Activation of molecular oxygen by a molybdenum complex for catalytic oxidation

Article abstract of DOI:10.1039/c5dt02931g

A sterically demanding molybdenum(vi) dioxo complex was found to catalytically activate molecular oxygen and to transfer its oxygen atoms to phosphines. Intermediate peroxo as well as reduced mono-oxo complexes were isolated and fully characterized. Monomeric Mo(iv) monooxo species proved to be of an unusual nature with the coordinated phosphine trans to the oxo group. The reduced molybdenum centers can activate O₂ to form a stable Mo(vi) oxo-peroxo complex unambiguously characterized by single crystal X-ray diffraction analysis. NMR experiments demonstrate that both oxygen atoms of the peroxo unit are transferred to an accepting substrate, generating the Mo(iv) intermediate and restarting the catalytic cycle.

Full text of DOI:10.1039/c5dt02931g

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Activation of molecular oxygen by a molybdenum
complex for catalytic oxidation†
Cite this: Dalton Trans., 2015, 44,
20514
a
a
a
b
Antoine Dupé,‡ Martina E. Judmaier,‡ Ferdinand Belaj, Klaus Zangger and
a
Nadia C. Mösch-Zanetti*
A sterically demanding molybdenum(VI) dioxo complex was found to catalytically activate molecular
oxygen and to transfer its oxygen atoms to phosphines. Intermediate peroxo as well as reduced mono-
oxo complexes were isolated and fully characterized. Monomeric Mo(IV) monooxo species proved to be
of an unusual nature with the coordinated phosphine trans to the oxo group. The reduced molybdenum
centers can activate O2 to form a stable Mo(VI) oxo–peroxo complex unambiguously characterized by
single crystal X-ray diffraction analysis. NMR experiments demonstrate that both oxygen atoms of the
peroxo unit are transferred to an accepting substrate, generating the Mo(IV) intermediate and restarting
the catalytic cycle.
Received 30th July 2015,
Accepted 30th October 2015
DOI: 10.1039/c5dt02931g
www.rsc.org/dalton
Although many iron, manganese and copper complexes are
known to react with dioxygen, only rare examples of mole-
Introduction
9
High-valent molybdenum-oxo complexes are of considerable cular oxygen activation by molybdenum have been previously
1
,2
10,11
12
interest as catalysts for oxidation reactions. In nature, molyb- reported,
including one by our group, and the reactivity
denum oxotransferases are a broad class of enzymes that trans- of these oxo–peroxo molybdenum is limited. Few homoge-
fer an oxygen atom to or from a substrate, using mainly water neously molybdenum catalyzed oxidations employing mole-
3
as a source of oxygen. The active site of these enzymes con- cular oxygen were reported. A bimetallic molybdenum/copper
sists of a molybdenum(VI) center substituted by at least one catalyst was disclosed by Osborn and coworkers for the oxi-
4
13
oxo group and one or two molybdopterin ligands. In order to dation of alcohols. It represents a “Wacker-type reaction”
mimic the activity of such enzymes, a wide range of model where the alcohol is oxidized by molybdenum but reoxidation
2
+
compounds containing a high valent [MoO ] core have been is mediated by the copper co-catalyst and not by molecular
2
5
prepared. While oxygen atom transfer (OAT) capability of such oxygen directly. Selected reports can be found in literature
1
4
complexes can be investigated using phosphines or sulfides as where O was used in Mo catalyzed oxidation chemistry but
2
oxygen accepting substrates, they are also commonly used as high catalyst loadings were needed and/or it remains unclear
catalysts for alkene epoxidation reactions and have thus found whether autooxidation is occurring.
6
wide applications in industry. In the most efficient systems,
In the course of our ongoing research on high-valent molyb-
tert-butyl hydroperoxide or hydrogen peroxide solutions are denum-oxo complexes bearing Schiff-base ligands and their
1
,7
15–17
used as oxidant. However, following the principles of sus- reactivity in OAT reactions,
we report herein the synthesis
tainable chemistry, the use of molecular oxygen as oxidant and characterization of a new molybdenum(VI) dioxo complex
would be highly desirable; it is abundant, cheap, readily avail- which can be reduced and subsequently activate dioxygen to
able and ideally does not lead to the formation of side- form
products. Despite these advantages, controlling the reactivity activation proceeds via the formation of a five-coordinated
of dioxygen with transition metal centers is a challenge. molybdenum(IV) mono-oxo complex or a highly unusual trans-
a new molybdenum(VI) oxo–peroxo complex. This
8
2
+
[MoO(PMe)
3
]
intermediate which was characterized by NMR
spectroscopy and elemental analysis. Furthermore, we
demonstrate that the molybdenum(VI) oxo–peroxo complex is
also active in OAT reaction and able to transfer both oxygen
atoms of the peroxo moiety to an accepting substrate.
a
Institute of Chemistry, University of Graz, Schubertstrasse 1, 8010 Graz, Austria.
E-mail: nadia.moesch@uni-graz.at
b
Institute of Chemistry, University of Graz, Heinrichstraße 28/II, 8010 Graz, Austria
†
Electronic supplementary information (ESI) available: Experimental pro-
18
Benchmark oxidation of tertiary phosphine confirms the
cedures, X-ray crystallographic data, characterization data. CCDC 1413968 for 1
and 1413969 for 4. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c5dt02931g
catalytic activity of the complexes. This reactivity is a new step
toward the use of oxygen as oxidant in molybdenum-catalyzed
oxidation reactions.
‡
These authors contributed equally to this work.
