuOOH as an oxidant catalysed by molybdenum complexes I, II, V and VI (Scheme 2)
FULL PAPER
Entry[a]
Complex
Olefin
Conv. (%)[b]
10 (%)[c] (trans/cis)
11 (%)[d]
Alcohols (%)[d]
Alcohols/epoxides
1
2
3
4
5
I
II
I
II
I
I
II
II
V
VI
V
VI
5
52 (27) (41)
36 (40) (8)
57 (39) (40)
34 (20) (20)
62 (35)
58 (35)
64 (46)
77 (45)
95 (92) (83)
15 (38)
79
22
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
2.0
4.0
4.0
6.0
Ϫ
Ϫ
2.0
0
Ϫ
Ϫ
Ϫ
Ϫ
6.0
0
10.0
26.0
Ϫ
Ϫ
15.0
0
Ϫ
Ϫ
Ϫ
Ϫ
0.1
0
0.2
0.5
Ϫ
Ϫ
0.2
0
5
6
6
(R)-7
(R)-7
(R)-7
(R)-7
5
54 (54:46)
54 (60:40)
50 (80:20)
45 (90:10)
Ϫ
[
e]
e]
6
7
8
9
1
1
1
[
0
1
2
5
Ϫ
(R)-7
(R)-7
62 (60:40)
22 (50:50)
[
a]
Catalytic conditions: 0.025 mmol of Mo (complexes I, II, V and VI), 1 mmol olefin (5؊7) and 1.5 mmol of tBuOOH (8  in water)
in 1 mL of toluene at room temp. for 22 h; 0.2Ϫ0.4 mmol of undecane (as external standard) was used to quantify the catalytic reaction.
[
b]
Conversion percentage based on the substrate determined by GC; in parentheses is the conversion obtained without solvent; in italics
[c]
[d]
the conversions obtained using tBuOOH in decane (5.5 ). In parentheses, the ratio of trans/cis of endocyclic epoxide 10. Determined
by GC. Reaction carried out without solvent.
[
e]
centre and catalytic olefin epoxidation. Despite heptacoord-
inated oxobis(peroxo)molybdenum complexes having been
reported to be fairly good epoxidation catalysts, the unex-
pected low activity [in the oxidation of cyclooctene and (R)-
limonene] observed for VI can be attributed to the nonla-
bility of the oxazolinylpyridine ligand, as observed by
NMR spectroscopy (see above). However, in the case of V,
the high activity obtained is in accordance with partial de-
coordination of the N,O-oxazolinylphenolate ligand, al-
[
9]
ready observed for its precursor I and for oxobis(peroxo)
[
19Ϫ22]
Thiel-type complexes.
The ability to generate a coor-
dination vacancy at the molybdenum atom can then be re-
sponsible for the differences in the catalytic behaviour of V
and VI. Furthermore, the remarkably higher activity ob-
tained with II compared with VI, with both systems con-
taining the nonlabile bidentate ligand 2, is due to the facile
formation of a vacant site in II by isothiocyanate dis-
sociation. Thus, metallic species with unsaturated coordi-
nation environments will favour either the olefin or the oxi-
dant to approach the metal. With regards to the equimolar
trans/cis-10 mixture obtained in the epoxidation of (R)-lim-
onene using monometallic catalysts, the species directly co-
ordinated to the molybdenum centre is probably tBuOOH
instead of the olefin.
