Molybdenum(VI) cis-dioxo complexes bearing (poly)pyrazolylmethane and -borate ligands: Syntheses, characterization and catalytic applications

Article abstract of DOI:10.1039/b009202i

Reaction of MoO₂Cl₂(THF)₂ with mono- (L₁) and bi-dentate (L₂) pyrazole type ligands leads to octahedral complexes of formula MoO₂Cl₂(L₁)₂ and MoO₂Cl₂(L₂) (1, L₁ = 3,5-Me₂pz; 2, L₂ = Me₂C(pz)₂). The structure of 2 has been determined crystallographically. Reaction of MoO₂X₂(THF)₂ (X = (Cl or Br) with tridentate ligands (L₃, e.g. tris(pyrazolyl)methane or tris(pyrazolyl)borate, leads to the replacement of both co-ordinated solvent molecules and one of the chloride ligands to give [MoO₂X(L₃)]X (3, X = Cl, L₃ = HC(pz)₃; 4, X = Br, L₃ = HC(pz)₃; 5, X = Cl, L₃ = HC(3,5-Me₂pz)₃) and MoO₂Cl{((pz)₃BH}) 6. Depending on the donor ability and the steric bulk of the ligand, rapid rotation is observed at room temperature in some cases. At lower temperatures the structures are less fluxional. Nevertheless, bidentate and tridentate pyrazolyl ligands produce distinctly different chemical shifts in the ⁹⁵Mo NMR spectra. The turnover frequencies of the described complexes in olefin epoxidation, with t-butyl hydroperoxide as oxidizing agent, are in the range of 150-460 [mol/(mol catalyst h)]. This activity is in the middle of the range observed for MoO₂X₂L₂ complexes with N-donor ligands.

Full text of DOI:10.1039/b009202i

View Article Online / Journal Homepage / Table of Contents for this issue
Molybdenum(VI) cis-dioxo complexes bearing (poly)pyrazolyl-
methane and -borate ligands: syntheses, characterization and
catalytic applications
Ana M. Santos,a Fritz E. Kühn,*a Kjerstin Bruus-Jensen,a Isabel Lucas,b Carlos C. Romão*b
and Eberhardt Herdtwecka
a Anorganisches-chemisches Institut der Technischen Universität München, Lichtenbergstr. 4,
D-85747 Garching b. München, Germany. E-mail: fritz.kuehn@ch.tum.de
b Instituto de Tecnologia Química e Biológica da Universidade Nova de Lisboa,
Quinta do Marquês, EAN, Apt. 127, 2781-901 Oeiras, Portugal. E-mail: ccr@itqb.unl.pt
Received 16th November 2000, Accepted 2nd March 2001
First published as an Advance Article on the web 30th March 2001
Reaction of MoO2Cl2(THF)2 with mono- (L1) and bi-dentate (L2) pyrazole type ligands leads to octahedral complexes
of formula MoO2Cl2(L1)2 and MoO2Cl2(L2) (1, L1 = 3,5-Me2pz; 2, L2 = Me2C(pz)2). The structure of 2 has been deter-
mined crystallographically. Reaction of MoO2X2(THF)2 (X = Cl or Br) with tridentate ligands (L3), e.g. tris(pyrazolyl)-
methane or tris(pyrazolyl)borate, leads to the replacement of both co-ordinated solvent molecules and one of the
chloride ligands to give [MoO2X(L3)]X (3, X = Cl, L3 = HC(pz)3; 4, X = Br, L3 = HC(pz)3; 5, X = Cl, L3 = HC(3,5-
Me2pz)3) and MoO2Cl{(pz)3BH} 6. Depending on the donor ability and the steric bulk of the ligand, rapid rotation
is observed at room temperature in some cases. At lower temperatures the structures are less fluxional. Nevertheless,
bidentate and tridentate pyrazolyl ligands produce distinctly different chemical shifts in the 95Mo NMR spectra. The
turnover frequencies of the described complexes in olefin epoxidation, with t-butyl hydroperoxide as oxidizing agent,
are in the range of 150–460 [mol/(mol catalyst h)]. This activity is in the middle of the range observed for MoO2X2L2
complexes with N–donor ligands.
only exception is the oxidation of Ph3P to Ph3PO, catalysed by
MoO2Cl{(3,5-Me2pz)3BH}] (3,5-Me2pz = 3,5-dimethylpyraz-
Introduction
Several molybdenum() complexes have proven to be useful
olyl) using Me2SO as oxidizing agent and a catalyst : substrate
catalysts for the epoxidation of olefins with hydroperoxides, e.g.
ratio of 1 : 20.15
t-butyl hydroperoxide (TBHP).1 Recently, we have presented a
In this work we report the synthesis and characterization
of several molybdenum() dioxo complexes bearing poly-
study on the catalytic activity of MoO2X2(L2) complexes in
which the ligands were mainly substituted 1,4-diazabutadienes.