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Results and discussion
2 2
The molybdenum-dioxo complex [MoO L ] 1 was synthesized
by addition of a solution of the ligand 2,4-di-tert-butyl-6-((tert-
1
9
butylimino)methyl)phenol (HL) in toluene to a solution of 1
equiv. of [MoO Cl ] in toluene in presence of an excess tri-
2
2
methylamine (Scheme 1). After stirring for 15 h, filtration and
washing with cold pentane, 1 was obtained as a orange solid Scheme 2 Synthesis of molybdenum(IV) complex 2.
1
in good yield (64%). The H NMR spectrum reveals the for-
mation of a single, symmetric isomer as only one set of reso-
nances for both coordinated ligands is observed. The
IR spectrum of 1 exhibits two strong νMovO bands at 885 and
structure refinements are presented in the Experimental
section. Complex 1 exhibits a six-coordinate metal center in a
distorted octahedral geometry.
12 cm− for both the symmetric and asymmetric stretching
1
9
2
+
mode of the cis-[MoO ] fragment, in good accordance with
2
1
6,20
Reactivity of the dioxo complex 1 in oxygen atom transfer
the literature.
lysis confirmed the formation of complex 1 as [MoO
Mass spectrometry as well as elemental ana-
].
(
OAT) reactions was studied using trimethylphosphine and tri-
2 2
L
phenylphosphine as oxygen acceptor substrates, allowing easy
This structure was confirmed by single crystal X-ray diffrac-
tion analysis. Suitable crystals were obtained from concen-
trated solution of 1 in toluene (Fig. 1). Crystal data and
3
1
monitoring by P NMR spectroscopy. Complex 1 was first
reacted with an excess (4–5 equiv.) trimethylphosphine in
toluene under inert conditions (Scheme 2).
The reaction was indicated by a quick change of color from
3
1
deep orange to crimson and P NMR measurements reveal
the formation of OPMe and further reduced molybdenum(IV)
3
complex 2. After evaporation of the solvent and excess PMe
3
then re-dissolution of the crude material in cold heptane,
OPMe3 is removed via subsequent filtration over a pad of
Celite. The pure reduced complex 2 was isolated as a highly
sensitive to moisture and air, crimson solid in excellent yield
1
13
(
87%). Interestingly, H and C NMR spectra revealed the for-
mation of a symmetric compound as only one set of reso-
nances for both coordinate ligands is observed (Table 1).
Scheme 1 Synthesis of Mo–dioxo complex 1.
1
Characteristic signals in the H NMR spectrum in benzene-d
6
are given at 8.33 ppm (imine proton), 7.42 and 7.19 ppm (aro-
3
1
matic protons). The P NMR spectrum of a concentrated solu-
tion of 2 in benzene-d points to the existence of a single Mo–
6
PMe species (−9.47 ppm) further evidenced by the occurrence
3
of a doublet at 0.91 ppm with the integration of nine protons
1
in the H NMR spectrum.
The symmetric coordination in complex 2 has not been
described in the literature up to now. In such type of OAT reac-
tions, either the formation of a dimeric Mo(V)–O–Mo(V) species
is observed, or the free coordination site is captured by a phos-
2
+
phine in cis position to the [MoO] metal core, forming com-
1
2,15,21
22
plexes of the type [MoO(PR )L ]
or [MoO(OPR )L ].
3 2
3
2
Such complexes render the geometry around the metal center
non-symmetric and therefore lead to two sets of resonances
Fig. 1 Molecular structure of complex 1. Thermal ellipsoids have been
drawn at 50% probability level. Hydrogen atoms are omitted for clarity.
Selected bonds lengths (Å) and angles (°): Mo(1)–O(2) 1.709(3); Mo(1)–
O(1) 1.720(3); Mo(1)–O(21) 2.037(3); Mo(1)–O(11) 2.105(3); Mo(1)–N(1)
1
Table 1 H NMR characteristic signals shifts in ppm for complexes 1, 2,
3
and 4 in benzene-d
6
2
.187(3); Mo(1)–N(2) 2.228(4); O(11)–C(11) 1.309(5); C(1)–N(1) 1.290(6);
N(1)–C(10) 1.518(5); O(21)–C(21) 1.345(5); C(2)–N(2) 1.311(5); N(2)–C(20)
.528(6); N(1)–Mo(1)–N(2) 171.91(13); O(1)–Mo(1)–O(11) 165.89(13);
Compd.
Ar–CHN
Ar–H
Ar–H
1
O(2)–Mo(1)–O(21) 159.04(13); C(11)–O(11)–Mo(1) 124.1(3); C(1)–N(1)–
C(10) 116.0(3); C(1)–N(1)–Mo(1) 116.9(3); C(10)–N(1)–Mo(1) 124.8(3);
C(21)–O(21)–Mo(1) 131.2(3); C(2)–N(2)–C(20) 116.0(3); C(2)–N(2)–Mo(1)
1
2
3
4
8.33
8.32
8.21
8.63, 8.55
7.66
7.40
7.47
7.60, 7.49
7.15
7.16
7.24
7.19, 7.19
1
21.8(3); C(20)–N(2)–Mo(1) 121.7(3).