Scheme 2. Cyclooctene (5), norbornene (6) and (R)-limonene (7)
epoxidation catalysed by I, II, V and VI using tBuOOH as an oxi-
dant; olefin/tBuOOH/Mo ϭ 40:60:1
Scheme 3. trans/cis-Limonene oxide (10) epoxidation catalysed by
II using tBuOOH as an oxidant; olefin/tBuOOH/Mo ϭ 40:60:1
(peroxo) complexes V and VI were tested for oxidation of
Information derived from NMR experiments and cata-
5
and (R)-7. It was found that V was the most active of all lytic reactivity patterns allows us to propose the mechanism
systems studied (Entries 9 and 11) achieving 95% cyclooc- shown in Scheme 4. The oxo(peroxo)molybdenum species
tene conversion (Entry 9). The higher activities observed for V can easily form a vacant site due to the hemilabile nature
oxo(peroxo) species V than for the dioxo compound I sug- of the oxazolinylphenolate ligand (A). Further coordination
gest that the former is the catalytically active species and I of tBuOOH (B) leads to the formation of a transition state
is its precursor. On the contrary, VI gave the lowest conver- (TS), in which the olefin approaches towards the tert-butyl
sions (Entries 10 and 12). Similar to the dioxo monometal- peroxide fragment, producing the epoxide and concomitant
lic complex I, diastereoisomeric induction of (R)-limonene elimination of tert-butyl alcohol, the active catalytic species
did not take place for either V or VI.
V being regenerated.
On the other hand, the unexpected selectivity observed
with the dimetallic catalytic system II in both oxidations,
Mechanistic Aspects of Olefin Epoxidation
The structural studies carried out provided evidence of a (R)-limonene (7) (Entries 7 and 8, Table 2) and limonene
certain relationship between the lability of the molybdenum epoxide 10 (Scheme 3), can only just be explained by the
4282
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
Eur. J. Inorg. Chem. 2004, 4278Ϫ4285

Structural Studies of Mono- and Dimetallic MoVI Complexes
FULL PAPER
8.87 (d, J ϭ 4.8 Hz, 1 H), 8.18 (pt, J ϭ 7.8 Hz, 1 H), 8.06 (d, J ϭ
7.6 Hz, 1 H), 7.4Ϫ7.2 (m, 5 H), 6.02 (d, J ϭ 7.0 Hz, 1 H), 4.45 (m,
1 H), 3.87 (m, 1 H), 3.78 (dd, J ϭ 10.0, 3.4 Hz,1 H), 3.40 (s, 3
H) ppm.
µ-Oxobis{[(3ЈS,4ЈS)-2-(4Ј-acetoxymethyl-3Ј,4Ј-dihydro-2Ј-oxazolyl-
3
Ј-phenyl)pyridine-N,N](isothiocyanato)dioxomolybdenum(VI)} (III):
An analogous procedure to that described for the preparation of
II was used, replacing with 3.Yield: 0.443 g (75%).
(1008.37): calcd. C 44.02, H 3.48, N 8.11, S
.18; found C 44.50, H 3.65, N 7.80, S 5.94. MS (FAB positive):
m/z ϭ 1010 [M], 922 [M Ϫ NCS Ϫ 2 O]. IR (KBr): ν˜ ϭ 2056 (st,
2
38 2 6 11 2
C H36Mo N O S
6
Ϫ1
CϭNNCS), 1658 (st, CϭN) and, 940 and 910 (MoϭO) cm
.
µ-Oxobis{[(3ЈS,4ЈS)-2-(3Ј,4Ј-dihydro-2Ј-oxazolyl-3Ј-phenyl-4Ј-trityl-
methyl)pyridine-N,N](isothiocyanato)dioxomolybdenum(VI)} (IV):
An analogous procedure to that described for the preparation of II
Scheme 4. Mechanistic proposal for olefin epoxidation catalysed
by seven-coordinate molybdenum species containing hemilabile li-
gands
was used, replacing
(1381.26): calcd. C 60.87, H 4.06, N 6.09, S
.64; found C 59.75, H 3.98, N 5.99, S 5.03. MS (FAB positive):
2
with 4. Yield: 0.302 g (68%).
70 2 6 9 2
C H56Mo N O S
mechanism described. Nevertheless, as stated above, the
4
nonlability of the oxazolinylpyridine ligand and the pres- m/z ϭ 780 [M Ϫ NCS Ϫ 3 O Ϫ 4]. IR (KBr): ν˜ ϭ 2043 (st, Cϭ
Ϫ1
ence of a vacant site can favour the direct olefin coordi- NNCS), 1658 (st, CϭN) and, 942 and 911 (MoϭO) cm
.
nation to the metal atom. Stereoselectivity control in the
organic process is then possible.
