Oxygen-transfer reactions of molybdenum- and tungstendioxo complexes containing η2-pyrazolate ligands

Article abstract of DOI:10.1002/adsc.200404265

Dioxomolybdenum and -tungsten compounds containing sterically demanding pyrazolate ligands have been synthesised by treatment of dioxometal halides with the potassium salts of 3,5-di-tert-butylpyrazole (t-Bu₂pzH) and 3,5-di-terf-butyl-4-bromopyrazole (t-Bu₂-4-BrpzH). The products [MoO₂Cl(η²-t-Bu₂pz)] (1), [MoO ₂(η²-t-Bu₂pz)₂] (2), [MoO ₂(η²-t-Bu₂-4-Brpz)₂] (3), [WO₂(η²-t-Bu₂pz)₂] (4) and [WO₂(η²-t-Bu₂-4-Brpz)₂] (5) were characterised by spectroscopic techniques. The X-ray structure of complex 3 reveals a distorted trigonal prismatic geometry with two η²-co- ordinated pyrazolate ligands. These high-valent compounds participate in oxygen-transfer reactions and catalyse the oxidation of PPh₃ with dimethyl sulphoxide. UV/VIS measurements of the oxo-transfer reactions and the kinetics of the catalytic process are described. By the reaction of 2 with three equivalents of PEt₃ or treatment of [MoOCl₂-(PMe ₃)₂] with two equivalents of f-Bu₂pzK mononuclear mono-oxo compounds of the type [MoO(t-Bu₂-pz) ₂(PR₃)₂] (R = Et 6, R = Me 7) were obtained and characterised by X-ray diffraction analyses. This points to biologically relevant mononuclear Mo(IV) intermediates in the catalytic process with this type of complex.

Full text of DOI:10.1002/adsc.200404265

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FULL PAPERS
Oxygen-Transfer Reactions of Molybdenum- and Tungstendioxo
2
Complexes Containing h -Pyrazolate Ligands
Kerstin Most, Jens Hoßbach, Denis Vidovi c´ , Jçrg Magull,
Nadia C. Mçsch-Zanetti*
Institut fꢀr Anorganische Chemie, Georg-August-Universitꢁt Gçttingen, Tammannstraße 4, 37077 Gçttingen, Germany
Fax: (þ49)-551-39-3373; e-mail: nmoesch@gwdg.de
Received: August 9, 2004; Accepted: November 22, 2004
th
Dedicated to Richard R. Schrock on the occasion of his 60 birthday
Supporting Information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: Dioxomolybdenum and -tungsten com- with dimethyl sulphoxide. UV/VIS measurements of
pounds containing sterically demanding pyrazolate the oxo-transfer reactions and the kinetics of the cat-
ligands have been synthesised by treatment of dioxo- alytic process are described. By the reaction of 2 with
metal halides with the potassium salts of 3,5-di- three equivalents of PEt or treatment of [MoOCl -
3
2
tert-butylpyrazole (t-Bu pzH) and 3,5-di-tert-butyl- (PMe ) ] with two equivalents of t-Bu pzK mononu-
2
3
2
2
4
-bromopyrazole (t-Bu -4-BrpzH). The products clear mono-oxo compounds of the type [MoO(t-Bu2-
2
2
2
[
[
MoO Cl(h -t-Bu pz)] (1), [MoO (h -t-Bu pz) ] (2), pz) (PR ) ] (R¼Et 6, R¼Me 7) were obtained and
2
2
2
2
2
2
3 2
2
2
MoO (h -t-Bu -4-Brpz) ] (3), [WO (h -t-Bu pz) ] (4) characterised by X-ray diffraction analyses. This
2
2
2
2
2
2
2
and [WO (h -t-Bu -4-Brpz) ] (5) were characterised points to biologically relevant mononuclear Mo(IV)
2
2
2
by spectroscopic techniques. The X-ray structure of intermediates in the catalytic process with this type
complex 3 reveals a distorted trigonal prismatic ge- of complex.
2
ometry with two h -co-ordinated pyrazolate ligands.
These high-valent compounds participate in oxygen- Keywords: homogeneous catalysis; molybdenum; N,N
transfer reactions and catalyse the oxidation of PPh3 ligands; oxo-transfer reactions; pyrazolates; tungsten
Introduction
sulphur-based ligand systems for molybdenum-cata-
lysed oxo-transfer reactions came from Holm and co-
[
14–16]
Molybdenum- and tungsten-catalysed oxygen-transfer workers.
Thereafter, many N,O-, N,S- and S,S-
[17,18]
reactions have attracted considerable interest in recent based ligands systems were investigated.
Signifi-
cantly less studied were anionic N,N-donors in spite of
Molybdenum is foundin a class of enzymes that are com- the fact that amido ligands have had a great impact on
[
1–3]
years due to their relevance in biological processes.
