Dioxomolybdenum(VI) and dioxotungsten(VI) complexes supported by an amido ligand

Article abstract of DOI:10.1039/b514873a

Stepwise addition of one equivalent of n-butyllithium and trimethylsilyl chloride to 2-tert-butylmercaptoaniline affords the new ligand 1-(Me ₃SiNH)-2-(t-BuS)C₆H₄ (LH), that reacts with one equivalent of butyllithium to it

Full text of DOI:10.1039/b514873a

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Dioxomolybdenum(VI) and dioxotungsten(VI) complexes supported
by an amido ligand
Ganna Lyashenko, Vojtech Jancik, Aritra Pal, Regine Herbst-Irmer and Nadia C. Mo¨sch-Zanetti*
Received 20th October 2005, Accepted 24th January 2006
First published as an Advance Article on the web 31st January 2006
DOI: 10.1039/b514873a
Stepwise addition of one equivalent of n-butyllithium and trimethylsilyl chloride to
2-tert-butylmercaptoaniline affords the new ligand 1-(Me3SiNH)-2-(t-BuS)C6H4 (LH), that reacts with
one equivalent of butyllithium to its lithium salt LLi. Dioxodichloromolybdenum [MoO2Cl2] and
dioxodichlorotungsten dimethoxyethane [WO2Cl2(dme)] react in tetrahydrofuran solution at low
temperature with two equivalents LLi to monomeric dioxomolybdenum(VI) [MoO2L2] (1) and
dioxotungsten(VI) complex [WO2L2] (2) employing two bidentate amido thioether ligands. The
crystallographic determination of the molecular structures of 1 and 2 show evidence for M · · · S
contacts. The reaction of [MoO2Cl2] with LLi in tetrahydrofuran solution at room temperature leads
next to 1 to two compounds where silyl group migration from nitrogen to oxygen atoms occurs forming
ꢀ
ꢀ
ꢀ
=
=
[Mo( NL )2(OSiMe)2] (3) and [Mo( NL )2(OSiMe3)L] (4, L = N-2-t-BuSC6H4) as determined by
NMR spectroscopy. Compound 4 was isolated in low yield and its molecular structure determined by
X-ray crystallography. Higher yields of a bisimido complex can be obtained by the direct reaction of
one equivalent of LLi with [Mo(NAr)2Cl2(dme)] (Ar = 2,6-Me2C6H4) forming [Mo(NAr)2LCl] (5).
[VO{N(SiMe3)2}2Br] rearranges into a imido siloxide species on
Introduction
heating for several hours.24 For group 6 metals, Wilkinson and
co-workers reported the formation of a low-melting compound
[Mo(NSiMe3)2(OSiMe3)2] and [W(NSiMe3)(OSiMe3)3Cl(py)] af-
ter reaction of [MO2Cl2] with HN(SiMe3)2, whereas with [CrO2Cl2]
the amido complex [CrO2{N(SiMe3)2}2] was obtained.25 Lee’s
molybdenum and tungsten complexes do not seem to rearrange.
This encouraged us to start our investigation with silyl substituted
anilido ligands. Due to the preference for sulfur ligands of these
metals we introduced a chelating thioether functionality.
Molybdenum and tungsten oxo compounds are of great impor-
tance in catalytic reactions involving oxidation reactions, both
in nature as well as in industrial processes.1–5 A wide variety
of dioxomolybdenum(VI) and -tungsten(VI) complexes have been
prepared to develop new catalysts and to model active sites
of molybdenum oxotransferase enzymes.6–15 We have recently
shown that the dioxomolybdenum complex that contains sterically
2
2
demanding g -pyrazolate ligands ([MoO2(g -Pz)2]; Pz = 3,5-t-
Bu2Pz) is a catalyst for oxygen atom transfer (OAT) reactions
from dimethyl sulfoxide to phosphines.16,17 This is interesting
in view of the abundance of nitrogen-based ligands in biolog-
ical systems. However, in oxotransferase enzymes sulfur-based
ligands (dithiolenes) are coordinated to the molybdenum oxo
cores.18 For this reason, MoO2 and WO2 complexes employing
mainly thiolate ligands with chelating amino functionalities were
prepared.3,4,19 Our success with pyrazolate ligands prompted us
to investigate the scope of amido molybdenum or tungsten dioxo
complexes as possible functional models for molybdoenzymes.