Further reactivity studies with dimetallic molybdenum
species involved in catalytic olefin epoxidations are in pro-
gress.
Bis[(4ЈR)-2-(4Ј-ethyl-3Ј,4Ј-dihydro-2Ј-oxazolyl)phenolato-N,O]oxo-
(
peroxo)molybdenum(VI) (V): MoO (0.285 g, 1.980 mmol) was ad-
3
ded in portions to an aqueous H O solution (30%, 5 mL) at room
2
2
temperature. The mixture was stirred for 20 min and warmed to 40
C for 40 min. The reaction mixture was filtered, the yellow solu-
tion was cooled to 10 °C and a solution of 1 (0.384 g, 2.00 mmol)
in CH OH was added. The mixture was stirred at 20 °C for 30 min
°
3
Experimental Section
and then at 0 °C for 16 h. It was concentrated under reduced press-
ure to approximately 5 mL and diethyl ether was added (20 mL).
The yellow precipitate obtained was filtered and recrystallized from
General Remarks: Solvents were purified by standard procedures
and distilled under nitrogen. Na
Strem), aqueous tBuOOH (8 ; Fluka), tBuOOH in decane solu-
tion (5.5 ; Fluka), cyclooctene (Fluka), norbornene (Aldrich) and
2
MoO
4
·2H
2
O (Probus), MoO
3
CH
2
Cl
2
/diethyl ether (3:1). Yield: 0.405 g (39%). M.p. 197 °C.
(524.38): calcd. C 50.39, H 4.61, N 5.34; found C
(
C
22
H
24MoN O
2 7
50.34, H4.85, N 5.17. MS (FAB positive): m/z ϭ 525. IR (KBr):
ν˜ ϭ 1630 (st, CϭN), 962 (st, MoϭO), 865 (st, OϪO), 758 (asymm.
st MoϪO), 582 (symm. st MoϪO) cm
[23]
(
2
R)-limonene (Aldrich) were used as purchased. Ligands 1,
Ϫ4[
24,25]
and complex I were prepared as described previously.
) were recorded with Bruker Avance600,
[9]
Ϫ1 1
. H NMR (500 MHz):
NMR spectra (in CDCl
3
δmajor ϭ 7.68 (dd, J ϭ 8.5, 1.5 Hz, 1 H), 7.53 (dd, J ϭ 8.0, 1.5 Hz,
Varian XL-500, Varian Gemini or Bruker DRX 250 spectrometers.
Chemical shifts are reported downfield from standards. IR spectra
1 H), 6.93 (m, 1 H), 6.88 (m, 1 H), 6.56 (m, 2 H), 6.04 (dd, J ϭ
8.5, 0.5 Hz, 1 H), 5.77 (dd, J ϭ 8.5, 0.5 Hz, 1 H), 5.73 (m, 1 H),
were recorded with a Nicolet 520 FT-IR or a Bruker IFS 55 FTIR 5.06 (m, 1 H), 4.92 (dd, J ϭ 8.5, 6.5 Hz, 1 H), 4.88 (dd, J ϭ 8.5,
spectrometer. FAB mass spectra were obtained with a Micromass
5.5 Hz, 1 H), 4.60 (dd, J ϭ 8.5, 5.5 Hz, 2 H), 4.52 (dd, J ϭ 8.5,
VG-Quattro instrument. The GC-MS analyses were performed 6.5 Hz, 1 H), 2.45 (m, 1 H), 2.20 (m, 1 H), 2.15 (m, 1 H), 2.02 (m,
with a HewlettϪPackard 5890 Series II gas chromatograph (50 m
Ultra 2 capillary column) interfaced with a HewlettϪPackard 5971
1 H); 1.07 (t, J ϭ 7.5 Hz,, 3 H), 1.01 (t, J ϭ 7.5 Hz, 3 H) ppm.
C NMR (75 MHz): δ ϭ 167.6 (CϭN), 167.4 (CϭN), 166.2 (C),
1
3
mass-selective detector. Elemental analyses were carried out by the 164.0 (C), 135.4 (CH), 135.0 (CH), 129.1 (CH), 128.5 (CH), 120.9
Serveis Cientifico-T e` cnics de la Universitat de Barcelona with an
Eager 1108 microanalyser.