[
19–22]
monly referred to as mononuclear molybdoenzymes or early transition metal chemistry.
oxotransferases, that catalyse oxygen atom transfer to oughly investigated tris(pyrazolyl)borate N-donor li-
Besides the thor-
[
23,24]
and from a substrate. Mononuclear tungsten enzymes gands by Enemark and co-workers,
producing a va-
[4]
are also known but, in contrast to the well developed riety of interesting Mo(IV), Mo(V) and Mo(VI) com-
molybdenum chemistry, tungsten-mediated atom trans- plexes, only a few other N,N-donor ligand systems
[
5–8]
[25,26]
fer reactions have been significantly less studied.
In were studied.
recent years, crystal structures of several molybdo- and
Pyrazolate ligands in general have been widely used to
[
9–13]
[27–30]
tungstoenzymes were determined.
In all cases, the synthesise a variety of transition metal complexes.
1
1
reaction centre contains a mononuclear metal centre The bridging co-ordination mode m-h :h of the pyrazo-
co-ordinated by one or two oxygen as well as one or late anion between two metal atoms is prevalent, lead-
two pterin ligands, whose dithiolene functionality co-or- ing mainly to dimeric or oligomeric structures. By intro-
dinates to the metal atom. Model chemistry has been fo- ducing sterically demanding groups in the 3- and 5-posi-
2
þ
cussed on the use of MO2 cores and on variation of the tions, mononuclear complexes of early transition metals
VI
2
co-ligands LL’ ([LL’M O ], M¼Mo, W) to influence were accessible that feature the unusual h -bonding
2
[
31–35]
2
the steric as well as the redox properties of the oxy- mode.
Until recently, h -co-ordination was limited
[
36,37]
gen-transfer reaction. Pioneering research designing to d-block metals with empty d-shells. We
and oth-
Adv. Synth. Catal. 2005, 347, 463–472
DOI: 10.1002/adsc.200404265
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[
38,39]
ers
have now demonstrated that reduced species are chloride starting material with potassium salts of the li-
2
accessible and that h -co-ordination is retained.
gands as shown in Scheme 1. Reaction of [MoO Cl ]
2 2
The lack of nitrogen-based systems as well as our gen- with one or two equivalents of potassium 3,5-di-tert-bu-
eral interest in pyrazolate chemistry prompted us to in- tylpyrazolate (t-Bu pzK) in toluene at room tempera-
2
2
þ
2
2
vestigate the use of such ligands to stabilise MO2 cores ture afforded [MoO Cl(h -t-Bu pz)] (1) and [MoO (h -
2
2
2
(
M¼Mo, W) and the reactivity of such compounds in t-Bu pz) ] (2) in good yields. Treatment of [MoO Cl ]
2
2
2
2
oxygen-transfer reactions. We recently communicated with two equivalents of potassium 3,5-di-tert-butyl-4-
the syntheses and catalytic activity of molybdenum com- bromopyrazolate (t-Bu -4-BrpzK) in toluene led to
2
2
[40]
2
pounds containing h -t-Bu pz ligands. We report here [MoO (h -t-Bu -4-Brpz) ] (3). The tungsten complexes
2
2
2
2
2
2
a more detailed investigation of such complexes and [WO (h -t-Bu pz) ] (4) and [WO (h -t-Bu -4-Brpz) ]
2
2
2
2
2
2
their oxygen-transfer properties, which includes analo- (5) were synthesised using the dimethoxyethane adduct
gous tungsten(VI) complexes as well as the syntheses [WO Cl (dme)], a more soluble starting material than
2
2
and crystallographic characterisation of a mononuclear the polymeric [WO Cl ].
2
2
mono-oxomolybdenum(IV) complex.
Compounds 1 and 2 were purified by sublimation at
20–1308C under reduced pressure to give pale yellow
and colourless 2, pointing to a thermal stability of these
1
1
Results
complexes, whereas 3–5 were purified by recrystallisa-
tion from concentrated toluene solutions. Attempts at
sublimation led to decomposition. The crystalline com-
plexes 1–5 are readily soluble in common organic sol-
vents such as toluene, tetrahydrofuran and pentane.
Synthesis of the Ligands and Complexes
The sterically demanding pyrazole ligand containing They are stable in dry air, whereas moisture leads to im-
tert-butyl groups in the 3- and 5-positions (t-Bu pzH) mediate decomposition forming blue intractable solids.
2
1
and its potassium salt (t-Bu pzK) were prepared accord-
The H NMR spectra of compounds 1–5 in toluene-d8
2
[
41,42]
ing to literature procedures.
The analogous pyra- between þ20 and ꢀ808C show for all five complexes
zole substituted by a bromine in the 4-position, 3,5-di- one set of resonances each for symmetrically co-ordinat-
tert-butyl-4-bromopyrazole (t-Bu -4-BrpzH), was pre- ed ligands (d¼1.12, 6.09 ppm for 1; 1.20, 6.29 ppm for 2;
2
pared according to an analogous procedure previously 1.30 ppm for 3; 1.18, 6.44 ppm for 4; and 1.28 ppm for 5).
[
43]
13
reported for 3,5-dimethyl-4-bromopyrazole.
