New insights into the reaction of t-butylhydroperoxide with dichloro- and dimethyl(dioxo)molybdenum(VI)

Article abstract of DOI:10.1016/S0022-328X(02)01134-8

Lewis-base adducts of dichlorodioxomolybdenum(VI) and dimethyldioxomolybdenum(VI) react in an equilibrium reaction with excess t-butylhydroperoxide (TBHP) under the formation of a seven-coordinated molybdenum(VI) complexes displaying a η¹-alkyl

Full text of DOI:10.1016/S0022-328X(02)01134-8

Journal of Organometallic Chemistry 649 (2002) 108–112
www.elsevier.com/locate/jorganchem
New insights into the reaction of t-butylhydroperoxide with
dichloro- and dimethyl(dioxo)molybdenum(VI)
a
a,b
a
a,
Michelle Groarke , Isabel S. Gon c¸ alves , Wolfgang A. Herrmann , Fritz E. K u¨ hn *
a
Anorganisches-chemisches Institut der Technischen Uni6ersit a¨ t M u¨ nchen, Lichtenbergstraße 4, D-85747 Garching b, Munich, Germany
b
Departamento de Qu ´ı mica, Uni6ersidade de A6eiro, Campus de Santiago, 3810-193 A6eiro, Portugal
Received 30 November 2001; accepted 2 December 2001
Abstract
Lewis-base adducts of dichlorodioxomolybdenum(VI) and dimethyldioxomolybdenum(VI) react in an equilibrium reaction with
excess t-butylhydroperoxide (TBHP) under the formation of a seven-coordinated molybdenum(VI) complexes displaying a
1
h -alkylperoxo-ligand. HCl/CH elimination or the protonation of the Lewis-base ligand is not observed, the TBHP hydrogen
4
atom is instead transferred to one of the terminal oxo ligands under the formation of a molybdenum bound ꢀOH moiety. The
peroxo species is assumed to be the active catalyst in olefin epoxidation. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Alkylperoxides; Epoxidation; Molybdenum
MoClO(O )L and MoO(O ) L are formed [3], it is not
1
. Introduction
2
2 2
clear to date, which kind of species could be formed in
the presence of TBHP. In this work we present results,
which give for the first time some closer insight into the
reaction of TBHP with MoO X L. In order to be able
Lewis-base adducts of dihalogenodioxomolybde-
num(VI) and dialkyldioxomolybdenum(VI) act as ac-
tive catalysts in the olefin epoxidation when treated
with high excess of t-butylhydroperoxide (TBHP) [1].
In the presence of hydrogen peroxide or triphenyl-
methyl-hydroperoxide they do not show catalytic activ-
ity, unless special Lewis bases are used [2]. While it is
known that in the presence of H O or Ph COOH
2
2
to follow the reaction in solution we synthesized a
soluble derivative MoO Cl L (1) and its alkyl homo-
2
2
logue, MoO (CH ) L (2), with L=4,4%-dihexyl-2,2%-
2
3 2
bipyridine).
2
2
3
2
h -mono- and bisperoxocomplexes of the types
2
. Results and discussion
Table 1
Aromatic 1H-NMR signals of 4,4%-bis(n-hexyl)bipyridine of selected
compounds (in CDCl3)
First we examined whether the Mo(VI) complexes 1
and 2 exchange some of their ligands in solution.
Excess of 4,4%-di-tert-butyl-2,2%-bipyridine was added to
a solution of complex 1 and 2, respectively, in CH Cl ,
Compound 1
+TBHP
Compound 2
+TBHP
9.39
8.82
9.38
8.85
8.22
8.54
8.75
8.52
8.06
8.01
8.04
7.98
7.50
7.56
8.34
8.21
7.49
7.30
7.51
7.32
7.14
7.52
7.38
7.09
2
2
1
the reaction mixtures were stirred for 4 h at 55° C and
1
2
then H-NMR and IR spectra were measured. No
N-oxide
ligand exchange was observed. Reacting a solution of
complex 1 with two equivalents of 2,2%-bipyridine, we
also found that no significant ligand exchange was
observed even after 2 days. The oxygens of the starting
molybdenum complex are also found not to exchange
with other terminal, metal bound oxygen ligands. This
was confirmed by the treatment of a solution of com-
MoO Cl (N-oxide)
2
2
Protonated ligand
Free ligand
*
Corresponding author. Tel.: +49-89-289-13080; fax: +49-89-
2
0
89-13473.