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2
3
for both coordinate ligands in NMR analysis. Our results point appropriate ligand in aqueous H
2
O
2
,
the activation of mole-
to the symmetric coordination of the PMe molecule in trans cular oxygen by molybdenum compounds is scarce in the lit-
3
position to the MovO moiety. Although an equilibrium erature. As previously described by our group, re-oxidation of
between a trans and cis species is in principle feasible, this is the molybdenum(IV) complex type [MoO(PMe
3 2
)L ] with mole-
2
+
very unlikely as we observe narrow peaks in NMR spectroscopy. cular oxygen either results in the formation of [MoO ] or
2
3
1
2+
12,15
Furthermore, low temperature
P NMR spectroscopy in [MoO(O
2
)] complexes,
depending on the steric demand
solutions of 2 and 3
toluene-d at −30 °C shows only one signal (see ESI†). This in the ligand backbone. When benzene-d
8
6
unusual arrangement is presumably caused by the high steric were exposed to dry molecular oxygen, color change from
demand of the ligand. Two isomers are conceivable with the crimson to orange for complex 2 and from purple to orange
1
13
phosphine and the oxo groups trans to each other, namely the for 3 occurred. H and C NMR spectra indicate the formation
trans-N,N isomer depicted in Scheme 2 and the trans-N,O of the same, non-symmetric compound in both reactions, as
isomer, but the latter seems sterically too congested to exist.
two sets of resonances for both coordinated ligands are
1
ESI-MS measurements and elemental analysis corroborates observed (e.g. in the H NMR spectra 2 × Ar–CHN at 8.55 and
the formation of a monomeric [MoO(PMe )L ] complex. Infor- 8.63 ppm, Table 1). Such behavior is in good accordance to the
3
2
mation about the molecular size of the complex were obtained literature regarding the formation of
from 2D DOSY diffusion measurements on a sample contain- complex, as the coordination of the O molecule in cis position
ing complex 2 and free ligand in benzene-d . For the free to the [MoO] renders the geometry around the metal center
2
a [MoO(O )]-type
2
2
+
6
2
−1
16
ligand (HL), a diffusion coefficient D = −8.805 (log(m
corresponding to 1.57 × 10 m s was found, while complex complex 4 can be synthesized directly by reacting 1 with a
s
)) non-symmetric.
2 2
It is noteworthy that this [MoO(O )L ]
−
9
2 −1
2
−1
2
1
diffuses with a D = −8.878 (log(m
s
)) corresponding to 5-fold excess phosphine under O2 atmosphere. After purifi-
.32 × 10− m s . Based on the Stokes–Einstein equation, the cation, 4 was isolated as an orange solid in 93% yield
9
2 −1
relative hydrodynamic radii of two components are related to (Scheme 3).
the diffusion coefficients by D /D = r /r . Therefore, the experi- Single crystals suitable for X-ray diffraction analyses of
1
2
2 1
mental diffusions coefficients of complex 2 and free ligand complex 4 were obtained from concentrated THF solutions
correspond to relative hydrodynamic radii of 1.18 : 1. Such an layered with pentane at room temperature. Crystal data and
increase of 18% of the diameter of the complex relative to HL structure refinements are presented in the Experimental
can only be explained by a monomer. In addition, the DOSY section. The structure reveals a hepta-coordinated metal center
analysis of a sample containing the dioxo complex 1 and the in a distorted pentagonal bi-pyramidal geometry (Fig. 2). The
2
reduced complex 2 showed similar diffusion for both com- metal center is ligated by one terminal oxygen atom, a η -
plexes (see ESI†).
peroxo moiety and two bidentate Schiff base ligands. The
in terminal oxygen atom is found to be in cis position to the η -
2
The OAT reaction of complex 1 with 2 equiv. PPh
3
benzene-d6 at room temperature as well results in a color peroxo moiety. All these features as well as bonds lengths and
2
+
2
change from deep orange to purple. After 5 h under stirring, an angles values are in good accordance with similar [MoO(O )]
1
13
10,12,24
NMR sample was taken. H and C NMR spectra exhibited complexes reported previously in the literature.
again one set of resonances for both coordinate ligands, indicat-
ing formation of a new symmetric compound [MoOL ] 3. with molecular O
The re-oxidation of the reduced [MoO(PMe )L ] complex 2
3
2
2
2
, yielding the [MoO(O
2
)L
2
] complex 4
1
31
6
Characteristic signals in H NMR in benzene-d (Table 1) are (Scheme 3), provided an interesting information. The P NMR
given at 8.21 ppm (imine proton), 7.47 and 7.24 ppm (aromatic spectra measured directly after exposition of 2 to O showed
2
31
protons). P NMR confirmed formation of OPPh
4.78 ppm) but showed that the other equivalent of PPh
not coordinate the molybdenum center and thus remains free tained under O atmosphere was submitted again for analysis
3
(signal at that the phosphine attached to the Mo is released as a free
2
3
does phosphine when 4 is formed. The same NMR sample, main-
2
3
1
(
signal at 5.41 ppm). Evaporation of the solvent led to a highly after 24 h. The P NMR spectra revealed that the equivalent of
air sensitive purple powder which contains 3, OPPh and PPh
3
3
.
Removal of OPPh3 was performed by filtration of a heptane
solution of the mixture over dry Celite, which however did not
remove residual PPh
00 °C was attempted to isolate 3, but only lead to decompo-
sition of the complex. Nevertheless, its sensitivity towards air as
3 3
. Thus, sublimation of residual PPh at
1
1
31
well as H and P NMR spectroscopy lead us to believe that the
monooxo [MoOL ] complex 3 is formed in situ rather than a
2
Mo(V) dimer or a Mo(IV) phosphino species. The monomeric
structure of 3 was confirmed by DOSY analysis, with a diffusion
coefficient similar to that obtained for complex 2.
2
+
2+
2 2 2
Although complexes with the [MoO(O )] or [MoO(O ) ]
cores are relatively abundant in the literature, as complexes
thereof are available from the reaction of [MoO ] and the Scheme 3 Synthesis of the oxo–peroxo Mo(VI) complex 4.