Reac- The C NMR spectra of the complexes show the signal
tion of t-Bu pzH with one equivalent of AgNO in etha- for the a-C, b-C, C(CH ) and C(CH ) (d¼30.05, 32.09,
2
3
3
3
3 3
nol led to an ill-defined silver salt of the ligand, which 112.46, 160.94 ppm for 1; 30.20, 32.37, 107.08,
was not further characterised. Treatment of this salt 162.30 ppm for 2; 28.35, 33.47, 98.51, 157.47 ppm for 3;
with bromine in chloroform gave t-Bu -4-BrpzH in and 30.05, 32.44, 108.57, 162.15 ppm for 4). On mass
2
þ
high yields [Eq.(1)].
spectrometry, the [M ] species with correct isotope pat-
tern are observed. The IR spectra display two strong
ꢀ
1
ꢀ1
nM¼O bands at 971 and 941 cm for 1, 951 and 922 cm
ð1Þ
3,5-Di-tert-butyl-4-bromopyrazole was isolated as a
slightly yellow solid, which is soluble in common organic
solvents such as toluene, tetrahydrofuran and pentane.
1
The H NMR spectrum shows one resonance for protons
of the tert-butyl groups (d¼1.34 ppm) and one broad
resonance for the NꢀH proton (d¼10.3 ppm).
Syntheses of Molybdenum(VI) and Tungsten(VI)
Complexes
Dioxomolybdenum(VI) and dioxotungsten(VI) com-
pounds were obtained by reacting the appropriate metal Scheme 1.
464
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2
Mo-, W-Dioxo Complexes Containing h -Pyrazolate Ligands
ꢀ
1
ꢀ1
[44]
1
for 2, 985 and 951 cm for 3, and 996 and 960 cm for 4, field. In contrast to compounds 1–5, the H NMR
characteristic for the symmetric and asymmetric vibra- spectra of 6 and 7 show resonances for two different t-
2
þ
tional modes, respectively, of the cis-[MO2] (M¼ Bu groups (d¼1.40 and 1.60 ppm for 6, 1.49 and
Mo, W) fragment. All compounds are monomeric in sol- 1.60 ppm for 7) and one for the pyrazolate ring proton
ution as confirmed by molecular mass determination by (d¼6.05 ppm for 6 and 6.00 ppm for 7) consistent with
3
1
vapour pressure osmometry in toluene. Solid state struc- the solid state structure. The P NMR spectrum of 7
tures of compounds 2 and 4 were determined by X-ray shows one resonance at d¼0.95 ppm. Compound 7 has
single crystal analyses (vide infra).
also been characterized by electron impact (EI) mass
þ
spectrometry and the [M – PMe ] species at m/z¼
3
5
48 with correct isotopic distribution pattern has been
Syntheses of Molybdenum(IV) Complexes
identified. An attempted X-ray diffraction analysis of
compound 7 confirms the connectivity and suggests an
By reacting three equivalents of PEt with 1 at 08C in isostructural relationship to 6but the dataset does not al-
3
pentane solution an immediate colour change to brown low a discussion of bond lengths and angles. Complex 7
was observed. After evaporation of the solvent to a min- reacts with excess DMSO to give the dioxo compound 2
1
imum amount and storage at ꢀ208C few green crystals which was confirmed spectroscopically. The H NMR
appeared in the solution along with an unidentified spectrum of a sample of 7 in benzene-d treated with ex-
6
brown precipitate. Single crystal X-ray analysis (vide in- cess DMSO-d shows the resonances for 2 and OPMe
6
3
3
1
fra) proved the green crystals to be the monomeric mo- and the only resonance found in the P NMR spectrum
lybdenum(IV) mono-oxo compound [MoO(t-Bu pz) - is that for OPMe . Electron impact mass spectrometry
2
2
3
(
PEt ) ] (6) (Scheme 2). The occurrence of a monomeric confirms the formation of 2.
3 2
Mo(IV)¼O species rather than a m-oxo-bridged
Mo(V) O species is of significant interest for the cur-
2
3
rent study as discussed below. In addition, only few mon- Molecular Structures of 3 and 6
omeric Mo(IV)¼O compounds have been crystallo-
2
graphically characterised and even fewer h -pyrazolate Figure 1 shows the molecular structure of compound 3.
complexes of reduced metal centres are known. Al- For selected bond lengths and angles as well as for the
though several attempts were made by varying the reac- crystallographic data and structure refinement see Ta-
tion conditions as well as the solvent, the yield could not bles 1 and 2 in the Supporting Information.
be increased, which is mainly due to the compoundꢃs
Compound 3 crystallises in the orthorhombic space
sensitivity towards oxygen and moisture but also due group Pna2 with one molecule in the asymmetric
1
to its high solubility in most organic solvents. Better unit. The molybdenum atom is surrounded by two oxy-
yields were obtained by the direct reaction of gen and four nitrogen atoms. The geometry is best de-
[
MoOCl (PMe ) ] with two equivalents of t-Bu pzK scribed as a distorted trigonal prism rather than as
2
3
3
2
leading to the analogous complex [MoO(t-Bu pz) - an octahedron, since the largest angles between the
2
2
(
(
PMe ) ] (7) with co-ordinated PMe rather than PEt
Schema 2).