(CH), 120.0 (CH), 117.8 (CH), 117.5 (CH), 112.7 (CϪO), 111.3
(CϪO), 73.1 (CH ϪO), 72.7 (CH ϪO), 70.0 (CH), 64.6 (CH), 28.1
CH ), 26.5 (CH ), 9.1 (CH ), 8.2 (CH ) ppm. δminor ϭ 8.08 (dd,
J ϭ 8, 1 Hz, 1 H), 7.78 (dd, J ϭ 8 Hz, 1 H), 7.65 (m, 1 H), 7.50
m, 1 H), 7.36 (pt, J ϭ 7.5 Hz 1 H), 7.05 (m, 2 H), 6.11 (d J ϭ
.5 Hz, 1 H), 4.69 (pt, J ϭ 10 Hz, 1 H), 4.44 (dd, J ϭ 8.5, 5 Hz, 1
H), 4.28 (pt, J ϭ 10 Hz, 1 H), 3.77 (dd, J ϭ 5.0, 11. 5 Hz, 1 H),
.90 (m, 2 H), 0.95 (t, J ϭ 7.5 Hz, 3 H), 0.88 (t, J ϭ 7.5 Hz, 3
2
2
(
2
2
3
3
µ-Oxobis{[(3ЈS,4ЈS)-2-(3Ј,4Ј-dihydro-4Ј-methoxymethyl-2Ј-oxazolyl-
3
Ј-phenyl)pyridine-N,N](isothiocyanato)dioxomolybdenum(VI)} (II):
HCl (1  1.7 mmol, 1.7 mL) was slowly added, with stirring, to a
mixture of aqueous solutions (10 mL) of Na MoO ·2H O (100 mg,
.413 mmol) and KNCS (160 mg, 1.652 mmol). After 15 min, the
oxazolinylpyridine 2 (110.8 mg, 0.413 mmol) in CH Cl (20 mL)
(
8
2
4
2
0
1
2
2
H) ppm.
was added to the yellow solution and vigorously stirred for 45 min.
The organic layer was extracted, washed with water (3 ϫ 10 mL)
[(3ЈS,4ЈS)-2-(3Ј,4Ј-Dihydro-4Ј-methoxymethyl-2Ј-oxazolyl-3Ј-phen-
and dried with Na
2
SO
4
. The solvent was removed under reduced
yl)pyridine-N,N]oxobis(peroxo)molybdenum(VI
)
(VI):
MoO
3
pressure. The solid obtained was washed with diethyl ether (3 ϫ 20
(0.252 g, 1.75 mmol) was added in portions to an aqueous H O
2
2
mL) and dried under vacuum. Yield: 0.161 g (77%).
solution (30%, 5 mL). The mixture was stirred for 20 min and
C
34
H32Mo
2
N
6
O
9
S
2
(924.68): calcd. C 43.68, H 3.46, N 9.09, S 6.93; warmed to 40 °C for 4 h. The reaction mixture was filtered and to
found C 43.62, H 3.29, N 8.90, S 6.84. MS (FAB positive): m/z ϭ the yellow solution was added 2 (0.470 g, 1.75 mmol) in CH OH
3
8
67 [M Ϫ NCS]. IR (KBr): ν˜ ϭ 2045 (st, CϭNNCS), 1658 (st, Cϭ
(20 mL). The mixture was stirred at room temperature for 1 h and
then concentrated under reduced pressure to 1 mL. Ethanol was
Ϫ1
1H NMR (600 MHz): δmajor
ϭ
N), 941 and 906 (MoϭO) cm
.