3
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observed and after t = 30 h 50%. This Mo intermediate exhibits
a single set of signal very similar to that of the Mo(IV) monooxo
complex 2 (e.g. signals at 8.34, 7.40 and 7.21 ppm). The small
1
differences between the H NMR signals of the observed Mo
intermediate and 2 can be explained by differences in the
coordination of the phosphine. In the intermediate, PMe is
3
only weakly interacting with Mo but is not fully coordinated.
1
This explanation is supported by the H NMR signal of the
weakly interacting PMe
different from free PMe
3
(0.86 ppm, JP–C = 4.2 Hz), which is
(0.81 ppm, JP–C < 2 Hz) and from the
3
fully bound PMe3 in 2 (0.91 ppm, JP–C = 7 Hz). During the
course of the reaction, no formation of a plausible Mo(VI)
Fig. 2 Molecular structure of complex 4. Thermal ellipsoids have been
drawn at 50% probability level. Hydrogen atoms are omitted for clarity. dioxo intermediate (complex 1) was observed, indicating that
Selected bond lengths (Å) and angles (°): Mo1–O1 1.6909(10); Mo1–O2
the transfer of the first oxygen atom from the peroxo unit is
1
.9487(10); Mo1–O3 1.9447(10); Mo1–O11 2.0494(9); Mo1–O31
.0269(9); Mo1–N1 2.2548(11); Mo1–N2 2.2162(11); O2–O3 1.4332(13);
O1–Mo1–O11 170.37(4); O2–Mo1–O31 155.69(4); O3–Mo1–O31
the rate determining step of the process, while transfer of the
second oxygen atom is either concerted or occurs much faster.
This was corroborated by kinetic experiments using UV-Vis
spectroscopy. The OAT reaction from complex 1 with a 100-fold
2
1
1
53.96(4); N1–Mo1–N3 165.72(4); O1–Mo1–O2 99.17(5); O1–Mo1–O3
01.82(5); O1–Mo1–O31 91.99(4); O1–Mo1–N1 91.57(4).
excess PMe
3
(pseudo-first order conditions) to form the
reduced complex 2 was found to have a rate constant k = 0.003
−
1
free PMe
to OPMe
Scheme 3, substitution of the phosphine by O occurred and phine was found to be orders of magnitude slower as hardly
3
formed during the re-oxidation had been converted
s
. The corresponding reaction of complex 4 with 100 equiv.
3
. Thus, when 2 reacted with O
2
as presented in PMe
3
and the same concentration for the complex and phos-
2
only subsequently PMe
complex 4 (see Fig. S3 in ESI†). These observations are note-
worthy as phosphine is usually strongly coordinated, so that OAT reaction, catalytic oxidation of PMe
prior oxidation is required for its displacement. Here, the the literature, the molybdenum complexes which activate O
steric demand of the ligand and the trans-effect resulting from could not catalytically oxidize phosphines. [MoO(O )(CN) ]-
the presence of the oxo group leads to a weakly coordinated [PPh is capable of oxidizing 3 mol PPh per 2 mol molyb-
PMe
rendering the system reactive. Such labilization of phos- denum in the presence of molecular O
phine ligands due to a trans-effect was already observed with dation of PPh was obtained due to conproportionation of the
other Mo complexes. The fact that the free PMe equivalent dioxo Mo(VI) and the monoxo Mo(IV) intermediates. Although
3
was oxidized to OPMe
3
by the any reaction occurred in the observed timeframe (see ESI†).
As both the dioxo and oxo–peroxo complexes are active in
was investigated. In
3
2
2
4
]
2
4
3
1
0
,
but no further oxi-
3
2
3
2
5
3
could be oxidized under O
us to investigate the role of the oxo–peroxo complex as possible and air, the stability of PMe
catalyst for the oxidation of phosphine using O , as described solutions was confirmed by running a blank experiment (see
2
atmosphere in presence of 4 lead PMe
3
is easily oxidized without any metal in presence of water
3
against dry O in dry benzene-d
2
6
2
thereafter. In order to exclude auto oxidation of PMe
prove the reactivity of the oxo–peroxo complex in OAT, the reac- benzene-d
tion of 4 with 2 equiv. PMe in exclusion of O was investigated Then 100 equivalents PMe
3
and to ESI†). For the catalytic run, a solution of 1 (10 mg, 14 μmol) in
was prepared and placed under O atmosphere.
were added and the reaction was
Scheme 4). Monitoring the reaction by H and P NMR spec- left to stir for 24 h (Scheme 5). The reaction was monitored by
6
2
3
2
3
1
31
(
1
31
troscopy revealed that the oxo–peroxo complex 4 is able to
3 3
H and P NMR analysis. Integration of PMe and OPMe
oxidize tertiary phosphines, but more importantly that both signals after 24 h showed the conversion of 19 equivalents
atoms of the peroxo group are involved in this transfer. PMe to OPMe3. After evaporation of the solvent and unreacted
Although the reaction does not go to completion, H NMR phosphine, mass balance confirmed formation of OPMe
spectra show formation of a new molybdenum species and besides the residue of the catalyst. Control experiments in the
OPMe in a 1 : 2 ratio, along with unreacted PMe
and complex presence of sodium molybdate or [MoO (acac) ] did not lead
. As shown in Fig. 3, after t = 5 h approx. 25% conversion is to formation of OPMe . The reactivity of 1 is limited by
3
1
3
3
3
2
2
4
3
2 6
Scheme 4 OAT reaction of the oxo–peroxo complex 4 in exclusion of O in benzene-d .
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1
Fig. 3 H NMR spectra in benzene-d
under O exclusion.