Both complexes are diamagnetic green solids that are N(3)¼140.35(15)8. Both pyrazolate ligands with bond
oxygen atoms and any other co-ordinating atom
3
2
3
3
are O(1)ꢀMo(1)ꢀN(2)¼140.07(14)8 and O(2)ꢀMo(1)ꢀ
readily soluble in common organic solvents. Spin-pair- lengths differences of 0.164 ꢄ for the N(1),N(2)-ligand
ing in mono-oxomolybdenum(IV) compounds is a com-
mon feature, as the MoꢀO bond dominates the ligand
2
Figure 1. Molecular structure of [MoO (h -t-Bu -4-Brpz) ]
2
2
2
Scheme 2.
(3).
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[
31,36,50]
plexes,
suggesting the possibility of p-donor inter-
actions between the two ligands and d-orbitals of the
metal.
2
In particular, the titanium(III) compound [Ti(h -t-
Bu pz) Cl(thf) ] displays a very similar overall structure
2
1
2
2
2
Figure 2. Molecular structure of [MoO(h -t-Bu pz)(h -t-Bu
2
2
with the two pz ligands and the titanium atom in one
plane and the two tetrahydrofuran molecules co-ordi-
pz)(PEt ) ] (6).
3
2
[
36]
nated to Ti above and below this plane. However, in
1
2
and 0.174 ꢄ for the N(3),N(4)-ligandcan be described as the d system both pz ligands co-ordinate h , although
2
ꢅslippedꢃ h -co-ordinated, a denotation previously ap- the metal-nitrogen bonds are in a similar range arguing
[
32]
plied for this kind of asymmetric bonding.
The against steric factors for the opening of one metal-ligand
Mo¼O bond lengths of 1.686(3) and 1.685(3) ꢄ are in bond in compound 6. Theoretical calculations on the ti-
[
40]
the same range to those found in 1. The dihedral an- tanium and molybdenum compounds confirming these
gles between the NꢀMoꢀN plane and NꢀCꢀCꢀCꢀN electronic features are currently being investigated.
best plane are 17.78 for the N(1),N(2)-ligand and 9.98
for the N(3),N(4)-ligand.
The solid state structure of complex 6 is shown in Fig- Molecular Structure of t-Bu -4-BrpzH
2
ure 2. Complex 6 crystallises in the monoclinic space
group P2 /c with two independent molecules in the The molecular structures of the complexes were com-
1
asymmetric unit. The two molecules have identical plemented by determination of the structure of the bro-
structural parameters within the standard deviation mine-substituted ligand t-Bu -4-BrpzH. Comparison to
2
(
see the table of selected bond lengths in the Supporting that of previouslycharacterised t-Bu pzH shows the bro-
2
[
51,52]
Information), so that only molecule 1 will be discussed. mine atom exerting little structural influence.
How-
The molybdenum atom is surrounded by four nitrogen ever, whereas the tert-butyl groups of t-Bu pzH are rota-
2
and two phosphorus atoms as well as one oxygen atom. tionally disordered, the additional bromine atom in t-
If the centre of the nitrogen-nitrogen bond in the Bu -4-BrpzH seems to prevent it as no disorder is ob-
2
N(1),N(2)-pyrazolate ligand is considered as a mono- served. The molecular structure is shown in Figure 3
dentate donor, the geometry about the molybdenum along with selected bond lengths and angles.
atom is best described as trigonal bipyramidal with the
two phosphorus atoms being in the apical positions
[
P(1)ꢀMo(1)–P(2)¼172.09(3)8]. Similar to 3, the
N(1),N(2)-ligand with a bond length difference of
0
2
.154 ꢄ
[Mo(1)ꢀN(1)¼2.337(3),
Mo(1)ꢀN(2)¼
2
.183(3) ꢄ] can be described as ꢅslippedꢃ h -co-ordinate.
1
In contrast, the N(3),N(4)-ligand is h -co-ordinate, since
the Mo(1)ꢀN(4) distance of 2.594 ꢄ is too long to as-
sume a bond. The Mo¼O distance of Mo1ꢀO1¼
1
.656(2) ꢄ is in the normal range for this type of com-
[44,45]
plex.
The trigonal bipyramidal geometry contrasts
[
45]
with that of [MoOCl (PMe ) ]
as well as that of
2
3 3
[
46]
[
MoO(acac) (PMe )] where distorted octahedral ge-
2 3
Figure 3. Molecular structure of t-Bu -4-BrpzH. Selected
2
ometries are observed. Only few mononuclear
bond lengths [ꢄ] and angles [8]: Br(1)ꢀC(2) 1.877(6),
Mo(IV)¼O compounds with co-ordinated phosphine li-
Br(2)ꢀC(13) 1.901(6), N(1)ꢀC(1) 1.338(8), N(3)ꢀC(12)
[
46–49]
gands have been structurally characterised.