(pyrazolyl)borate and poly(pyrazolyl)methane ligands. The
This study is now extended to include poly(pyrazolyl)-borate
complex MoO2Cl2(3,5-Me2pz)2 is also synthesized for com-
and -methane ligands.
parison purposes. The fluxional behaviour of selected
Since Trofimenko2 synthesized the poly(pyrazolyl)borate
1
complexes is studied via temperature-dependent H NMR and
ligands, they have widely been used in inorganic and organo-
all the complexes are tested for their catalytic activity towards
metallic chemistry, especially with f and d transition elements.3–7
the epoxidation of cyclooctene with TBHP as oxidant under
Also, tris(pyrazolyl)borate and related tripodal ligands have
catalytic conditions.
had a significant impact on the modeling of the active centre
of molybdenum enzymes.8 The related neutral ligands,
Results and discussion
poly(pyrazolyl)methanes, have been studied much less, since
the few metal complexes were first reported by Trofimenko in
1970.9 Recently, an improved synthesis, that delivers good
yields of highly pure ligands, was developed by Jameson and
Castellano.10 This procedure unlocks the coordination
chemistry of poly(pyrazolyl)methane with various metals and
enables comparison with related boron complexes. So far, this
type of ligand was used more in the chemistry of low oxidation
state metals like SnII,11 PdII 12 and YIII.13 In fact, to the best
of our knowledge, there are no examples in the literature of
molybdenum() dioxo complexes bearing this type of ligand.
Moreover, these ligands are interesting due to the fact that
their fluxional behaviour has been established,13,14 especially
in the case of the tridentate ligands. It would appear that the
presence of a relatively weakly coordinating ligand in the
complex positively influences the catalytic activity for epoxid-
ation reactions.1a Thus, the use of these ligands in molyb-
denum() dioxo complexes should, in principle, lead to good
catalytic results. So far, related complexes have not widely been
tested as far as their catalytic properties are concerned. The
Synthesis
Preparation of the new complexes 1–5 (see Chart 1) is carried
out by the stoichiometric combination of MoO2Cl2(THF)2
with the appropriate ligand. Complex 6 was prepared by direct
metathesis of MoO2Cl2(THF)2 and K[HB(pz)3]. An alternative
synthesis using MoO2Cl2(OPPh3)2 also provides the compounds
in good yields. All the reactions proceed in THF in good to
almost quantitative yields. These complexes are soluble in
co-ordinating solvents like THF and NCCH3, moderately
soluble in CH2Cl2 and insoluble in diethyl ether and n-hexane.
All can be handled briefly in air. Complex 1 decomposes under
these conditions within a few minutes whereas 2–6 decompose
within a few hours, as indicated by the appearance of pyrazole
or 3,5-dimethylpyrazole in the NMR spectra.
Infrared spectra
The infrared spectra of the compounds exhibit two strong
ν(Mo᎐O) bands at 930–950 and 900–920 cmϪ1 characteristic
᎐
1332
J. Chem. Soc., Dalton Trans., 2001, 1332–1337
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b009202i

View Article Online
Table 1 Selected catalytic and spectroscopic data for the complexes
1–6
N–H of the pyrazole ring. The signal at δ 5.9 is due to the
pyrazole ring protons, that at δ 2.28 to the methyl groups on
the one and three position of the pyrazole rings. Apparently,
a quick exchange of the hydrogen atom between the two
nitrogens takes place. Therefore, the two different methyl
groups become equivalent, as we only see their averaged reson-
ance. Owing to the presence of the electron withdrawing Lewis-
acidic molybdenum() centre, the ligand signals are, in general,
shifted to lower field in comparison to those of the free ligand
(Table 2). The N–H proton can be observed as a very broad
signal at ca. δ 14. Cooling the dissolved complex results in a
freezing of the ligand exchange. The N–H proton resolves to a
sharp singlet at δ 15.5 at Ϫ90 ЊC. The other two resonances do
not change with temperature. Therefore, the exchange process
seems to remain at low temperature, taking place less quickly
than at higher temperatures.
TOF [mol/
ν(Mo᎐O)/
᎐
Compound
(mol catalyst h)]a
cmϪ1
δ(95Mo)
1
2
3
4
5
6
240
460
270
150
300
210
943, 906
943, 916
953, 920
948, 918
946, 912
935, 905
157
144
87
158
87
80
a The TOF values were calculated for all examined complexes after 5
minutes reaction time and are defined as mol epoxide/[(mol catalyst ×
time (h)].
For complex 2 four resonances are present at ambient tem-
perature with a relative intensity of 1 : 1 : 1 : 3 at δ 8.77 and 7.95
(H-3,5), 6.55 (H-4) and 2.25 (CH3), respectively. These signals
are slightly shifted to lower field when compared to those of the
free ligand (Table 2). By lowering the temperature to Ϫ90 ЊC no
significant dynamic behaviour is detected, only the signals at
δ 8.77 and 7.95 resolve into doublets with a coupling constant
of 2.22 Hz. The different behaviour of compounds 1 and 2 is
certainly due to the absence of N–H bonds in 2 and also to the
higher rigidity arising from the bidentate ligand system in 2.
At ambient temperature complex 3 shows equivalent pyr-
azole rings with resonances at δ 9.17, 8.31 (3,5-H) and 6.62
(H-4); the C–H resonance is found at δ 10.83. Once again, the
resonances are shifted to lower field in relation to those of the
free ligand (see Table 2). At room temperature all three nitrogen
atoms interacting with molybdenum are equivalent due to their
rapid rotation. At Ϫ90 ЊC each type of hydrogen atom of the
pyrazole ring shows two resonances in a 1 : 2 ratio. The coal-
escence temperature is Ϫ40 ЊC. Two of the three nitrogen donor
atoms are located trans to the oxygens, in equatorial positions,
whereas the other nitrogen atom is in axial position, trans to the
chloride ligand. At ambient temperature all the pyrazole
rings of the ligand are equivalent, while at low temperature
the motion is hindered and the complex probably ‘freezes’ in
Cs symmetry. This is analogous to the related compounds
MoO2Cl[{(3,5-Me2pz)3BH}]15 or MoO2(CH3)[{(3,5-Me2pz)3-
BH}],16 where the pyrazole rings are in a facial arrangement. At
room temperature, however, the axial and equatorial positions
become equivalent due to rotation of the ligand. The bromine
derivative 4 shows a very similar behaviour. The 1H NMR
signals are only slightly shifted in comparison to those of the Cl
derivative 3. However, it is clear from the observed resonances
that the better donor capability of the bromine ligand is
reflected in the observed chemical shifts (see Table 2).