6
at t = 5 h (left) and t = 30 h (right) of the reaction of molybdenum oxo–peroxo complex 4 with 2 equiv. PMe
3
2
Experimental section
All reactions have been carried out under nitrogen using stan-
dard Schlenk or glovebox techniques. [MoO Cl ] was pur-
2
2
chased from Sigma-Aldrich and used in the Glovebox without
further purification. The Schiff base ligand was synthesized
1
9
according to previously published literature. Solvents were
purified via a Pure-Solv MD-4-EN solvent purification system
1
13
from Innovative Technology, Inc. The H and C NMR spectra
were recorded on a Bruker Optics Instrument 300 MHz. Peaks
are denoted as singlet (s), doublet (d), doublet of doublets
Scheme 5 Conversion of PMe
3
3 2
to OPMe catalyzed by 1 using O .
(
dd), triplet (t) and multiplet (m), Ar denotes aromatic
protons. Chemical shifts are reported in ppm and are refer-
enced using the residual solvent peak. To obtain self-diffusion
decomposition of the complex, but is a successful progress coefficients we used two-dimensional diffusion ordered
2
6
toward the use of molecular oxygen as sole oxidant in catalytic spectroscopy (DOSY). The employed pulse sequence was a
OAT reactions.
bipolar pulse pair longitudinal eddy current delay (BPP-LED)
sequence, using 32 scans per increment, 60 ms diffusion delay
time, 1 ms gradient pulses and variation of the gradient
strength in 32 increments, linearly varied between 2 and 95%
of maximum (which is 53.5 G cm− ). DOSY analysis was per-
formed using the Bruker DOSY package of TopSpin 3.1. All
1
Conclusion
We were able to prepare a new molybdenum(VI) dioxo complex DOSY measurements were carried out at 300 K on a Bruker
which is active in oxygen atom transfer reaction to tertiary Avance III 500 MHz NMR spectrometer using a 5 mm TXI
phosphines, allowing the isolation of the unusual trans-[MoO- probe with z-axis gradients. IR spectra were measured as solid
PMe )L ] complex 2. The activation of molecular oxygen by samples on a Bruker Alpha P Diamond FTIR-ATR spectrometer
1
(
3
2
the reduced species yields the oxo–peroxo complex [MoO(O
] 4 in excellent yield. 4 is also active in OAT and NMR MF in situ spectrometer using a glass fiber optic probe. ESI-MS
2
)- or as liquid samples in benzene on a Bruker FT-MIR matrix
L
2
spectroscopy studies show that both atoms of the peroxo spectra were recorded in acetonitrile on an Agilent 1100 Series
group are transferred to phosphine. Hence, the molybdenum LCMSD (SL type). Elemental analyses were carried out using a
(
VI) dioxo complex 1 exhibits promising results as catalyst for Heraeus Vario Elementar automatic analyzer at the Institute of
oxidations using O as the oxidant.
Inorganic Chemistry at the University of Technology in Graz.
2
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[
MoO
Ligand HL (500 mg, 1.7 mmol, 2 equiv.) was dissolved in 5 ml
of dry toluene and slowly added to a suspension of [MoO Cl
180 mg, 0.9 mmol, 1 equiv.) in 5 mL of dry toluene under
inert conditions. Dry triethylamine (0.26 mL, 1.9 mmol,
.2 equiv.) was then added. The formation of the complex was
2
L
2
] (1)
Anal. Calcd for MoO
Found: C, 64.25; H, 9.23; N, 3.38%.
3
N
2
PC41
H
69: C, 64.38; H, 9.09; N, 3.66.
2
2
]
[
2
MoOL ] (3)
(
[
MoO L ] (100 mg, 0.14 mmol, 1 equiv.) was dissolved in dry
2
2
2
3
toluene and an excess PPh (74 mg, 0.28 mmol, 2 equiv.) was
immediately indicated by a quick change of color from pale
yellow to deep red. To ensure complete complex formation, the
solution was stirred 5 h at room temperature. The mixture was
then filtered with a cannula and the solvent was removed
in vacuo. The residue was washed thrice with 10 ml of cold
pentane to afford complex 1 as an orange solid in 64% yield
added. The OAT was indicated by a quick change of color from
orange to purple. After stirring for 15 hours, the solvent was
removed under reduced pressure. The obtained material was
re-dissolved in cold dry heptane (5 mL) and filtered over a pad
of celite. After evaporation of the heptane, [MoOL ] was
2
obtained as a purple solid in 60% yield, with partial decompo-
(
407 mg). Single crystals suitable for X-ray diffraction analyses
sition to the free ligand and traces of triphenylposphine.
1
were obtained by slow evaporation from concentrated toluene
solution at room temperature.
H NMR (300 MHz, benzene-d , ppm) δ = 8.21 (s, 2H,
6
4
4
Ar–CHN), 7.47 (d, JH–H = 2.6 Hz, 2H, Ar–H), 7.24 (d, JH–H
=
1
H NMR (300 MHz, benzene-d , ppm) δ = 8.33 (s, 1H, Ar–
6
2.6 Hz, 2H, Ar–H), 1.61 (s, 18H, N–C(CH ) ), 1.34 (s, 18H,
3
3
4
4
CHN), 7.66 (d, JH–H = 2.7 Hz, 1H, Ar–H), 7.15 (d, JH–H = 2.7
Hz, 1H, Ar–H), 1.56 (s, 9H, NC(CH ) ), 1.33 (s, 9H, Ar–C(CH ) ),
Ar–C(CH ) ), 1.33 (s, 18H, Ar–C(CH ) ).