Eur. J. Inorg. Chem. 2004, 4278Ϫ4285
www.eurjic.org
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4283

M. G o´ mez, H. Teruel et al.
FULL PAPER
Table 3. Crystal data for V and VI
V
VI
Empirical formula
Formula mass
Crystal size [mm]
Temperature [K]
Crystal system
Space group
C
524.37
0.50 ϫ 0.16 ϫ 0.10
298(2)
orthorhombic
P2(1)2(1)2(1)
6.7608(8)
22
H
24MoN
2
O
7
C
444.25
0.27 ϫ 0.10 ϫ 0.08
298(2)
monoclinic
P2(1)
7.2720(4)
14.6377(8)
8.4005(4)
16
H
16MoN
2 7
O
˚
a [A]
˚
b [A]
9.9119(11)
33.456(4)
˚
c [A]
β [°]
90
2242.0(4)
90.6780(10)
894.13(8)
˚
3
V [A ]
Z
4
2
Ϫ3
Density (calculated) [Mg·m
Absorption coefficient [mm
]
]
1.554
0.631
1.650
0.774
Ϫ1
θ range for data collection [°]
Reflections collected
2.14Ϫ26.38
12971
2.42Ϫ28.28
5751
Data/restraints/parameters
Independent reflections
Max./min. transmission
Final R(int) [I Ͼ 2σ(I)][
R int (all data)
4560/0/299
4560 [R(int) ϭ 0.0294]
0.9396/0.7433
R1 ϭ 0.0388, wR2 ϭ 0.1020
R1 ϭ 0.0482, wR2 ϭ 0.107
1.061
3392/1/235
3392 [R(int) ϭ 0.0162]
0.9407/0.8182
R1 ϭ 0.0211, wR2 ϭ 0.0526
R1 ϭ 0.0227, wR2 ϭ 0.0535
1.025
a]
[b]
[b]
GOF on F2
Absolute structure parameter
Largest diff. peak/hole [e·A
Ϫ0.02(5)
0.565/Ϫ0.481
0.00(3)
0.201/Ϫ0.490
˚
Ϫ3
]
[
a]
2
Ϫ F 2 ) ]}1/2. [b] The weighting scheme employed was w ϭ [σ (F
2 2
2
2
2
Ϫ1
R1 ϭ Σ||F
o
| Ϫ |F
c
|| and wR2 ϭ {Σ[w(F
o
c
)]/Σ[w(F ) ϩ (aP) ϩ bP]
o
o
2
2
and P ϭ (|F
o c
| ϩ 2|F | )/3.
added (2 ϫ 20 mL) and removed each time. The yellow solid ob- lected over a hemisphere of the reciprocal space by combination of
tained was recrystallized from CH
2
Cl
2
/hexane (9:1). Yield: 0.234 g
(444.25): calcd. C 43.26, H
three exposure sets. Each frame covered 0.3° in ω and the first 50
frames were recollected at the end of data collection to monitor
(30%). M.p. 146 °C. C16
H16MoN
2
O
7
3
.63, N 6.31; found C 47.28, H 5.05, N 6.31. MS (ES): m/z ϭ 445 crystal decay. The crystals used for the diffraction studies showed
]. IR (KBr): ν˜ ϭ 1636 (st, CϭN), 962 (st,
no decomposition during data collection. The crystallographic data
MoϭO), 863 (st, OϪO), 769 (asymm. st MoϪO) and 581 (symm. for V and VI are summarised in Table 3. Absorption corrections
[M ϩ 1], 415 [M Ϫ O
2
Ϫ1
1
[26]
st MoϪO) cm . H NMR (400 MHz): δ ϭ 8.39 (pd, J ϭ 5.2 Hz,
were applied using the SADABS programme.
The structures
1
1
H), 7.94 (m, J ϭ 1.6 Hz, 8 Hz, 2 H), 7.55 (m, J ϭ 2 Hz, 1.6 Hz, were solved using the Bruker SHELXTL-PC software by direct
2
[27]
H), 7.49 (s, 5 H), 6.34 (d, J ϭ 6.8 Hz, 1 H), 5.01 (m, 1 H), 4.30
methods and refined by full-matrix least-squares methods on F .