17
E-mail address: fritz.kuehn@ch.tum.de (F.E. K u¨ hn).
pound 1 in CDCl with O labelled MTO, which is
3
022-328X/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(02)01134-8

M. Groarke et al. / Journal of Organometallic Chemistry 649 (2002) 108–112
109
known to readily exchange its oxo functionality via
intermolecular oxygen bridges [4]. After 2 days, NMR
spectroscopy showed only the labelled rhenium oxygen
signal.
compound 1 and seem to confirm the conclusions
1
drawn above. Unfortunately, the CH - H-NMR signal
3
was shifted to lower field (to ca. 0.9 ppm) in the case of
the reaction of 2 with TBHP so that it could not be
integrated due to heavy overlap with the n-hexyl groups
of the Lewis-base ligands.
It is also interesting to know whether the catalytically
active species is formed in a reversible or irreversible
manner with TBHP. Both complexes 1 and 2 were
therefore reacted with a 300-fold excess of TBHP (5.5
M in n-decane solution, the same conditions as applied
in our previous catalytic examinations) in CH Cl .
17
O-NMR experiments were also found to be quite
informative on the reaction between compounds 1 and
17
2 with TBHP. O labelled complex 1 exhibits a signal
at 995 ppm with a linewidth of 800 Hz, compound 2
displays a signal at 845 ppm (linewidths 720 Hz). Upon
addition of 0.5 equivalents of TBHP to compound 1, a
new small signal appears, at 563 ppm with a linewidth
of 100 Hz (measurement time was ca. 15 min). In the
case of compound 2 the new signal appears at 527 ppm
(linewidths 140 Hz). The signal increases in size on
addition of more TBHP and longer waiting times in
both cases. The MꢁO signal does not change signifi-
cantly on the incremental addition of TBHP and the
linewidth remains constant. Unlabelled TBHP itself
reveals two broad signals at 251 and 202 ppm with
linewidths of 800 and 1050 Hz, respectively, which were
2
2
When working up the reaction mixture we found that
we obtained the starting complex unchanged in quanti-
tative yield. This provides additional support for the
conclusions drawn in a previous paper [1a] that the
MoꢀX bonds are not cleaved during the reaction with
TBHP and indicates that the catalytically active species
exists in an equilibrium with the starting complex in
solution.
The reaction of compounds 1 and 2 with TBHP was
then examined using spectroscopic tools. The bipyridine
1
aromatic H-NMR signals were used as diagnostic fea-
tures in the elucidation of information. The addition of
17
0.5 equivalents of TBHP to complex 1 leads to new
not observed in the O-NMR spectra. Molybdenum
peroxo complexes exhibit their peroxo signals in a
range of 400–600 ppm with linewidths of 1200–2000
Hz [5]. In our experiments, neither the chemical shift
nor the linewidth was found to change for the signal at
563 and 527 ppm, respectively. Considering the fact
that the TBHP oxygens are not evident in the NMR
spectra, then it is highly unlikely that the peak at 563
(and 527, respectively) ppm is due to a minor amount
of TBHP coordinated to the Mo centre. Therefore, we
assume that this oxygen signal is due to a labelled
oxygen of the complex. We propose that the proton of
the TBHP is transferred to one of the terminal oxygens
of the complex upon coordination of the TBHP to the
molybdenum core. From literature precedent, the
linewidth of ca. 100 Hz is obviously too narrow for a
signals of very low intensity (see Table 1). Increasing
the amount of TBHP leads to a slow increase in
intensity of these new signals. During a period of more
1
than 1 h new H-NMR signals grow when a 1:6 excess
of TBHP is applied, while the original signals lose
integral intensity with respect to an internal standard.
The development with higher amounts of TBHP is
difficult to follow because of the large TBHP and
solvent signals. However, applying a reaction tempera-
ture of 55° C leads to a faster reaction and a higher
intensity of the product signals after a given time.120
We were initially concerned that the new signals were
due to ligand oxidation. However, this was proven not
to be the case (see Table 1). Moreover, it was found
that adding alkylbipy-bis oxide ligands to MoO Cl in a
2
2
17
1
:1 fashion, leads to a different compound, the aro-
molybdenum peroxo species and a Moꢀ OꢀH is a
1
matic H-NMR signals are also given in Table 1. We
were also considering that the TBHP proton was trans-
ferred to the Lewis-base ligand. Thus, the free ligand
reasonable assignment. Unfortunately, to the best of
17
our knowledge, no O signal of a similar complex has
been reported in the literature to date.