3
3
3 3
1
3
3
3
3 3
6
C NMR (75 MHz, benzene-d , ppm) δ = 165.66 (Ar–CHN),
1
.30 (s, 9H, Ar–C(CH ) ).
3 3
163.68 (Ar–O), 139.01 (Ar), 138.52 (Ar), 132.05 (Ar–H), 129.07
1
3
C NMR (75 MHz, benzene-d
6
, ppm) δ = 167.48 (Ar–CHN),
(
Ar–H), 119.06 (Ar), 65.97 (N–C(CH ) ), 35.80 (Ar–C(CH ) ),
3 3 3 3
1
64.31 (Ar–O), 139.32 (Ar), 139.00 (Ar), 131.03 (Ar–H), 129.28
3 3 3 3 3 3
34.27 (Ar–C(CH ) ), 31.69 (Ar–C(CH ) ), 30.83 (N–C(CH ) ),
(
(
(
Ar–H), 123.39 (Ar), 64.96 (NC(CH ) ), 35.28 (Ar–C(CH ) ), 34.18
3
3
3 3
30.14 (Ar–C(CH ) ).
3 3
Ar–C(CH
3
)
3
), 31.59 (Ar–C(CH
3
)
3
3 3
), 31.24 (N(CH ) ), 29.57
Ar–C(CH ) ).
3
3
[
MoO(O )L ] (4)
2 2
IR (ATR, cm− ): ν = 1613 (m, CvN), 1587 (m, CvN), 1534
1
[
MoO L ] (100 mg, 0.14 mmol, 1 equiv.) was dissolved in dry
2 2
(
m), 1387 (m), 1255 (m), 1169 (s), 913 (s, MovO), 883 (s,
benzene (5 mL) followed by the addition of excess PMe
53 mg, 70 µL, 0.70 mmol, 5 equiv.). The OAT was indicated by
3
MovO), 836 (m), 745 (m), 533 (m).
(
Anal. Calcd for MoO N C H : C, 64.75; H, 8.58; N, 3.97.
4
2 38 60
a quick change of color from orange to dark red. The solution
was stirred for 30 minutes, followed by exposure of this solu-
tion to dry molecular oxygen for 5 minutes. The reaction solu-
tion quickly changed the color from dark violet to orange. The
Found: C, 64.29; H, 8.42; N, 3.73%.
[MoO(PMe
3 2
)L ] (2)
[
MoO
2
L
2
] (100 mg, 0.14 mmol, 1 equiv.) was dissolved in dry mixture was left to stir for 30 minutes and the solvent was
(70 μL, 0.7 mmol, 5 equiv.) was removed in vacuo. The product was dissolved in heptane
added. The OAT was indicated by a quick change of color from (5 mL) and twice filtered over a pad of Celite. [MoO(O )L ] was
toluene and excess PMe
3
2
2
orange to dark red. After stirring for 3 hours, the solvent and obtained as an orange solid. Yield: 87 mg (93%). For elemental
excess PMe were removed under reduced pressure. The analyses, the complex was dissolved in minimum amount of
obtained material was re-dissolved in cold dry heptane (5 mL) CH Cl and again filtered over a pad of Celite. Single crystals
3
2
2
and filtered over a pad of Celite. After evaporation of the suitable for X-ray diffraction analyses were obtained from con-
solvent, complex 2 was obtained as dark red solid. Yield: centrated THF solutions layered with pentane.
1
9
5 mg (87%).
H NMR (300 MHz, benzene-d , ppm) δ = 8.63 (s, 1H, Ar–
6
1
4
H NMR (300 MHz, benzene-d
6
, ppm) δ = 8.32 (s, 2H, Ar– CHN), 8.55 (s, 1H, Ar–CHN), 7.60 (d, JH–H = 2.4 Hz, 1H, Ar–H),
CHN), 7.40 (d, JH–H = 2.6 Hz, 2H, Ar–H), 7.16 (m, 2H, Ar–H), 7.49 (d, JH–H = 2.4 Hz, 1H, Ar–H), 7.19 (d, JH–H = 2.4 Hz, 2H,
4
4
4
1
.65 (s, 18H, N–C(CH ) ), 1.34 (s, 18H, Ar–C(CH ) ), 1.31 (s, Ar–H), 1.87 (s, 9H, NC(CH ) ), 1.74 (s, 9H, NC(CH ) ), 1.41 (s,
3 3 3 3 3 3 3 3
1
8H, Ar–C(CH
3
)
3
), 0.91 (d, JP–H = 7 Hz, 9H, OPMe
C NMR (75 MHz, benzene-d , ppm) δ = 164.57 (Ar–CHN), Ar–C(CH
62.96 (Ar–O), 138.78 (Ar), 136.58 (Ar), 131.55 (Ar–H), 128.90
3
).
9H, Ar–C(CH
3
)
3
), 1.27 (s, 9H, Ar–C(CH
3
)
3
), 1.23 (s, 9H,
1
3
6
3
)
3
), 1.09 (s, 9H, Ar–C(CH
3 3
) ).
1
3
1
C NMR (75 MHz, benzene-d , ppm) δ = 166.36 (overlap-
6
(
3
3
Ar–H), 122.59 (Ar), 64.47 (N–C(CH
3
)
3
), 35.67 (Ar–C(CH
4.05 (Ar–C(CH ), 31.81 (N–C(CH
0.60 (Ar–C(CH ) ), 17.55 (d, J = 17.9 Hz, Mo–P(CH3)3).