(
1
(
dd, J ϭ 3.6 Hz; J ϭ 10.8 Hz, 1 H), 3.98 (dd, J ϭ 2.8 Hz; J ϭ All non-hydrogen atoms were refined anisotropically. Hydrogen
0.8 Hz, 1 H), 3.57 (s, 3 H) ppm. 13C NMR (75 MHz): δ ϭ 169.8
CϭN), 148.0 (CH), 139.2 (CH), 139.1 (C), 135.9 (C), 130.4 (CH),
atoms were included in calculated positions and refined using the
riding mode. CCDC-236207 (V) and -236208 (VI) contain the sup-
129.6 (CH), 129.3 (CH), 126.5 (CH), 124.6 (CH), 89.9 (CH), 73.4 plementary crystallographic data for this paper. These data can be
(
CH), 70.7 (CH
2
ϪO), 59.6 (CH
3
ϪO) ppm.
obtained free of charge at www.ccdc.cam.ac.uk/conts/retriev-
ing.html [or from the Cambridge Crystallographic Data Centre, 12,
Union Road, Cambridge CB2 1EZ, UK; Fax: (internat.) ϩ 44-
Molybdenum-Catalysed Olefin Epoxidation: The precursor
(
0.025 mmol of complex: 12.7 mg for I, 11.6 mg for II, 13.1 mg for
V and 11.1 mg for VI) was dissolved in toluene (1 mL). The olefin
1 mmol, 110 mg for 5, 94 mg for 6 and 139 mg for 7) was added,
1223-336-033; E-mail: deposit@ccdc.cam.ac.uk].
(
followed by a tert-butyl hydroperoxide aqueous solution (8 , 0.19
mL, 1.50 mmol). The mixture was stirred at room temperature for
Acknowledgments
2
2 h and then quenched by addition of ethyl acetate and filtered
We would like to thank the Spanish Ministerio de Ciencia y Tecnol-
og ´ı a (BQU 2001-3358), Generalitat de Catalunya and the Decanato
through silica. Undecane was added as an external standard
(
0.2Ϫ0.4 mmol). Analyses of organic products were performed by de Investigaci o´ n y Desarrollo (Universidad Sim o´ n Bol ´ı var) for fin-
gas chromatography. Catalytic reactions were also performed with-
ancial support. The authors thank Dr. Xavier Santos for the GC
out solvent according to the above mentioned experimental pro- analysis. J. B. thanks the Universitat de Barcelona for a two-year
cedure.
pre-doctoral grant.
X-ray Crystallographic Studies: Single crystals were grown by al-
lowing diethyl ether to diffuse into a dichloromethane solution. The
crystal data for V and VI were collected using a Bruker SMART
CCD based diffractometer operating at room temperature. Intensit-
[
1]
R. H. Holm, Coord. Chem. Rev. 1990, 100, 183Ϫ221 and refer-
ences cited therein.
R. C. Maurya, D. D. Mishra, S. N. Rao, R. Verma, N. N. Rao,
Ind. J. Chem. 1997, 36, 599.
H. Mimoun, I. Seree de Roch, L. Sajus, Tetrahedron 1970, 26,
37Ϫ50.
[2]
[3]
ies were collected with graphite-monochromated Mo-K
α
radiation
˚
(λ ϭ 0.71073 A) generated at 50 kV and 30 mA, using the ω/2θ
scan technique. A total of 1271 frames of intensity data were col-
4284
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
Eur. J. Inorg. Chem. 2004, 4278Ϫ4285

Structural Studies of Mono- and Dimetallic MoVI Complexes
FULL PAPER
E. O. Schlemper, G. N. Schrauzer, I. A. Hugues, Polyhedron
1984, 3, 377Ϫ380.
W. A. Herrmann, W. R. Thiel, J. G. Kuchlerf, J. Behm, E.
Herdtweck, Chem. Ber. 1990, 123, 1963Ϫ1970.
W. R. Thiel, M. Angstl, T. Preirmeier, Chem. Ber. 1994, 127,
2373Ϫ2379.
[
[
4]
5]
[17]
H. Mimoun, Angew. Chem. Int. Ed. Engl. 1982, 21, 734Ϫ750.
K. B. Sharpless, J. M. Townsen, D. R. Williams, J. Am. Chem.
Soc. 1972, 94, 295Ϫ296.