1
95
was protonated with TFA, but the aromatic H-NMR
Mo-NMR was explored as a tool to elucidate the
signals of the protonated species were found at different
NMR shifts (Table 1). Therefore, protonation of the
Lewis-base ligand may be excluded. It can also be
assumed that protonation of the Lewis-base ligand
would destabilize the complex resulting in its fast de-
composition, a phenomenon, which was not observed
to a significant extent. The overall up field shift of the
electronic differences of the Mo core on reaction with
TBHP. This involved sequential addition of TBHP
solution in n-decane to a CDCl solution of the com-
3
plex. Up to a tenfold excess of TBHP was added. The
molybdenum signal of complex 1 occurred at 192 ppm,
displaying a half width of ca. 800 Hz (for complex 2 at
424 ppm, half width ca. 740 Hz). Addition of 0.5
equivalents of TBHP leads to a broadening of the
signal in both cases but no significant shift change
occurs. Adding more TBHP during ca. 1 h leads to
further broadening (up to ca. 1500 Hz with three
equivalents of TBHP). Until ten equivalents of TBHP
are added, the signal is slowly shifted to 206 ppm in the
1
aromatic H signals from the free complex to the
anomalous signals observed on treatment with TBHP
could indicate a somewhat less Lewis acidic molybde-
num core. This may be expected in case of coordination
of the TBHP to the molybdenum. The results obtained
for compound 2 were analogous to those obtained for

110
M. Groarke et al. / Journal of Organometallic Chemistry 649 (2002) 108–112
case of compound 1, the linewidth remains very broad.
A similar small downfield shift change is observed in
band was conspicuously absent. Thus, it is reasonable
−
1
to assume that the 3300 cm
band can be ascribed to
95
the case of the methyl derivative (Dl( Mo)=16 ppm
for complex 2). An explanation with respect to the
results reported above may be that an equilibrium
between the starting material and the reaction product
with TBHP exists, which is located on the side of the
a MoꢀOH vibration. The results obtained for derivative
2 were analogous. Additionally, the MoꢀC IR-vibra-
−
1
tions at 495 cm
cm
(asymmetric vibrations) and 470
(symmetric vibrations) remained observable dur-
ing all our experiments and were shifted less than 20
−
1
−
1
−1
9
5
cm
(to 479 and 457 cm , respectively), so the
starting materials. The mediated Mo-NMR signal
therefore strongly resembles that of the starting mate-
rial, but is much broader.
cleavage of these particular bonds seems not to be of
major concern under the conditions applied. The MoꢀC
interaction, however, seems to weaken. The intensity
loss of the bands might also be attributed to the
changes in the structure of the complex due to the
reaction with TBHP.
Solution phase IR and Raman spectroscopies were
used to examine the changes in stretching frequencies
on reaction of complexes 1 and 2 with TBHP solutions
in THF and CH Cl . The terminal MoꢁO bonds of
2
2
We were also interested in gaining information on
the mode of coordination of the TBHP to the molybde-
num centre, i.e. whether initial protonation of a MoꢁO
complex 1 show characteristic cis MoꢁO stretching
−
1
frequencies of 942 and 915 cm . Small spectral
changes are observed after addition of 0.5 equivalents
of TBHP, which become very clear and pronounced
after adding a tenfold excess of TBHP to the complex
and an appropriate waiting time for establishing the
t
−
occurs followed by coordination of the BuOO or
coordination of the TBHP to the Mo is the first step
followed by proton transfer to the MꢁO. We examined
the reaction between complex 1 and TFA and it was
−
1
equilibrium. A band at 978 cm
increases in intensity on addition of TBHP to MoO Cl2
is observed, which
1
found that no changes occurred in the H-NMR spec-
2
trum even after 2 days. Complex decomposition was
not observed. This suggests that the MoꢁO is not
sufficiently basic to be protonated even by such a
strong acid. It is also argued that the triflate anion has
an insufficient coordinating ability due to the electron
(
8
alkylbipy) solution. Additionally, two other bands at
−
1
85 and 593 cm
appear in both the IR and Raman
spectra. The peaks attributable to the MoꢁO stretching
frequencies are found to decrease in intensity. The new
1
bands can be reasonably assigned to the h -coordinated
withdrawing capacity of the CF group. This strongly
t
−
3
BuOO species and a protonated MoꢁO. The new
suggests that coordination of the TBHP is a prerequi-
−
1
band at 980 cm
8
correlates to w(MꢁO), the band at
site for proton transfer. Considering the pK of TBHP
−
1
−1
a
85 cm
to w(OꢀO) and the signal at 593 cm
to the
(
ca. 11), it was decided to compare its coordination
w(MoꢀO). Mimoun et al. reports similar bands for the
with an alcohol of similar pK . Phenol was chosen,
1
t
a
vanadium(V) h -peroxo complex, (dipic)VO(OO Bu)
having a pKa of 10. It was found that the phenol
[
dipic=2,6-pyridinedicarboxylate] [6]. The latter com-
1
H-NMR signals remained unchanged on reaction with
−
1
plex exhibits a band at 980 cm
due to the terminal
complex 1 and the phenolic OꢀH was still evident. It
might be that the phenol is too sterically demanding to
coordinate to the molybdenum core and, as proton
transfer is not observed, then it may be assumed that
coordination of the alcohol is required before the
MoꢁO may be protonated. It can, in principle, not be
totally excluded that instead of an ionic transfer mecha-
nism a radicalic transfer of the proton from TBHP
takes place. To elucidate if the mechanism involves
radical pathways an experiment was carried out with
complexes 1 and 2 in the presence of a radical scav-
enger added at the beginning of the reaction (equimolar
amounts of cyclooctene and 2,6-di-tert-butyl-4-methyl-
phenol). The reaction rate was not significantly af-
fected, therefore not supporting the involvement of
radical species, which are not extremely short lived.