The
1
.362(8),
C(1)ꢀC(2)
1.392(8),
C(2)ꢀC(3)
1.410(8),
ꢀ
two pz ligands in 6 are approximately coplanar with
ꢀ
ꢀ
N(2) C(3) 1.353(8), N(1) N(2) 1.357(7), N(3) N(4)
each other as well as with the metal atom. This structural 1.354(7), C(1)ꢀN(1)ꢀN(2) 110.4(5), C(1)ꢀC(2)ꢀC(3)
2
feature is found in several h -co-ordinate azolate com- 107.0(5).
466
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Mo-, W-Dioxo Complexes Containing h -Pyrazolate Ligands
Oxo-Transfer Reactions
The reaction of [MoO (t-Bu pz) ] (2) with one equiva-
2
2
2
lent of triphenylphosphine in toluene-d at 808C was
8
31
[40]
monitored by P NMR spectroscopy. Over the course
of 3 hours, the reaction solution turned gradually brown
and remained clear throughout the reaction time. The
only two resonances observed were those for PPh and
3
OPPh . The phosphinewas oxidised by the molybdenum
3
compound, but the reaction did not proceed after 80%
conversion. In addition, oxo-transfer from 2 to triphe-
nylphosphine was monitored by UV/VIS spectropho-
tometry as shown in Figure 4. In the UV/VIS reactions Figure 6. Reduction of 2 in toluene at 808C. Spectra recorded
the ratio between Mo:PPh was 1:50. All reaction solu- from t¼260 to 950 minutes.
3
tions turned gradually brown and remained clear
throughout the reaction time. The maximum at 450 nm
increases for the 2.6 hours as the reaction proceeds (Fig- tion. However, the overlay of the two processes prevent-
ure 5). Thereafter, the maximum at 450 decreases and ed the analysis of the kinetics.
the one at 753 nm increases (Figure 6). For the first proc-
ess, a clean isosbestic point is found at 345 nm whereas
for the second process isosbestic points at 334, 422 and Catalysis of the Oxo-Transfer
640 nm are found. The chosen ratio between Mo:PPh3
of 1:50 would allow a first order treatment of the reac- The reduction of complex 2 by oxo-transfer can be cou-
pled to its reoxidation by the reaction with dimethyl
sulphoxide leading to catalytic transfers. Ten equiva-
lents of P(p-C H F) in DMSO in the presence of 2 are
6
4
3
completely converted to OP(p-C H F) . Reaction solu-
6
4
3
tions remain colourless during the entire catalysis. The
1
9
progress of the reaction was monitored by F NMR
spectroscopy at four different temperatures. NMR spec-
tra show the decrease of the resonance for the fluorine in
P(p-C H F) and the increase of that in OP(p-C H F)
3
6
4
3
6
4
(
Figure 7). No other resonances were detected. DMSO
and P(p-C H F) do not react under these conditions
6
4
3
without catalyst.
Because DMSO was in excess over P(p-C H F) and 2,
6
4
3
the reaction should be first-order in the concentration of
the phosphine. Phosphine oxidation is the rate-limiting
step, so that reoxidation of the molybdenum complex
is probably fast and its concentration may be assumed
constant. The rate law shown in Eq. (2) or (3) is there-
fore applicable and plots of ln({c[P(p-C H F) ]þ
Figure 4. Reduction of 2 in toluene at 808C. Thin and thick
lines indicate two processes at different times (thin lines first
process, thick lines second process).
6
4
3
c[OP(p-C H F) ]}/c{c[P(p-C H F) ] vs. t should give lin-
6
4
3
6
4
3
ear relationships whose slopes are consistent with k div-
ided by the concentration of the metal complex.
ð2Þ
ð3Þ
The catalytic reactions with compound 2 gave linear re-
lationships over the entire period. Values for k·c([MoO2
(
t-Bu pz) ])¼k determined are given in Table 1.
2
2
cat
Figure 5. Reduction of 2 in toluene at 808C. Spectra recorded
from t¼0 to 160 minutes.
The Eyring plot (ln(h·k /k ·T)·R vs. ꢀ1/T (h¼
cat
B
Planckꢃs constant, k ¼Boltzmann constant, R gas con-
B
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Discussion
Structural Aspects
Several crystal structures of DMSO reductase from
Rhodobacter sphaeroides or Rhodobacter capsulatus
have revealed considerable variability in the structure
of the active site, so that mechanistic aspects are still un-
[
10–12]
clear.
In this regard, the position of the pterin-sul-
[
11,12,53]
phur atoms is controversially discussed.
However,
a recent high resolution X-ray structure of DMSO re-
ductase (1.3 ꢄ) suggests that the co-ordination geome-
try of the oxidised enzyme seems to be best described
as distorted trigonal prismatic with four sulphur and
1
9
Figure 7. F NMR spectra recorded at T¼333 K in toluene
ꢀ
4
initially containing 3.9·10
mol/L P(p-C H F) and 3.9·
6 4 3
ꢀ
5
10
mol/L [MoO (t-Bu pz) ], every 15 minutes.