P–C
3
)
)
3
), ping signals, 2 × Ar–CHN), 161.74 (Ar–O), 159.65 (Ar–O), 140.22
), (Ar), 139.79 (Ar), 138.97 (Ar), 137.78 (Ar), 131.83 (Ar–H), 130.86
(Ar–H), 130.16 (Ar–H), 130.37 (Ar–H), 124.45 (Ar), 122.63 (Ar),
3 3 3 3
) ), 32.07 (Ar–C(CH )
3
3
3
3
3
1
P NMR (121 MHz, benzene-d
P(CH ).
IR (FT-IR, benzene, cm ): ν 1605 (m, CvN), 1542 (m), 1460 (NC(CH ) ), 31.60 (NC(CH ) ), 31.54 (2 × Ar–C(CH ) ), 30.25
6
, 298 K, ppm) δ −9.47 (Mo − 67.96 (NC(CH
3
)
3
), 67.49 (NC(CH
3
)
3
), 35.44 (Ar–C(CH
3
)
3
), 34.77
3
)
3
(Ar–C(CH ), 34.13 (Ar–C(CH
3
)
3
3
)
3
3 3
), 34.06 (Ar–C(CH ) ), 32.78
−
1
3
3
3 3
3 3
(
(
3 3 3 3
s), 1436 (s), 1396 (s), 1362 (m), 1307 (s), 1256 (s), 1165 (s), 932 (Ar–C(CH ) ), 29.91 (Ar–C(CH ) ).
s, MovO), 836 (m).
IR (ATR, cm− ): ν 1606 (s, CvN), 1541 (m), 1255 (s), 1168
1
ESI-MS (50 V) m/z (%): 690.3 (100) [M − HP(CH ) ].
(s), 931 (s, MovO), 838 (s), 777 (s), 747 (m), 542 (s, Mo–O).
3
3
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Dalton Transactions
Anal. Calcd for MoO
.23; N, 3.78. Found: C, 62.18; H, 8.44; N, 3.90%.
5
N
2
C
38
H
60·0.23 CH
2
Cl
2
: C, 62.00; H, Table 2 Crystallographic data and structure refinements for complexes
1
and 4
8
1
4
Oxygen atom transfer reactivity of [MoO(O )L ] (4)
2
2
[
MoO(O )L ] (25 mg, 0.035 mmol, 1 equiv.) was dissolved in Empirical formula
C
38
H
60MoN
2
O
4
2MoO
5
N
O·
2
C
38
H
60
·
2
2
xC
H
4 8
dry benzene-d
equivalent PMe
6
under inert conditions in a Young NMR tube. 2
(8 μL, 0.07 mmol) was added using a micro-
(
1 − x)C
5
H
12
;
3
x = 0.786(6)
1513.75
1
31
pipette. The OAT reaction was monitored by H and P NMR Formula weight
Crystal description
spectroscopy.
704.82
Plate, red
0.27 × 0.17 × 0.05 mm
Monoclinic,
P21/n
Block, red
Crystal size (mm)
0.32 × 0.25 × 0.25
Monoclinic,
C2/c
Crystal system, space
group
Catalytic oxidation of trimethylphosphine
Unit cell dimensions
a (Å)
b (Å)
[
MoO
2 2
L ] (complex 1, 10 mg, 14 µmol) was placed in a Schlenk
10.3863(4)
11.4627(5)
33.1945(13)
90
98.6499(17)
90
24.3085(13)
15.8102(8)
20.8035(11)
90
94.112(2)
90
flask in the glovebox. The Schlenk flask was evacuated then
refilled with dry O . Dry benzene-d (2 mL) and trimethyl- c (Å)
2
6
α (°)
β (°)
γ (°)
phosphine (108 mg, 0.15 µL, 1.4 mmol, 100 equiv.) were added
and the reaction was left to stir under O atmosphere for 24 h.
2
31
1
3
The reaction was monitored by H and P NMR at t = 4 h, 20 h Volume (Å )
3907.0(3)
4, 1.198
7974.7(7)
4, 1.261
Z, calculated density
and 24 h. After removal of the solvent, the mass balance was
calculated and the yield of OPMe confirmed by integration of
−
3
(
g cm
)
3
F(000)
1504
0.373
3233.9
0.373
1
the signals in the H NMR spectrum at t = 24 h (yield = 19%). Linear absorption−
1
coefficient μ (mm
)
3 2
To confirm the stability of PMe under O atmosphere, a blank
Absorption correction
Semi-empirical
from equiv.
100
0.71073
2.46 to 28.45
Semi-empirical
from equiv.
100
0.71073
1.79 to 30.00
experiment was performed. A Schlenk flask was evacuated
then refilled with dry O . Dry benzene-d (2 mL) and trimethyl- Temperature (K)
2
6
α
Wavelength (Mo K ) (Å)
phosphine (108 mg, 0.15 µL, 1.4 mmol, 100 equiv.) were added
and the reaction was left to stir under O atmosphere for 24 h.
Theta range for data
collection (°)
2
31
1
The reaction was monitored by H and P NMR at t = 4 h, 20 h Limiting indices
−12 ≤ h ≤ 12
−34 ≤ h ≤ 29
−22 ≤ k ≤ 22
−29 ≤ l ≤ 29
46 647/11 648
−
5 ≤ k ≤ 14
40 ≤ l ≤ 40
and 24 h. at t = 24 h, a signal for OPMe could be observed in
the H NMR (1% yield) but no signal was observed in
3
−
1
31
P
Reflections collected/
26 791/7661
NMR. After 24 h, the solvent and free PMe were evaporated, unique
no solid OPMe could be isolated.
3
3
Reflections with I > 2σ(I)
R(int), R(sigma)
5550
0.0549, 0.0723
0.999
10 882
0.0187, 0.0148
0.999
Completeness to theta
max.