In summary, based on the experimental data pre-
sented above, it seems reasonable to assume that the
reaction of Lewis-base adducts of dichloro- and
dimethyl(dioxo)molybdenum(VI) with excess TBHP
produce products of the composition shown in Eq. (1).
Additional kinetic and theoretic examinations are cur-
VꢁO, whereas the w(OꢀO) stretching band occurs at 890
−
1
−1
cm
and the w(VꢀO) at 580 cm . Molybdenum
2
mono- and bis-h -peroxo species have also been re-
ported in the literature [2,7]. The complex
MoO(O )Cl L (L=DMF, HPMT) shows the w(OꢀO)
2
2
−
1
band at 920 cm
whilst two bands at 550 and 600
−
1
cm
MoO(O ) L (L=range of mono and bidentate N–O
are found for the Mo(O ) stretching frequencies.
2
2
2 n
−
1
ligands) exhibit their peroxo w(OꢀO) at 880 cm
and
the w(MoꢀO) is highlighted by two bands at 500 and
−
1
6
00 cm . Analogous bands are not found in our
experiments, therefore, we can confidently exclude the
2
possibility of the formation of a h -mono- or bisperoxo
−
1
species. A shoulder at approximately 3270 cm
ap-
pears in the IR spectra on addition of one equivalent of
−
1
TBHP, which broadens (3300 cm ) after the addition
of excess TBHP to the complex solution. This is sugges-
tive of a different OH species to the TBHP–OH. We
were concerned that this might merely be due to mois-
ture in the sample. However, the presence of moisture
−
1
would be confirmed by a band at ca. 1600 cm . This

M. Groarke et al. / Journal of Organometallic Chemistry 649 (2002) 108–112
111
rently under way in our laboratory and will be pub-
lished elsewhere.
3.2. Preparation of MoO Cl (n-hexylbipy) (1)
2
2
4
,4%-Dihexyl-2,2%-bipyridine (0.84 g, 2.60 mmol) was
added to a suspension of MoO Cl (0.47 g, 2.36 mmol)
2
2
in CH Cl (5 ml) and the resulting solution was stirred
2
2
for 30 min. The solution was filtered and concentrated
to 1 ml and hexane was added to precipitate the
colourless product, which was purified by washing with
hexane. The colourless solid was collected and dried in
vacuo to yield the title complex, 1.19g (96%). Calc. for
(
1)
−
1
C H Cl MoN O (525.36 g mol ): C, 50.49; H, 6.16;
22
34
2
2
2
N, 5.35. Found: C, 50.80; H, 6.40; N, 5.16%. Selected
−
1
3
. Experimental
IR data: (KBr, w cm ): 3071 w, 2939 s, 1610 s, 937 vs,
1
9
12 vs, 895 m, 623 w, 344 m, 237 m; H-NMR (400
All preparations and manipulations were done with
MHz, CDCl ); 0.92 (6H, t, CH ), 1.36 (8H, m, (CH ) ),
3
3
2 2
standard Schlenk techniques under an atmosphere of
nitrogen. Solvents were dried by standard procedures,
1
.43 (4H, m, CH ), 1.76 (4H, m, CH ), 2.84 (4H, t,
2 2
CH ), 7.50 (2H, d, pyꢀH), 8.06 (2H, s, pyꢀH), 9.39 (2H,
2
distilled under Ar and kept over 4 A
for NCMe). Microanalyses were performed at the
,
molecular sieves (3
d, pyꢀH).