Beside tris(pyrazolyl)borate N-donor ligands that have rendered
a significant impact in modelling such enzymes,20–22 compounds
of the type [MoO2L2] where L represents an anionic nitrogen
ligand are extremely rare. To the best of our knowledge next
to our pyrazolate system only one other report can be found
in the literature.23 Lee et al. synthesized a series of dioxo-
molybdenum and -tungsten compounds stabilized by pyridine-
functionalised amido ligands.23 Surprisingly, they employ silyl
amido ligands despite the fact that silyl migration from the
nitrogen to the oxygen atom forming imido complexes is a well
documented phenomenon.24–26 Thus, the vanadium(V) complex
In this paper we report our attempts to generate molybdenum-
and tungsten-dioxo complexes with 2-thioether functionalised
anilido ligands (Fig. 1). Surprisingly, such ligands have not been
previously employed in any metal compounds in contrast to
similar ether functionalised molecules.27–31
Fig. 1
Here we report the synthesis and molecular structure of
several complexes containing the anilido ligand shown in Fig. 1
with R = t-Bu. In contrast to the work reported for pyridine-
functionalised amido ligands,23 we observe silyl group migration
in the molybdenum but not in the tungsten compound.
Results and discussion
Synthesis of the ligand
Institut fu¨r Anorganische Chemie, Georg-August-Universita¨t Go¨ttingen,
Tammannstraße 4, D-37077, Go¨ttingen, Germany. E-mail: nmoesch@
gwdg.de
The reaction of 2-tert-butylmercaptoaniline32 with one equivalent
of n-butyllithium and subsequent treatment with trimethylsilyl
1294 | Dalton Trans., 2006, 1294–1301
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chloride gives the desired product LH as a colourless viscous
liquid.
occurs. They are readily soluble in common organic solvents
including pentane. The 1H and 13C NMR spectra in C6D6 of both
compounds show signals for one type of ligand, which implies a
symmetrical coordination of both ligands as shown in Scheme 2.
This is consistent with their solid-state structures determined by
single-crystal X-ray diffraction (see below). The IR spectrum of 2
exhibits moderate absorptions at 901 and 945 cm−1 attributable to
the characteristic tungsten oxo stretching vibration.23,35–37 Electron
impact mass spectrometry revealed a peak at m/z 634 and 720 for
1 and 2, respectively, for the molecular ions with correct isotopic
contributions.
The amine can be conveniently purified by distillation at 73 ◦C
1
under reduced pressure. H and 13C NMR spectra are consistent
with the structure shown in Scheme 1. Deprotonation of the
amine H-atom by one equivalent of n-BuLi in pentane leads
to the formation of a white precipitate of the lithium salt that
can be isolated and spectroscopically characterized. 1H NMR
spectroscopy showed two singlets at d 0.30 and 1.07 ppm with
equal intensity, shifted from those in the spectrum for LH (d 0.16
and 1.20 ppm) indicating complete deprotonation. However, we
generally synthesized the LLi in situ by addition of n-BuLi to the
The reaction of [MoO2Cl2] with 2 equiv. of LLi at room
◦
1
amine LH in tetrahydrofuran at −20 C and subsequent stirring
temperature was investigated by H NMR spectroscopy. Thus,
for an additional 30 min at room temperature (Scheme 1). Such
solutions were directly used in the reactions described below.
the two reagents were directly dissolved in a 5 : 1 mixture of
deuterated benzene (C6D6) and deuterated tetrahydrofuran (THF-
D8) at room temperature. The immediate recording of the 1H
NMR spectrum of this mixture revealed resonances for two
compounds with symmetrically coordinated ligands in the ratio
2 : 1. Resonances at 0.42 (s, 9H, SiMe3), 1.15 (s, 9H, t-Bu), 6.5 (m,
1H, Ar) and 6.95 (m, 3H, Ar) ppm are assignable to compound
1 as an independently measured spectrum of isolated 1 in this
solvent mixture shows identical shifts. This 1H NMR experiment
revealed signals for a second species 3 (Scheme 3) at 0.11 (s, 9H,
SiMe3), 1.24 (s, 9H, t-Bu), 6.74 (m, 1H, Ar), 7.02 (m, 1H, Ar), 7.31
(m, 1H, Ar) and 7.60 (m, 1H, Ar) that are different from those
of LLi. The significant shift of the resonance for the SiMe3 group
form 0.48 for 1 to 0.11 ppm for the additional species points to
the formation of a OSiMe3 group, as this is the typical region for
this type of silicon substituents.25
Scheme 1
Synthesis of the complexes
The reaction of [MoO2Cl2]33 or [WO2Cl2(dme)]34 (dme =
dimethoxyethane) in tetrahydrofuran with two equivalents of LLi
in ethereal solvents at low temperature leads after work up by
extraction with pentane and crystallisation at −35 ◦C to yellow
crystalline compounds [MoO2L2] (1) and [WO2L2] (2) as shown in
Scheme 2.
Scheme 3 Formation of compounds 3 and 4.
Scheme 2 Synthesis of bisamido complexes.