2 2 2
[11]
two oxygen atoms co-ordinated to the metal. This
structural situation should be compared with that of
compound [MoO (t-Bu -4-Brpz) ] (3). Although the
2
2
2
co-ordinating atoms differ from those in the natural sys-
tem, the co-ordination geometry of 3 is comparable. The
distorted trigonal prism approaches the situation in the
structure of DMSO reductase. In both geometries the
oxo ligands are not trans to any sulphur or nitrogen li-
gand, respectively. This situation is different from most
other model compounds for this type of enzyme as
they usually provoke distorted octahedral geometries
[
54]
with ligands trans to the oxo groups. In the pyrazolate
complexes, the small bite angle of the ligand prevents an
Figure 8. Typical plot of ln(A /A) vs. time, shown for T¼353
0
K {A ¼c[P(p-C H F) ]þc[OP(p-C H F) ], A¼c[P(p-C H - octahedral geometry. Therefore, the pyrazolate com-
0
6
4
3
6
4
3
6
4
F) ]}.
3
plex represents a rare example of a co-ordinational
model for DMSO reductase.
stant) gives a linear relationship and its slope and axis in-
tercept lead to activation enthalpy and entropy of Oxo-Transfer Reactivity
#
#
DH ¼77.7 kJ/mol and DS ¼ ꢀ81 J/K·mol.
The analogous reaction of the tungsten compound Oxo-transfer is accompanied by the formation of a m-
[
WO (t-Bu pz) ] with P(p-C H F) in DMSO at 808C oxo dimer as shown in Scheme 3, unless it is prevented
2 2 2 6 4 3
led also to the oxidation of the phosphine, although sig- by sterically demanding ligands as is found in nature.
nificantly slower than its molybdenum analogue. In typ- Dimerisations are found to be reversible and irrever-
[
2]
ical experiments containing c{[WO (t-Bu pz) ]}¼3.9· sible. Reversible reactions performed with a molybde-
2
2
2
ꢀ
4
ꢀ3
10
M and c[P(C H F) ]¼3.9·10 M values for k
num to phosphine ratio of 1:1 lead to complete oxida-
and 4.92 M s , respectively. tion of the phosphine, whereas irreversible reactions
6
4
3
cat
ꢀ
4
ꢀ1
ꢀ1 ꢀ1
and k are 1.92·10
s
In the absence of DMSO, no oxo-transfer reaction oc- lead only to 50% conversion. Therefore, to answer the
curred from [WO (t-Bu pz) ] to P(C H F) at up to question whether a m-oxo compound is formed as an in-
2
2
2
6
4
3
808C.
termediate, ultimately the isolation and structural char-
acterisation of the reduced species is necessary.
We were able to isolate and structurally characterise
the reduced compound [MoO(t-Bu pz) (PEt ) ] (6)
2
2
3 2
Table 1. Rate constants kcat and k for the oxidation of P(p-C6 which proved to be mononuclear. This strongly suggests
H F) with DMSO catalysed by [MoO (t-Bu pz) ] at differ-
4
3
2
2
2
ent temperatures.
ꢀ
1
ꢀ1
ꢀ1
T [K]
kcat [s
]
k [s ·M ]
ꢀ
ꢀ
ꢀ
ꢀ
4
4
4
3
223
233
243
253
1.09·10
3.05·10
6.40·10
1.44·10
4.87
7.82
16.41
36.92
Scheme 3.
468
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2
Mo-, W-Dioxo Complexes Containing h -Pyrazolate Ligands
the occurrence of a mononuclear intermediate in oxo-
transfer reactions of this system. However, kinetic stud-
ies employing low as well as high concentrations of phos-
phine vs. molybdenum point to two different pathways.
In oxo-transfer reactions employing a 1:1 ratio of 2 and
PR 80% conversion to OPR is found which contrasts
3
3
the theoretical 33% in the case where [MoO(t-Bu pz) -
2
2
Scheme 4. Catalytic oxo-transfer showing the presumed inter-
mediates.
(
PR ) ] represents the only species formed. We believe
3 2
that the low phosphine concentration leads to competi-
tive formation of the m-oxo dimer, which is presumably
unstable under the employed reaction conditions plex. The isolation of such an OPR adduct has recently
3
[57]
(
808C). Therefore, several attempts to isolate a m-oxo been reported in a trispyrazolylborate complex. Un-
compound failed. At high concentrations of phosphine der excess DMSO conditions the phosphine oxide is
excess PPh vs. molybdenum), two stepwise processes substituted forming a sulphoxide complex that cannot
(
3
are involved both showing clean isosbestic points in be isolated as it converts quickly to 2.
the UV/VIS spectra. Whereas in the first process a spe-
In comparison to molybdenum, oxo-transfer reactions
cies is formed showing a maximum at 450 nm, it disap- employing tungsten compounds are far less investigat-
[
5,7,17,58–60]
pears in the second process. This points to the reduction ed.