Chart 1
of the cis-[MoO2]2ϩ fragment asymmetric and symmetric
vibrations respectively. Although there are no dramatic ligand
dependent changes in 1–6 in the IR region for this moiety, there
are distinctive differences (see Table 1), notably in the case of
the borate derivative 6 where these bands are shifted to lower
wavenumbers. This observation can easily be explained by
considering the fact that complex 6 bears the only negatively
charged tripod ligand of the examined series. It also shows the
typical band for ν(B–H) at 2485 cmϪ1 indicating a stronger B–H
bond as in the case of MoO2Cl{(Me2pz)3BH} where this band
In complex 5 two of the protons of each pyrazole ligand (in 3
and 5 positions) are replaced by methyl groups, thus generating
a bulkier ligand. Accordingly, complex 5 shows two different
types of resonances for the 3,5-dimethylpyrazole ring of the
HC(3,5-Me2pz)3 ligand, even at ambient temperature. In this
case the rotation of the ligand is slower than in complex 3 due
to the steric hindrance caused by the presence of the methyl
groups. Lowering the temperature to Ϫ90 ЊC does not produce
significant changes in the 1H NMR spectrum, as would be
expected. The rigidity of the methylated ligand system has been
noted.17
appears at 2550 cmϪ1 15
.
NMR Spectra
The H, 95Mo and 11B NMR spectra of compounds 1–6 were
recorded in CD2Cl2 at room temperature. Temperature-
dependent H NMR spectra were also recorded for some of
the complexes in order to study their fluxional behaviour. It
has been observed previously that many poly(pyrazolyl)borate
complexes show dynamic behaviour on the NMR timescale.13,14
The fluxional behaviour of their neutral analogues poly(pyr-
azolyl)methanes was also investigated.
At ambient temperature, complex 1 presents two sets of
resonances of relative intensity 1 : 6 (at δ 5.9 and 2.28, respect-
ively), and a very broad peak at δ ca. 14 corresponding to the
1
1
The borate complex 6 also shows a fluxional behaviour.
Three proton signals can be observed for the pyrazole ligand
(see Table 2), and another for the B–H. Even at Ϫ90 ЊC the
system remains flexible. The signals broaden but do not split in
a 1 : 2 ratio.
The compounds exhibit 95Mo NMR resonances with chem-
ical shifts close to the centre of the chemical shift range of this
nucleus (see Table 1). Complexes 1 and 2 show their 95Mo NMR
chemical shifts at δ 157 and 144, respectively (see also Table 1).
J. Chem. Soc., Dalton Trans., 2001, 1332–1337
1333

View Article Online
Table 2 1H NMR chemical shifts of compounds 1–7 and the free ligands. All measurements were performed in methylene chloride at room
temperature
Compound
N–H
CH3/H-3
H-4
CH3/H-5
B–H/C–H
1
14.0
12.1
2.28
2.22
5.93
5.75
2.28
2.22
Ligand
2
8.77
7.54
6.55
6.28
7.95
7.48
2.25
2.28
Ligand
3
9.17
7.64
6.62
6.34
8.31
7.55
10.83
8.42
Ligand
4
8.35
2.79/2.76 (1 : 2)
2.16
7.67
7.52
6.64
6.32/6.26 (1 : 2)
5.90
6.06
6.1
8.07
2.73; 2.79
1.98
7.23
7.25
9.06
2.70; 8.05
8.08
8.0
4.7
5
Ligand
6
Ligand
Fig. 2 Cyclooctene epoxide yields after 4 and 24 h, respectively. Com-
pounds 1–6 have been used as catalysts. See text for more details.
Fig. 1 Typical catalytic run in cyclooctene epoxidation with com-
pound 2 as the catalyst. For details see text.
species must be formed shortly after the addition of the
peroxide. The coordinating ligands profoundly influence the
product yield. The catalytic activities of compounds 1–6 are
shown in Fig. 2, and the turnover frequencies (TOF) are given
in Table 1.