[
[
[
[
18]
19]
20]
21]
[
[
[
[
6]
7]
8]
9]
K. A. Jørgensen, R. Hoffmann, Acta Chem. Scand., Ser. B
1986, 40, 411Ϫ419.
P. Macchi, A. J. Schultz, F. K. Larsen, D. B. Iversen, J. Phys.
Chem. A 2001, 105, 9231Ϫ9242.
W. R. Thiel, T. Preirmeier, Angew. Chem. Int. Ed. Engl. 1995,
A. Hroch, G. Gemmecker, W. R. Thiel, Eur. J. Inorg. Chem.
34, 1737Ϫ1738.
2
000, 1107Ϫ1114.
H. Glass, M. Spiegler, W. R. Thiel, Eur. J. Inorg. Chem. 1998,
275Ϫ281.
W. R. Thiel, J. Eppinger, Chem. Eur. J. 1997, 3, 696Ϫ705.
M. G o´ mez, S. Jansat, G. Muller, D. Panyella, M. Font-Bard ´ı a,
X. Solans, J. Chem. Soc., Dalton Trans. 1997, 3755Ϫ3764.
G. Balavoine, J. C. Clinet, I. Lellouche, Tetrahedron Lett. 1989,
30, 5141Ϫ5144.
J. M. Canal, M. G o´ mez, F. Jim e´ nez, M. Rocamora, G. Muller,
E. Du n˜ ach, D. Franco, A. Jim e´ nez, F. H. Cano, Organometall-
ics 2000, 19, 966Ϫ978.
G. M. Sheldrick, SADABS, A Program for Empirical Absorp-
tion Correction of Area Detector Data, University of Göttingen,
Germany, 1996, based on the method of Robert Blessing: R.
H. Blessing, Acta Crystallogr., Sect. A 1995, 51, 33.
G. M. Sheldrick, SHELXS-97, A Program for Solving Crystal
Structures and Crystal Structure Refinement, University of
Göttingen, Germany, 1997.
M. G o´ mez, S. Jansat, G. Muller, G. Noguera, H. Teruel, V.
Moliner, E. Cerrada, M. Hursthouse, Eur. J. Inorg. Chem.
[
[
22]
23]
2
001, 1071Ϫ1076.
[
10]
M. G o´ mez, G. Muller, M. Rocamora, Coord. Chem. Rev. 1999,
93Ϫ195, 769Ϫ835.
[24]
1
[
[
11]
12]
G. Helmchen, A. Pfaltz, Acc. Chem. Res. 2000, 33, 336Ϫ345.
Catalytic Asymmetric Synthesis (Ed.; I. Ojima), 2nd ed., John
Wiley & Sons, New York, 2000.
H. Arzoumanian, R. Bakhtchadjian, G. Agrifoglio, H. Krentz-
ien, J.-C. Daran, Eur. J. Inorg. Chem. 1999, 2255Ϫ2259.
Bis(oxazolines) used were: 1,2-bis[(4R)-4-ethyl-∆ -1,3-oxazolin-
[25]
[
[
13]
14]
[
[
26]
27]
2
2
2
-yl)]benzene and 1,2-bis[(4R)-4-ethyl-∆ -1,3-oxazolin-2-yl)]e-
thane: M. G o´ mez, S. Jansat, G. Muller, M. A. Maestro, J.
Mah ´ı a, M. Font-Bard ´ı a, X. Solans, J. Chem. Soc., Dalton
Trans. 2001, 1432Ϫ1439.
R. J. Cross, P. D. Newman, R. D. Peacock, D. Stirling, J. Mol.
Catal. A: Chem. 1999, 144, 273Ϫ284.
R. Bandyopadhyay, S. Biswas, S. Guha, A. K. Mukherjee, R.
Bhattacharyya, Chem. Commun. 1999, 1627Ϫ1628.
[
[
15]
16]
Received April 26, 2004
Early View Article
Published Online August 26, 2004
Eur. J. Inorg. Chem. 2004, 4278Ϫ4285
www.eurjic.org
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4285