of [MoO (t-Bu pz) ] to [MoO(t-Bu pz) (PPh ) ] in the or do not occur at all, which is certainly due to the higher
Conversions are usually significantly slower
2
2
2
2
2
3 2
first process, but under the reaction conditions (excess redox potential found in the higher homologue. It is
phosphine) further reduction occurs to an as yet un- therefore of interests that the analogous tungsten
known molybdenum compound. For this reason, rate compound [WO (t-Bu pz) ] catalyses the oxidation of
2
2
2
laws could not be determined. The kinetic investigation P(C H F) to OP(C H F) albeit much slower than the
6
4
3
6
4
3,
together with the isolation and crystallographic charac- Mo analogue. This represents a rare example of a nitro-
terisation of [MoO(t-Bu pz) (PEt ) ] suggest that oxo- gen-based tungsten system that catalyses the oxidation
2
2
3 2
transfer is possible in the here reported system forming of phosphines. The reactivity towards better oxygen ac-
initially a mononuclear molybdenum pyrazolate com- ceptors that might allow the isolation of reduced tung-
plex but two different decomposition pathways occur sten compound is currently being investigated in our
depending on the phosphine concentration.
laboratory.
Catalytic Oxo-Transfer Reactivity
Conclusion
In contrast to the oxo-transfer discussed above, catalytic Sterically demanding pyrazolate ligands have been
oxo-transfer reactions are not hampered by decomposi- found to stabilise oxomolybdenum and oxotungsten
tion reactions. Complex [MoO (t-Bu pz) ] cleanly catal- compounds. The ligands co-ordinate either in the unusu-
2
2
2
2
1
yses the oxidation of trisubstituted phosphines. Oxo- al h -or h -mode depending onthe steric situation within
transfer occurs from dimethyl sulphoxide to the phos- the complex. The dioxo compounds are capable of oxi-
phine forming dimethyl sulphide and the oxidised phos- dising phosphines by oxo-transfer reaction from dimeth-
phine. By using excess DMSO, pseudo-first-order condi- yl sulphoxide. The use of DMSO as oxo donor in this
tions are observed and kinetic measurements show line- study has biological relevance to molybdenum-contain-
ar relationships ln[c(PR )] vs. time throughout the reac- ing DMSO-reductases, which are able to utilise a variety
3
tion pointing to the catalystꢃs stability under these con- of dialkyl and alkyl aryl sulphoxides as oxidizing sub-
ditions. By varying the temperature, activation strates. Reduction of [MoO (t-Bu pz) ] by addition of
2
2
2
#
#
parameters were obtained (DH ¼77.7 kJ/mol, DS ¼ PEt leads to [MoO(t-Bu pz) (PEt ) ] (6), which allows
3
2
2
3 2
ꢀ
81 J/K·mol). The large negative value for the entropy the conclusion that a monomeric Mo(IV)¼O intermedi-
suggests an associative mechanism similar to previously ate is involved in the catalytic cycle under appropriate
[
18,55,56]
described systems.
The rate-limiting step is pre- conditions (co-ordinating solvents or excess phosphine).
sumably the oxidation of the phosphine indicated by
the colour of the reaction solution which remained col-
ourless. Fast oxidation of the reduced molybdenum spe-
cies by DMSO reforming 2 has been experimentally es-
tablished (Scheme 2). The catalytic cycle shown in
Scheme 4 may be considered.
Experimental Section
General Remarks
We believe that in the catalytic reaction a species of
All manipulations were carried out under dry nitrogen using
the type [MoO(t-Bu pz) (L) ] is formed. The first step standard Schlenk-line or glove-box techniques. All solvents
2
2
2
represents the formation of an OPR co-ordinated com- were purified by standard methods and distilled under a nitro-
3
Adv. Synth. Catal. 2005, 347, 463–472
asc.wiley-vch.de
ꢂ 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
469

DEDICATED CLUSTER
FULL PAPERS
Kerstin Most et al.
gen atmosphere immediately prior to use. The starting materi-
uct was isolated. Recrystallisation from toluene gave the pure
compounds in the 70 to 89% yields. See the Supporting Infor-
mation for the characterisation data for compounds 1–5.
[61]
[62]
[63]
als [MoO Cl ],
[WO Cl (dme)],
[MoOCl (PMe ) ],
2
2
2
2
2 3 3
[
64]
[42]
P(C H F) , and t-Bu pzK were prepared according to lit-
6
4
3
2
erature procedures. All other chemicals mentioned were
used as purchased from commercial sources.
Samples for infrared spectroscopy were prepared as toluene
solutions and measured on a BIO-RAD Digilab FTS-7, mass
spectrometer were recorded with a Finnigan MAT 95 and all
NMR spectra on a Bruker Avance 500 or 200. UV/VIS samples
were prepared in a well-maintained grove-box with dry sol-
vents and measured on the spectrophotometer analytikjena
AG Specord S 100 equipped with a thermostat heating block.
Elemental analyses were performed by the Analytisches-
Chemisches Laboratorium des Instituts fꢀr Anorganische
Chemie, Gçttingen.