These values are in accordance with those in the literature for
octahedral MoO2X2L complexes, with L = bidentate Lewis base
N donor ligand and X = Cl or Br.1a,18 Complexes 3, 5, 6 display
their 95Mo NMR chemical shift between δ 80 and 90, in good
agreement with values from the literature for oxohalogeno-
molybdenum() complexes with tripodal N donor ligands.15
The borate complex 6 is, as expected and consistent with the IR
data, the most electron rich complex. However, the chemical
shift difference between compounds 5 and 6 is rather small
(7 ppm). For 3 and 4 the observed chemical shifts exhibit an
inverse halogen dependence. Inverse halogen dependencies of
chemical shifts have also been observed for other oxohalogeno-
molybdenum() complexes.1a,19
Comparing the activity of compounds 3 and 4, the bromine
derivative shows a lower turnover frequency than the chlorine
derivative (see Table 1). This is in good agreement with observ-
ations made from comparable complexes of formula MoX2O2-
L2.1a However, after a longer reaction time (4 and 24 h, see
Fig. 2) the difference is within the experimental error of these
measurements. This might be due to the fact that both com-
plexes appear to be stable under the catalytic conditions so that,
finally, nearly quantitative epoxide yields are reached in both
cases. The differences caused by the changes of the organic
ligand are much more pronounced. In fact, compound 6, bear-
ing the most electron donating ligand [HB(pz)3]Ϫ, shows much
lower activity than 3 and 5. In fact, among compounds 3–6, the
latter is the only one that has a negatively charged ligand which,
ultimately, increases the electron density at the metal centre, as
can be seen from both IR and NMR data (see Table 1). The fact
that the epoxide yield with complex 6 as catalyst precursor did
not increase very much from 4 to 24 h reaction time indicates
that an oxidation of the catalyst occurs, as is expected for a
B–H bond under such conditions. It should be noted, in this
context, that the complexes 3 and 6 have very similar structures
and steric bulk around the Mo but have different charges. A
comparison of their catalytic behaviour, however, is difficult to
make due to the decomposition of 6. Compound 1 has, accord-
ing to our NMR examinations, two relatively weakly bound
ligands and is also less stable than 2–5. The chelate effect of the
ligand in complex 2 obviously has a big influence on the catalyst
stability and subsequently on the catalytic activity. This is even
more conspicuous if one examines the bis(pyrazole) derivative
of complex 1: (i) this compound is too unstable to be isolated;
(ii) it decomposes well below room temperature. The activity
Complexes 1–6 in oxidation catalysis
The complexes were tested for the catalytic epoxidation of
cyclooctene with t-butyl hydroperoxide. The TBHP is used as a
5.5 M solution in n-decane, and after its addition homogeneous
conditions are obtained. The applied temperature was 55 ЊC
and the catalyst : substrate : oxidant ratio 1 : 100 : 200. Further
details are given in the Experimental section. Blank reactions
were performed. Without catalyst no significant epoxide form-
ation (<5%) was observed under these conditions. Significant
by-product formation, e.g. diol instead of epoxide, was never
observed in the uncatalysed or catalysed runs.
In general, the compounds show an overall yield between 40
and 60% after 4 h. If the reaction time is increased to 24 h the
yield increases significantly being almost quantitative in the
cases of 2–5. This indicates that the catalysts have a high stab-
ility under the reaction conditions and that the conversions
after 4 h are mainly due to a relatively slow reaction and not to
decomposition of the catalyst. In Fig. 1 a typical catalytic run is
shown for compound 2. Within the experimental limit defined
by the time of the initial measurement there is no indication of
an induction period and, therefore, the catalytically active
1334
J. Chem. Soc., Dalton Trans., 2001, 1332–1337

View Article Online
difference between compounds 3 and 5 is very small, despite the
fact that the methylated ligand of 5 is more electron rich and
bulkier than that of 3. This means that the activity growth can
be related to the electron density at the metal centre at least to a
certain degree. Electron donating ligands decrease the activity
somewhat. Also, steric hindrance might contribute to a lower
activity, as it can make the co-ordination of the peroxide to the
metal centre more difficult, and also hinders the approach of
the olefin. However, with respect to compounds 3 and 5 this
effect does not appear to be very pronounced. Although it
cannot totally be ruled out that the catalytic activity is due to
a significantly different species derived from the precursor
complexes, it seems beyond reasonable doubt that the organic
ligands remain, at least partially, attached to the metal in the
catalytically active centre and determine its activity.
The complexes examined in this work display turnover
frequencies which are within the range of the TOFs of MoX2-
O2L2 complexes already described. The range of the TOFs of
complexes of general formula MoX2O2L2 is ca. 20–600 [mol/
(mol catalyst h)], mainly depending on the ligand L.1a The most
active complexes with ligands of the 1,4-diaza-1,3-butadiene
type have TOFs up to 600 [mol/(mol catalyst h)].1a Complexes
with bipyridine ligands show TOFs below 100 [mol/(mol
catalyst h)].1a The solvent ligated complexes of formula
MoX2O2(Solv)2 (Solv) = THF, RCN, etc. display TOFs between
ca. 100 and 200 [mol/(mol catalyst h)],18 the alkylated deriv-
atives MoR2O2L2 (R = Me or Et) between 20 and 180 [mol/(mol
catalyst h)].20 The well known catalyst MoO2(acac)2 displays
a TOF of 105 [mol/(mol catalyst h)], Mo(CO)6 of ca. 75 [mol/
(mol catalyst h)].1a Therefore the compounds examined in this
work are medium active MoVIO2 catalysts. However, they are
of good stability and the yields obtained after 4 and 24 h
are comparable with those obtained with other systems of
higher TOFs. A comparison with other related systems, e.g.
MoCp*O2Cl, described by Trost and Bergmann21 (which can
be regarded as analogous to the complexes 3–5), and MoO2L4
and WO2L4, described by Herrmann et al.22 is more difficult.
Either no turnover frequencies are given in the original
literature and/or the reactions have been performed under
different conditions, e.g. at different temperatures. It is clear,
however, that significantly more active epoxidation catalysts
exist, e.g. molybdenum() monoperoxo complexes of formula
Fig. 3 PLATON26f plot of the solid state structure of complex 2.
Thermal ellipsoids are at the 50% probability level. Hydrogen atoms are
omitted for clarity.