The formation of 3 involves a SiMe3 group migration from the
nitrogen to the oxygen atom. This type of migration has previously
been previously described by Wilkinson and co-workers in group
6 metal oxo chemistry.25
For the synthesis of 1 it is critical that the temperature during
the reaction, as well as during work-up, remains low (approx.
−5 ◦C). Higher temperatures lead to mixtures of 1 and two other
compounds described below. The isolated compounds 1 and 2 can
be stored under a nitrogen atmosphere at room temperature for an
unlimited amount of time, but by exposing the yellow crystals or
solutions thereof to the laboratory atmosphere quick hydrolysis
Further evidence for the silyl group migration gives the crys-
ꢀ
=
tal structures analysis of a bisimido compound [Mo( NL )2-
(OSiMe3)L] (4, Lꢀ = N-2-t-BuSC6H4) that is derived from
compound 3. Single crystals of 4 suitable for X-ray diffraction
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were obtained from reactions of [MoO2Cl2] and LLi in THF
at room temperature, subsequent extraction with pentane and
crystallisation at −35 ◦C. The thus obtained orange crystalline
material contained 1, 3 and 4 in various ratios depending on
the crystallizing conditions, evidenced by 1H NMR spectroscopy.
Mass spectrometry shows peaks at m/z 634 and 797 with correct
isotopic distribution for 1 (or 3) and 4, respectively. Changing the
ratio of [MoO2Cl2]/LLi to 1 : 3 according to the stoichiometry
of the reaction slightly increased the amount of 4 in comparison
to 1 and 3, however pure 4 could not be obtained. This points
to equilibria of the involved reactions, so that slight changes
of the reaction times and crystallizing conditions lead to varied
distributions of 1, 3 and 4. The mechanism of formation of 3
and 4 is as yet unknown. However, a high-temperature 1H NMR
spectrum of isolated 1 in THF-D8 shows only resonances for
1 and also the addition of 1 equiv. of LLi to a sample of 1
shows no evidence of 3 or 4. Thus, silyl group migration must
occur before the formation of 1. We assume that 3 is formed
stepwise after substitution of one chlorine atom of [MoO2Cl2] by
the nitrogen ligand followed by silyl group migration. The driving
force for the substitution with a third equivalent of LLi in 4 is
whether the steric bulk of the t-Bu group is stabilizing compounds
1 and 2 and whether increased silyl group migration hampers the
isolation with analogous anilido ligands.
We were interested why the remaining OSiMe3 group in 4 is
not substituted by a second amido ligand under elimination of a
further equivalent of LiOSiMe3. For this reason we investigated the
reaction of a preformed imido complex with various amounts of
39
=
LLi. Treatment of [Mo( NAr)2Cl2(dme)] (Ar = 2,6-Me2C6H3)
with one equivalent of LLi in diethyl ether at room temperature
overnight and usual work-up by extraction with toluene gave 5 as
bright orange crystals (eqn (1)).
(1)
The 1HNMRspectrum of 5 shows one resonance for the protons
of the SiMe3 group (d 0.36 ppm) and one for those of the t-Bu
group (d 1.35 ppm). The protons of the methyl groups at the
aromatic ring systems appear as one broad singlet (d 1.78 ppm)
and multiplets for the 10 aromatic protons can be found between
d 6.60 and 7.48 ppm. The broad resonance for the four methyl
groups, indicative of a dynamic process, splits into two broad
resonances at 1.9 and 2.4 ppm upon cooling to −80 ◦C. This can be
explained by a structural situation where the two imido groups are
inequivalent but rotation along N–aryl bond still occurs. At room
temperature the additional dynamic process most likely involves
reversible S–Mo bond breaking and formation. Similar behaviour
has been found in bisimido complexes with bidentate phenolate
38
likely the high oxophilicity of lithium forming LiOSiMe3 after
the disappearance of the oxo groups by rearrangement to the
bisimido form. It is somewhat surprising, that the second siloxide
ligand is not substituted. However, this can be explained by steric
considerations (see below).
With pyridine-functionalised amido ligands no silyl group
migration is reported.23 The difference in reactivity is interesting.
The conversion of the amido into the imido group involves a
substantial movement of the aromatic ring as shown in Scheme 4.
Imido groups in d0 complexes show usually linear M–N–C units
with sp hybridised N atoms.25 Once 1 is formed this movement
is less likely due to steric reasons. In the pyridine-functionalised
amido complexes, the stronger bond to the pyridyl nitrogen atom
in comparison to the crowded sulfur atom presumably prevents
this movement. Whether silyl group migration occurs or not is
apparently dependant on a delicate equilibrium between the bond
strengths within the involved species. For this reason, tungsten
compound 2 is formed in the absence of silyl group migration.