2
1
[
MoO(h -t-Bu pz)(h -t-Bu pz)(PEt ) ] (6)
2 2 3 2
To a solution of 2 (0.20 g, 0.41 mmol) in pentane, PEt (0.15 g, 3
3
equivs., 1.23 mmol) was added slowly at 08C. The reaction mix-
ture was stirred for 15 h at ambient temperature. After the vol-
ume of solvent was reduced under vacuum to approximately
5
mL, the solution was stored at ꢀ248C to obtain green crys-
1
tals. H NMR (C D ): d¼0. 98 (br s, 18H, CH CH ), 1.43 (br
6
6
2
3
s, 12H, CH CH ). 1.40 (s, 18H, t-BuH), 1.60 (s, 18H, t-BuH),
2
3
6
.05 (s, 2H, ring-H).
2
1
[
MoO(h -t-Bu pz)(h -t-Bu pz)(PMe ) ] (7)
2
2
3 2
3
,5-Di-tert-butyl-4-bromopyrazole (t-Bu -4-BrpzH)
2
A solution of [MoOCl (PMe ) ] (0.28 g, 0.68 mmol) in toluene
2
3 3
In analogy to the procedure reported for the synthesis of silver
[43]
(20 mL) was added to a suspension of t-Bu pzK (0.30 g, 2
2
3,5-dimethylpyrazolate, 3,5-di-tert-butylpyrazole (1.00 g, 1.0
equivs., 1.36 mmol) in toluene (10 mL) and stirred for 15 h. Af-
ter filtration through a 2.5-cm pad of Celite, the volume of sol-
vent was reduced to approximately 5 mL and the solution was
stored at ꢀ248C to get green crystals of 7; yield: 0.15 g (35%).
equiv., 5.43 mmol) was suspended in ethanol (50 mL) and
treated with 1 mL of NH (conc.). Silver nitrate (0.922 g, 1.0
equiv., 5.43 mmol) was added to the solution and the reaction
mixture was stirred for 12 h at ambient temperature. The sol-
vent was removed under reduced pressure and the colourless
residue was extracted with water. After filtration and evapora-
3
1
H NMR (C D ): d¼0.72 (s, 18H, MeH, 1.49 (s, 18H, t-BuH),
6
6
1
(
.60 (s, 18H, t-BuH), 6.05 (s, 2H, ring-H); MS-EI: m/z¼548
þ þ þ
M
– PMe ), 532 (M – PMe – O), 472 (M – 2 PMe );
3 3 3
tion of the solvent t-Bu -4-BrpzAg was obtained as colourless
crystals; yield: 91%.
2
anal. calcd. for C H MoN OP : C 53.55, H 8.83, N 9.30; found:
2
8
56
4
2
C 54.01, H 9.07, N 9.00.
To a solution of t-Bu -4-BrpzAg (3.00 g, 1 equiv., 10.5 mmol)
2
in CH Cl (75 mL) bromine (1.84 g, 1.1 equiv., 11.5 mmol) was
2
2
added and the reaction mixture was refluxed for 16 h. After fil-
tration, the filtrate was washed with an aqueous solution of
X-Ray Crystallography
NaOH and the organic layer dried with MgSO . Evaporation
4
Crystallographic data (excluding structure factors) for the
structures reported in this paper have been deposited with
the Cambridge Crystallographic Data Centre as supplementa-
ry publication nos. CCDC-246863 (3), CCDC-246862 (6) and
of the solvent afforded t-Bu -4-BrpzH as yellow crystals; yield:
2
1
9
0%. H NMR (C D ): d¼1.34 (s, 18H, t-Bu), 10.3 (br s, 1H,
6
6
1
3
NH); C NMR (C D ): d¼152.0 (s, 4C, a-C), 97.6 (s, 2C, b-
6
6
C), 32.7 [s, 4C, C(CH ) ), 28.7 [s, 12C, C(CH ) ); MS: m/z¼
3
3
3 3
CCDC-246864 (t-Bu -4-BrpzH). Copies of the data can be ob-
þ
þ
2
2
58 (M ), 243 (M – Me); anal. calcd. for C H N Br: C
11 19 2
tained free of charge on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK [fax.: (internat.) þ44 1223/336-033;
e-mail: deposit@ccdc.cam.ac.uk].
50.97, H 7.39, N, 10.81; found: C 49.7, H 7.2, N, 10.4.
See the Supporting Information for the X-ray crystallogra-
phy of compounds 3 and 6 and selected crystallographic data.
Potassium 3,5-Di-tert-butyl-4-bromo-pyrazolate
t-Bu -4-BrpzK)
(
2
A solution of t-Bu -4-BrpzH (3 g, 11.6 mmol) in 30 mL of tet-
2
Acknowledgements
rahydrofuran was treated with excess KH (1.85 g, 46.3 mmol)
and stirred at room temperature for 8 hours. Filtration through
a pad of Celite and evaporation of the solvent gave the product
This research was supported by a Grant from the Deutsche For-
schungsgemeinschaft MO 963/2.
as a colourless, microcrystalline material; yield: 2.45 g (71%).
1
H NMR (CD CN): d¼1.33 (s, 18H, t-Bu).
3
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