Table 3 Selected interatomic distances (Å) and angles (deg) for com-
pound 2
Mo–Cl1
Mo–Cl2
Mo–O1
Mo–O2
2.3570(7)
2.3877(5)
1.693(2)
1.690(2)
Mo–N12
Mo–N22
N11–N12
N21–N22
2.301(2)
2.325(2)
1.362(3)
1.368(3)
Cl1–Mo–Cl2
Cl1–Mo–O1
Cl1–Mo–O2
Cl1–Mo–N12
Cl1–Mo–N22
Cl2–Mo–O1
Cl2–Mo–O2
Cl2–Mo–N12
163.36(2)
95.76(6)
95.22(6)
83.85(5)
88.12(5)
93.37(6)
95.80(6)
82.26(5)
Cl2–Mo–N22
O1–Mo–O2
O1–Mo–N12
O1–Mo–N22
O2–Mo–N12
O2–Mo–N22
N12–Mo–N22
79.97(5)
105.09(9)
89.98(8)
166.45(8)
164.91(8)
87.42(7)
77.50(6)
the appropriate ligands with solvent adducts of dihalogeno-
dioxomolybdenum(). In the case of mono- and bi-dentate
pyrazole derivatives, both solvent molecules co-ordinated to the
starting material are replaced by the ligands, thus forming
distorted octahedral complexes. Tridentate pyrazolyl deriv-
atives, additionally replace one of the halogeno ligands. The
co-ordination strength of the ligand depends on its electron
donor capability and steric bulk. Weak donors, lacking
substituents at the pyrazolyl moieties, rotate quickly about the
Mo. Axial and equatorial positions are only distinguishable at
lower temperatures. The described complexes can be utilized
in the oxidation of olefins to epoxides without concomitant
formation of diols. Tris(pyrazolyl)methane and its methylated
derivatives form quite active and stable molybdenum()
catalysts. Different substituents at the pyrazole rings may, there-
fore, further improve the catalytic activity of these systems.
1f,23
24
MoO2(O2)L2
or the extensively examined Re(CH3)O3 and
its adducts of formula Re(CH3)O3L and Re(CH3)O3L2.25 Never-
theless, these established systems have other disadvantages.
While Re(CH3)O3 leads to significant diol formation, the
last two complex types require a ligand excess of about 10 : 1
[L/L2 : Re(CH3)O3] to be efficient epoxidation catalysts.
Crystal structure of MoO2Cl2(C9H12N4) 2
The crystal structure of compound 2 involves discrete
molecules without short interatomic contacts. The molecular
structure is shown in Fig. 3; selected bond lengths and angles
are listed in Table 3. The molecule has nearly Cs symmetry in
the solid state but it exhibits no real crystallographic symmetry.
The co-ordination around the metal centre is best described as
a distorted octahedron. All bond lengths and angles of the
MoO2Cl2N2 moiety do not vary significantly and are compar-
able to those observed in other structural fully characterized
molybdenum compounds with the same core geometry.26g
NMR and IR data are consistent with these observations.
All crystallograpically characterised complexes with a 2,2-
bis(pyrazolyl)propane ligand exhibit a very similar geometry
in the organic building block. Distances and angles in the mol-
ecule are unexceptional and do not require further comment.
Experimental
All preparations and manipulations were carried out with
standard Schlenk techniques under an oxygen free and water
free nitrogen or argon atmosphere. Solvents were dried by
standard procedures, distilled and kept under argon over 4 Å
molecular sieves. Microanalyses and mass spectra were
performed at the TUM (Garching) laboratories. IR spectra
were measured on a Perkin-Elmer FTIR spectrometer, mass
spectra with a Finnigan MAT 311 A and a MAT 90
1
spectrometer and H (400.13), 13C (100.62), 95Mo (26.09) and
11B NMR (128.37 MHz) on a Bruker Avance DPX-400. NMR
spectra of the compounds recorded at 55 ЊC do not vary signifi-
cantly from those at 25 ЊC and are therefore not commented
upon. The starting material MoO2Cl2(THF)2,18 potassium
Conclusion
Molybdenum() cis-dioxo complexes bearing (poly)pyrazolyl-
methane and -borate ligands can readily be prepared by treating
hydrotris(1-pyrazolyl)borate,2b
2,2-di(1-pyrazolyl)propane,27
tris(1-pyrazolyl)methane10 and tris(3,5-dimethyl-1-pyrazolyl)-
J. Chem. Soc., Dalton Trans., 2001, 1332–1337 1335

View Article Online
Table 4 Summary of data for crystal structure analysis of compound 2
methane27 were prepared as described. All other chemicals
mentioned were used as purchased from Aldrich.
Formula
C9H12Cl2MoN4O2
Formula weight
Crystal system
Space group
a/Å
375.05
Preparations
Orthorhombic
Pbca (no. 61)
12.0163(2)
15.8867(4)
14.4079(3)
2750.5(1)
8
1.341
0.71073
173
24230
MoO2Cl2(C10H14N4) 1. To a solution of MoO2Cl2(THF)2
(1307 mg, 3.81 mmol) in THF (15 mL), 3,5-dimethylpyrazole
(732 mg, 7.62 mmol) is added at room temperature. The solu-
tion turns from colourless to yellow and then to orange within
a few minutes. The mixture is allowed to react for 25 min. The
volume is reduced to ca. 2 mL and 10 mL of diethyl ether are
added under stirring. The solution is taken to dryness until
a foamy orange solid is formed. The solid is washed twice with
n-hexane and then dried. Yield: 1298 mg (87%). Calc. for
C10H14Cl2MoN4O2: C 30.87, H 3.63, N 14.40. Found: C 30.59,
H 3.67, N 14.31%. IR (KBr, ν/cmϪ1): 3118m, 1603vs, 1283s,
943vs (νasym(Mo᎐O)), 906s (ν (Mo᎐O)), 902s, 713m, 572m. 1H
b/Å
c/Å
V/Å3
Z
µ/mmϪ1
λ/Å
T/K
No. reflections collected
No. independent reflections/(Rint
No. observed reflections (I > 2σI)
R1, wR2 indices (I > 2σI)
)
2519/0.039
2275
0.0243, 0.0585
0.0277, 0.0602
᎐
᎐
sym
R1, wR2 indices (all data)
NMR: see Table 2. 13C NMR (CD2Cl2, RT): δ 12.05 (CH3–pz),
13.96 (CH3–pz), 106.01 (4-C), 141.66 (3-C), 144.32 (5-C). 95Mo
NMR (CD2Cl2, RT): δ 156.8.