OAT kinetic study
Refinement method
Full matrix least
squares on F
7661/432/0
Full matrix least
squares on F
11 648/486/12
In the glovebox, a solution of 7 mg of complex 1 in 5 mL
2
2
toluene (2 × 10− mol L ) and a solution of 15 mg PMe
3
−1
in
3
Data/restraints/
−
1
1
mL toluene (0.2 mol L ) were prepared. Using a micro- parameters
2
Goodness-of-fit on F
Final R wR [I > 2σ(I)]
1.107
1.100
pipette, 100 µL of the solution containing the complex were
placed in a quartz cuvette and 2 mL toluene were added. Then
1
2
R
1
= 0.0585
wR = 0.1401
= 0.0869
wR = 0.1507
1.484 and −2.767
R
1
= 0.0277
wR = 0.0689
= 0.0304
wR = 0.0709
1.283 and −0.599
2
2
1
00 µL of the solution containing PMe were added and the R indices (all data)
R
1
R
1
3
2
2
cuvette was sealed with a Teflon stopper and parafilm. The
sample was removed from the glovebox and the measurement
immediately started. The reaction was monitored by the diss-
apearance of the signal at 430 nm, at room temperature using a
Cary50 Conc. UV-Vis spectrophotometer under PC control using
Largest diff. peak and
−3
hole (e Å
)
the kinetics program “Scan” included with the instrument soft- Cambridge Crystallographic Data Center [CCDC 1413968 for 1,
ware. The same procedure was followed preparing a solution of CCDC 1413969 for 4].
−3
−1
7
mg of complex 4 in 5 mL toluene (2 × 10 mol L ).
Crystal structure determination of 1. The structure was
solved by direct methods (SHELXS-97) and refined by full-
matrix least-squares techniques against F (SHELXL-2014/6).
Structure determination
2
For X-ray structure analyses the crystals were mounted onto The non-hydrogen atoms were refined with anisotropic dis-
the tip of glass fiber and data collection was performed at placement parameters without any constraints. The H atoms
1
0
00 K using graphite monochromated Mo K
.71073 Å) with a BRUKER-AXS SMART APEX II diffractometer atom of a CvN double bond were put at the external bisectors
α
radiation (λ = of the phenyl rings as well as the H atoms bonded to the C
equipped with a CCD detector. Essential details of the crystal- of the C–C–X angles at C–H distances of 0.95 Å and common
data and structure refinements for compounds 1 and 4 are isotropic displacement parameters were refined for these H
summarized in Table 2. Crystallographic data for the struc- atoms of the same ligand. The H atoms of the methyl groups
tures of compounds 1 and 4 have been deposited with the were refined with common isotropic displacement parameters
20520 | Dalton Trans., 2015, 44, 20514–20522
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Paper
for the H atoms of the same tert-butyl group and idealized
geometries with tetrahedral angles, enabling rotation around
the C–C bonds, and C–H distances of 0.98 Å.
Crystal structure determination of 4. The structure could be
solved (SHELXS-97) by interpretation of the patterson map
5 J. H. Enemark, J. J. A. Cooney, J.-J. Wang and R. H. Holm,
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(second solution) in the non-centrosymmetric space group C2,
but not in centrosymmetric C2/c. After completion of the mole-
cule an inversion center could be detected and the structure
was refined by full-matrix least-squares techniques against
2
F
(SHELXL-2014/6) in the centric space group C2/c after an
3
appropriate shift of the origin. A void of approx. 188 Å is occu-
pied by a THF molecule disordered over two orientations lying
near a center of symmetry or by a n-pentane molecule at the
inversion center. The ratio of the refined occupation factors is
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0
.768(6) to 0.232(6). The non-hydrogen atoms of the solvent
molecules were refined with isotropic displacement parameters
with some restraints. The H atoms of the solvent molecules
were included at calculated positions with their isotropic dis-
placement parameters fixed to 1.2 times Ueq of the C atom they
are bonded to. The non-hydrogen atoms of the metal complex
were refined with anisotropic displacement parameters without
any constraints. The H atoms of the phenyl rings were put at
the external bisectors of the C–C–C angles at C–H distances of
7 S. A. Hauser, M. Cokoja and F. E. Kühn, Catal. Sci. Technol.,
2013, 3, 552.
8 I. W. C. E. Arends and R. A. Sheldon, in Modern Oxidation
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KGaA, Weinheim, Germany, 2010, pp. 147–185;
W. B. Tolman, Activation of small molecules. Organometallic
and bioinorganic perspectives, Wiley-VCH Verlag GmbH &
Co. KGaA, Weinheim, Germany, 2006.
0
.95 Å and common isotropic displacement parameters were
9 J. Serrano-Plana, I. Garcia-Bosch, A. Company and
M. Costas, Acc. Chem. Res., 2015, 48, 2397; W. Nam, Acc.
Chem. Res., 2015, 48, 2415; E. I. Solomon, D. E. Heppner,
E. M. Johnston, J. W. Ginsbach, J. Cirera, M. Qayyum,
M. T. Kieber-Emmons, C. H. Kjaergaard, R. G. Hadt and
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H30 were put at the external bisector of the C–C–C angle at a
C–H distance of 0.95 Å but the individual isotropic displace-
ment parameters were free to refine. The H atoms of the methyl
groups were refined with common isotropic displacement para-
meters for the H atoms of the same group and idealized geome-
tries with tetrahedral angles, enabling rotation around the C–C
bond, and C–H distances of 0.98 Å.
7
2
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The authors gratefully acknowledge financial support from the
Austrian Science Fund (FWF): project number P26264 and
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