A
,
Mikroanalytische Labor of the Technische Universit a¨ t
3
.3. Preparation of MoO (CH ) (n-hexylbipy) (2)
2
3 2
1
M u¨ nchen in Garching (M. Barth). H-NMR spectra
were recorded at 400 MHz in a Bruker Avance DPX-
To a suspension of 0.53 g (1.0 mmol) 1 in Et O at
2
1
7
4
5
00 spectrometers. O-NMR spectra were measured at
4.14 MHz in a Bruker Avance DPX-400, and Mo-
−
25° C (isopropanol bath), 2.1 equivalents of a 1.0
9
5
molar solution MeMgCl dissolved in Et O was slowly
2
NMR spectra were measured at 26.07 MHz. Infrared
spectra of solid samples (in the form of KBr pellets and
Nujol mulls) were measured in a Bio-Rad FTS-60A.
The far infrared spectra of the complexes were recorded
as Nujol mulls in a Bio-Rad FTS-175A system using a
added via syringe. The reaction was allowed to warm-
up to r.t. and was stirred for 90 min. The dark red
suspension was dried and distilled water was added.
The product was extracted with CH Cl and the or-
2
2
ganic phase was dried over anhydrous MgSO . The
4
6
mm Mylar beam splitter. The Raman spectra were
solvent was dried and the yellow residue was recrys-
also performed with a Bio-Rad dedicated FT-Raman
spectrometer using the 1064 nm excitation of an Nd-
YAG laser.
tallised from CH Cl –Et O. Yield: 0.25 g (52%). Calc.
2
2
2
−
1
for C H MoN O (482.26 g mol ): C, 59.74; H, 7.94;
22
38
2
2
N, 6.81. Found: C, 60.06; H, 7.99; N, 6.74%. IR (KBr,
−
1
w cm ): 3055 m, 2953 vs, 2907 s, 2871 s, 1611 s, 930
3.1. Preparation of 4,4%-bishexyl-2,2%-bipyridine
vs, 906, vs, w(MoꢁO), 880 m, 618 s. 495 m, 470 w.
1
H-NMR (400 MHz, CDCl ); 0.59 (s, 6H), 0.90 (6H, t,
3
n-BuLi in hexane (17.5 ml, 28 mmol, 1.6 M) was
CH ), 1.35 (8H, m, (CH ) ), 1.40 (4H, m, CH ), 1.72
3
2 2
2
added dropwise to a solution of diisopropyl amine (3.9
ml, 28 mmol) in THF (10 ml) at 0° C. 4,4%-Dimethyl-
(4H, m, CH ), 2.79 (4H, t, CH ), 7.51 (2H, d, pyꢀH),
2
2
8.04 (2H, s, pyꢀH), 9.38 (2H, d, pyꢀH).
2,2%-bipyridine (2 g, 11 mmol) dissolved in THF (50 ml)
17
was added slowly to the reaction mixture and the
resulting red solution was stirred at 0° C for 3 h. A
solution of amyl bromide (3.5 ml, 28 mmol) was then
added and the reaction mixture was stirred overnight,
reaching slowly the room temperature (r.t.). The reac-
tion was quenched by the addition of MeOH and the
solution was poured onto ice. The crude product was
3.4. O-labelling studies
1
7
The labelled complex Mo O Cl (DMF) was pre-
2
2
2
1
7
pared according to the published procedure using H O
[8] [1b]. Labelled complex Mo O Cl (THF) was pre-
2
1
7
2
2
2
1
7
pared by recrystallisation of Mo O Cl (DMF) from
2
2
2
1
7
THF. Labelled complex Mo O X L (X=Cl, Me) was
2
2
extracted with Et O and purified by chromatography
on silica using EtOAc–n-hexane (40–60) as eluent to
prepared using the usual method with the labelled
starting material Mo O Cl (NCMe) .
2 2 2
2
17
yield the pure product as a pale yellow oil, 2.5 g (70%).
−
1
Calc. for C H N (324.51 g mol ): C, 81.43; H, 9.94;
2
2
32
2
1
N, 8.63. Found: C, 81.12; H, 9.84; N, 9.04%. H-NMR
400 MHz, CDCl ), 0.84 (6H, m, CH ), 1.19 (4H, m,
Acknowledgements
(
3
3
CH ), 1.27 (8H, m, (CH ) ), 1.65 (4H, m, CH ), 2.64
I.S.G. acknowledges the Bayerische Forschungs-
stiftung for a research grant. M.G. thank the Alexander
von Humboldt-foundation for a postdoctoral research
2
2 2
2
(
4H, m, CH ), 7.09 (2H, m, pyꢀH), 8.21 (2H, m, pyꢀH),
2
8.52 (2H, m, pyꢀH).

112
M. Groarke et al. / Journal of Organometallic Chemistry 649 (2002) 108–112
´
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(
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[
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