δ 73.05, 107.56, 132.75, 145.11. 95Mo NMR (CD2Cl2, RT):
δ 157.79.
MoO2Cl2(C9H12N4) 2. To a solution of MoO2Cl2(THF)2 (686
mg, 2 mmol) in THF (10 mL) 2,2-di(1-pyrazolyl)propane (350
mg, 2 mmol) is added at room temperature. The solution turns
yellow. After reacting for 15 min, it is filtered into another
Schlenk tube. Then the volume is reduced to ca. 2 mL by oil
pump vacuum. Diethyl ether is then added (10 mL) and the
formation of a fine white precipitate occurs. The remaining
solution is filtered off. After taking the residue to dryness a
yellowish oil is obtained. This oil is dissolved in CH3CN
(10 mL) and diethyl ether (10 mL) added, leading to the form-
ation of a white precipitate, which is then isolated by filtration
and drying of the filtrate. Yield: 393 mg (52.5%). Calc. for
C9H12Cl2MoN4O2: C 28.82, H 3.22, N 14.94. Found: C 29.17, H
2.96, N 14.80%. IR (KBr, ν/(cmϪ1): 3144s, 1510s, 1442s, 1308s,
943vs (νasym(Mo᎐O)), 916s (ν (Mo᎐O)), 902s, 713m, 572m. 1H
MoO2Cl2(C16H22N6) 5. To a solution of MoO2Cl2(THF)2
(314 mg, 0.918 mmol) in THF (10 mL), tris(3,5-dimethyl-1-
pyrazolyl)methane (277 mg, 0.918 mmol) is added at room
temperature. The colourless solution becomes orange and after
ca. 10 min a yellow precipitate forms. The mixture is allowed to
react for 15 min. The volume is reduced to ca. 3 mL and the
remaining solution filtered off. The precipitate is washed twice
with diethyl ether and taken to dryness. Yield: 421 mg (92%).
Calc. for C16H22Cl2MoN6O2: C 38.66, H 4.42, N 16.90. Found:
C 37.91, H 4.40, N 16.91%. IR (KBr, ν/cmϪ1): 3132s, 1564vs,
1457s, 1262s, 1046vs, 946vs (νasym(Mo᎐O)), 912s (ν (Mo᎐O)),
᎐
᎐
sym
705vs. 1H NMR: see Table 2. 95Mo NMR (CD2Cl2, RT): δ 87.4.
MoO2Cl(BC9H10N6) 6. To a solution of MoO2Cl2(THF)2
(820 mg, 2.39 mmol) in 10 mL of THF, potassium hydrotris-
(1-pyrazolyl)borate (602 mg, 2.39 mmol) is added at room
temperature and allowed to react for 90 min. The colourless
solution turns to orange. The volume is reduced to ca. 2 mL and
10 mL of diethyl ether were added, leading to the formation
of a yellow precipitate. The solution is filtered off and the pre-
cipitate taken to dryness, washed twice with diethyl ether and
recrystallized from CH2Cl2. Yield: 690 mg (77%). Calc. for
C9H10ClMoN6O2: C 28.69, H 2.65, N 22.30. Found: C 27.93, H
2.60, N 22.01%. IR (KBr,ν/cmϪ1): 3120 (N–H), 2485 (B–H),
935vs (νasym(Mo᎐O)), 905vs (ν (Mo᎐O)), 774vs, 604s. 1H
᎐
᎐
sym
NMR: see Table 2. 13C NMR (CD2Cl2, RT): δ 30.24, 108.8,
132.82, 150.25. 95Mo NMR (CD2Cl2, RT): δ 144. CI-MS: m/z
(%) = 358(0.34) [Mϩ Ϫ CH3; 95Mo, 35Cl], 197 (7.24) [Mϩ Ϫ
(CH3)2C(pz)2].
MoO2Cl2(C10H10N6) 3. To a solution of MoO2Cl2(THF)2
(765 mg, 2.23 mmol) in THF (10 mL) tris(1-pyrazolyl)methane
(477 mg, 2.23 mmol) is added at room temperature and allowed
to react for 30 min. The yellow solution obtained is then filtered
into another Schlenk and concentrated under oil pump vacuum
to ca. 2 mL. To this solution 10 mL of diethyl ether are added
with the subsequent formation of a slightly yellow precipitate;
the solution is filtered into another Schlenk and the precipitate
taken to dryness. To the solution more diethyl ether is added to
precipitate the complex. The solution is filtered off and the
precipitate taken to dryness and washed twice with diethyl
ether. Yield: 561 mg (61%). Calc. for C10H10Cl2MoN6O2: C
᎐
᎐
sym
NMR: see Table 2. 11B NMR (CD2Cl2, RT): δ Ϫ4.3. 95Mo NMR
(CD2Cl2, RT): δ 80.3.
Catalytic reactions with compounds 1–6 as catalysts
800 mg (7.3 mmol) cis-cyclooctene, 800 mg n-dibutyl ether
(internal standard), 1 mol% (73 µmol) 1–6 (as catalyst, added as
a solid) and 2 ml 5.5 M t-butyl hydroperoxide in n-decane are
added to a thermostated reaction vessel and stirred for 4 h at
55 ЊC. The course of the reaction is monitored by quantitative
GC analysis. Samples are taken every thirty minutes, diluted
with dichloromethane, and chilled in an ice-bath. For the
destruction of hydroperoxide and removal of water, a catalytic
amount of manganese dioxide and magnesium sulfate are
added. After the gas evolution ceases, the resulting slurry is
filtered over a filter equipped Pasteur pipette and the filtrate
injected in the GC column. The conversion of cyclooctene and
formation of cyclooctene oxide are calculated from a calibra-
tion curve (r2 = 0.999) recorded prior to the reaction course.
29.08,
H 2.44, N 20.34. Found: C 28.82, H 2.31, N
20.07%. IR (KBr, ν/cmϪ1): 3102s (N–H), 1409vs, 1067s, 953vs
(νasym(Mo᎐O)), 920s (ν (Mo᎐O)), 860s, 783vs, 603s. 1H NMR:
᎐
᎐
sym
see Table 2. 95Mo NMR (CD2Cl2, RT): δ 86.8.
MoO2Br2(C10H10N6) 4. To a solution of MoO2Br2(THF)2
(811 mg, 1.88 mmol) in 40 mL of THF at 70 ЊC tris(1-
pyrazolyl)methane (400 mg, 1.88 mmol) is added and the
mixture allowed to react for 15 min. The solution obtained
is filtered into another Schlenk and concentrated to ca. 2 mL.
n-Hexane (5 mL) is then added leading to the formation of a
bright yellow precipitate. This precipitate is taken to dryness,
washed three times with n-hexane and finally with diethyl ether
and taken to dryness again. Yield: 658 mg (70%). Calc. for
C10H10Br2MoN6O2: C 23.93, H 2.01, N 16.74. Found: C 23.77,
H 2.15, N 16.59%. IR (KBr, ν/cmϪ1): 3105m, 2947m, 1508s,
Structure determination of complex 2
Details of the X-ray experiment, data reduction, and final
structure refinement are summarized in Table 4. Crystals of
complex 2 were grown by slow evaporation of a saturated
1408vs, 1098s, 1064s, 948vs (νasym(Mo᎐O)), 918vs (ν (Mo᎐O)),
᎐
᎐
sym
1
774vs, 604s. H NMR: see Table 2. 13C NMR (CD2Cl2, RT):
1336 J. Chem. Soc., Dalton Trans., 2001, 1332–1337

View Article Online
7 I. A. Degnan, W. A. Herrmann and E. Herdtweck, Chem. Ber., 1990,
123, 1347; I. A. Degnan, J. Behm, M. R. Cook and W. A. Herrmann,
Inorg. Chem., 1991, 30, 2165.
8 E. I. Stiefel, C. Coucovanis and W. E. Edds, ACS Symp. Ser., 1993,
535; J. H. Enemark and C. G. Young, Adv. Inorg. Chem., 1993, 40, 1;
R. S. Pilato and E. I. Stiefel, in Inorganic Catalysis, ed. J. Reedijk,
Marcel Dekker, New York, 1993, p. 131.
9 S. Trofimenko, J. Am. Chem. Soc., 1970, 92, 5118.
10 D. L. Jameson and R. K. Castellano, Inorg. Synth. Proc., 1998, 32,
51.
11 D. L. Reger, S. J. Knox, M. F. Huff, A. L. Rheingold and B. S.
Haggerty, Inorg. Chem., 1991, 30, 1754.
12 S. Tsuji, D. C. Swenson and R. F. Jordan, Organometallics, 1999, 18,
4758.
13 D. L. Reger, J. A. Lindemann and L. Lebioda, Inorg. Chem., 1988,
27, 3923.
14 D. L. Reger and S. S. Mason, Inorg. Chem., 1993, 32, 5216.
15 S. A. Roberts, C. G. Young, C. A. Kipke, W. E. Cleland, Jr.,
K. Yamanouchi, M. D. Carducci and J. H. Enemark, Inorg. Chem.,
1990, 29, 3650.
16 J. Sündermeyer, J. Putterlik and H. Pritzkow, Chem. Ber., 1993, 126,
289.
17 M. Onishi, K. Ikemoto, K. Hiraki and R. Koga, Bull. Chem. Soc.
Jpn., 1993, 66, 1849.
18 F. E. Kühn, E. Herdtweck, J. J. Haider, W. A. Herrmann, I. S.
Gonçalves, A. D. Lopes and C. C. Romão, J. Organomet. Chem.,
1999, 583, 3.
19 M. Minelli, C. J. Young and J. H. Enemark, Inorg. Chem., 1985, 24,
1111.
20 F. E. Kühn, A. M. Santos, A. D. Lopes, I. S. Gonçalves,
E. Herdtweck and C. C. Romão, J. Mol. Catal., 2001, 15, 43.
21 M. K. Trost and R. G. Bergmann, Organometallics, 1991, 10, 1172.
22 W. A. Herrmann, G. M. Lobmaier, Th. Priermeier, M. Mattner and
B. Scharbert, J. Mol. Catal., 1997, 117, 455; J. J. Haider, Ph.D.
Thesis, Technische Universität München, 1998.
23 W. R. Thiel, M. Angstl and N. Hansen, J. Mol. Catal., 1995, 103, 5.
24 F. E. Kühn and W. A. Herrmann, Struct. Bonding (Berlin), 2000,
596, 177; C. C. Romão, F. E. Kühn and W. A. Herrmann, Chem.
Rev., 1997, 97, 3197.
25 F. E. Kühn, A. M. Santos, P. W. Roesky, E. Herdtweck, W. Scherer,
P. Gisdakis, I. V. Yudanov, C. Di Valentin and N. Rösch, Chem. Eur.
J., 1999, 3603; M. C. A. van Vilet, I. W. C. Arends and R. A.
Sheldon, Chem. Commun., 1999, 821; W. D. Wang and J. H.
Espenson, J. Am. Chem. Soc., 1998, 120, 11335; J. Rudolph, K. L.
Reddy, J. P. Chiang and K. B. Sharpless, J. Am. Chem. Soc., 1997,
119, 6189.
26 (a) Collect, Data collection software, R. Hooft, Nonius B. V., Delft,
1998; (b) Z. Otwinowski and W. Minor, Methods Enzymol., 1997,
276, 307; (c) A. Altomare, G. Cascarano, C. Giacovazzo,
A. Guagliardi, M. C. Burla, G. Polidori and M. Camalli, SIR 92,
J. Appl. Crystallogr., 1994, 27, 435; (d) G. M. Sheldrick, SHELXL
97, University of Göttingen, 1997; (e) International Tables for
Crystallography, ed. A. J. C. Wilson, Kluwer Academic Publishers,
Dordrecht, 1992, vol. C, Tables 6.1.1.4 (pp. 500–502), 4.2.6.8
(pp. 219–222), and 4.2.4.2 (pp. 193–199); ( f ) A. L. Spek, PLATON,
A Multipurpose Crystallographic Tool, Utrecht University, 1999;
(g) 3d Search and research Using the Cambridge Structural
Database, F. H. Allen and O. Kennard, Chem. Des. Automat. News,
1993, 8, 1, 31. The refcodes are as follows: for the MoO2Cl2N2
core, HAGDOI, HAGDUO, PHNOMO, TEPKII, and VOCTAI;
for the organic ligand C9H12N4; FEWYOV, KOYGEK, SIKYOA,
TEMWIR, VONMIU, ZOFHIL, and ZOFHOR.
solution of it in a mixture of diethyl ether, acetonitrile and
dichloromethane. Preliminary examination and data collection
were carried out on a Nonius Kappa CCD system area-detector
diffractometer at the window of a rotating anode generator
(NONIUS FR591; 50 kV; 60 mA; 3.0 kW) using graphite
monochromated Mo-Kα radiation. Data collection was
controlled by the Collect software package.26a Collected images
were processed using Denzo.26b The unit cell parameters were
obtained by full-matrix least-squares refinements of 42799
reflections.26b The structure was solved by a combination of
direct methods and Fourier-difference syntheses.26c All non-
hydrogen atoms of the asymmetric unit were refined with
anisotropic thermal displacement parameters. All hydrogen
atoms were found in the Fourier difference map and refined
freely with individual isotropic thermal displacement param-
eters. Full-matrix least-squares refinements were carried out by
minimizing Σw(Fo2 Ϫ Fc2)2 with SHELXL 97.26d Neutral atom
scattering factors for all atoms and anomalous dispersion
corrections for the non-hydrogen atoms were taken from ref.
26(e). All other calculations (including ORTEP graphics)
were done with the program PLATON.26f Calculations were
performed on a PC workstation (Intel Pentium II) running
LINUX.
CCDC reference number 154166.
See http://www.rsc.org/suppdata/dt/b0/b009202i/ for crystal-
lographic data in CIF or other electronic format.
Acknowledgements
We are greatly indebted to Professor Dr W. A. Herrmann for
continuous and generous support. This work was supported by
the DAAD (Acções Integradas Program), the BMFT/JNICT
Protokoll, and the Fonds der Chemischen Industrie. AMS
thanks the Bayerische Forschungsstiftung for a Ph.D. grant.
References
1 (a) F. E. Kühn, A. D. Lopes, A. M. Santos, E. Herdtweck,
J. J. Haider, C. C. Romão and A. G. Santos, J. Mol. Cat. A, 2000,
151, 14; (b) N. Indictor and W. F. Brill, J. Org. Chem., 1965, 29,
2075; (c) D. D. Agarwal, Inorg. Chem., 1986, 25, 65; (d) E. P. Talsi,
K. V. Shalyaev and K. I. Zamaraev, J. Mol. Catal., 1993, 83,
347; (e) R. H. Holm, Chem. Rev., 1987, 87, 1401; ( f ) W. R.
Thiel and T. Priermeier, Angew. Chem., Int. Ed. Engl., 1995, 34,
1737.
2 (a) S. Trofimenko, J. Am. Chem. Soc., 1967, 89, 6288; (b)
S. Trofimenko, J. Am. Chem. Soc., 1967, 89, 3170.
3 M. A. Moss and C. J. J. Jones, J. Chem. Soc., Dalton Trans., 1990,
581 (and references therein).
4 M. V. R. Steiner and J. Takats, Inorg. Chem., 1982, 21, 4056; V. D.
Moffat, M. V. R. Steiner and J. Takats, Inorg. Chim. Acta, 1987, 139,
75.
5 D. L. Reger, C. A. Swift and L. Lebioda, Inorg. Chem., 1988, 27,
3923; D. L. Reger, M. F. Huff, A. Rheingold and B. S. Haggerty,
J. Am. Chem. Soc., 1992, 114, 579.
6 M. J. Abrams, A. Davidson and A. G. Jones, Inorg. Chim. Acta,
1984, 82, 125; (b) J. A. Thomas and A. Davidson, Inorg. Chim. Acta,
1991, 190, 231.
27 S. Julia, J. M. Del Mazo and L. Avila, Org. Prep. Proced., 1984, 16,
299.
J. Chem. Soc., Dalton Trans., 2001, 1332–1337
1337