=
ligands [Mo( NAr)2(ON)X], where the four ortho-substituents
on the imido aryl groups are found to be equivalent at room
temperature.40 Electron impact mass spectrometry (peak at m/z
623 for the molecular ion) and elemental analysis support the
formation of 5. In addition, X-ray diffraction analysis of a suitable
single crystal confirms the structure shown in eqn (1).
Interestingly, treatment of [Mo(NAr)2Cl2(dme)] with two or
more equivalents of LLi does not lead to the substitution of two
chloride ligands but rather compound 5 is isolated in all attempts.
Apparently, the steric bulk of the amido ligand prevents a second
substitution. This provides an explanation for the question why
the second siloxide ligand in the bisimido compound 4 is not
substituted by L.
Crystallographic studies
The crystal structures of complexes 1, 2, 4 and 5 were determined
by single-crystal X-ray techniques. The molecular structures are
displayed in Fig. 2–5. Selected bond lengths and angles are
presented in Table 1, while crystal data and refinement details
are given in Table 2.
Scheme 4 Comparison to pyridine-functionalised amido ligands.
Complexes 1 and 2 are isostructural and exhibit distorted
octahedral metal centres coordinated by two mutually cis terminal
oxo and two bidentate amido ligands. The two nitrogen atoms are
transoid to each other with N1–M–N2 angles of 146.1(1)◦ in 1 and
146.7(1)◦ in 2 thus rendering the sulfur atom approximately trans
It is worth noting that with analogous ligands to L having 2-
MeS or 2-MeO groups instead of 2-t-BuS, we were not able to
isolate any pure compounds, although mass spectrometry pointed
to the existence of [MO2L2] species. At this point it is not clear
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◦
ꢀ
˚
Table 1 Selected bond distances (A) and angles ( ) for complexes [MoO2(L)2] (1), [WO2(L)2] (2), [Mo(NL )2(OSiMe3)L] (4) and [Mo(NAr)2(L)Cl] (5)
1
Mo1–S1
Mo1–S2
2.805(1)
2.775(1)
Mo1–N1
Mo1–N2
2.044(2)
2.059(2)
Mo1–O1
1.709(2)
Mo1–O2
1.704(2)
O2–Mo1–N1
O1–Mo1–N2
O2–Mo1–N2
S2–Mo1–S1
97.3(1)
96.7(1)
103.5(1)
78.8(1)
O1–Mo1–O2
O1–Mo1–N1
N1–Mo1–N2
O1–Mo1–S2
104.9(1)
103.4(1)
146.1(1)
165.0(1)
O2–Mo1–S2
N2–Mo1–S2
N1–Mo1–S2
O1–Mo1–S1
89.0(1)
74.1(1)
80.0(1)
88.1(1)
O2–Mo1–S1
N1–Mo1–S1
N2–Mo1–S1
165.9(1)
73.9(1)
79.9(1)
2
W1–S1
W1–S2
2.787(1)
2.766(1)
W1–N1
W1–N2
2.038(2)
2.055(2)
W1–O1
1.720(2)
W1–O2
1.725(2)
O2–W1–N1
O1–W1–N2
O2–W1–N2
S2–W1–S1
103.1(8)
103.4(8)
96.8(8)
78.2(3)
O1–W1–O2
O1–W1–N1
N1–W1–N2
O1–W1–S2
104.3(1)
97.3(1)
146.7(1)
89.6(1)
O2–W1–S2
N2–W1–S2
N1–W1–S2
O1–W1–S1
165.1(1)
74.2(1)
80.1(6)
166.0(5)
O2–W1–S1
N1–W1–S1
N2–W1–S1
88.6(6)
74.2(6)
79.9(6)
4
Mo1–N1
Mo1–N2
1.747(2)
1.763(2)
Mo1–N3
Mo1–O1
2.009(2)
1.920(1)
Mo1–S3
Mo1–S1
2.906(1)
4.074(1)
Mo1–S2
4.487(1)
Mo1–N1–C4
Mo1–N2–C14
N1–Mo1–N2
168.0(1)
152.3(1)
106.1(1)
N1–Mo1–O1
N2–Mo1–N3
N1–Mo1–N3
116.5(1)
98.3(1)
104.7(1)
N1–Mo1–S3
N2–Mo1–O1
O1–Mo1–N3
76.9(1)
104.7(1)
123.8(1)
N2–Mo1–S3
O1–Mo1–S3
N3–Mo1–S3
172.0(1)
80.1(1)
73.7(1)
5
Mo1–N1
Mo1–N2
2.020(2)
1.753(2)
Mo1–N3
1.754(2)
Mo1–S1
2.737(1)
Mo1–Cl1
2.382(1)
Mo1–N2–C7
Mo1–N3–C8
N2–Mo1–S1
168.3(2)
153.6(2)
167.0(1)
N2–Mo1–N1
N3–Mo1–N1
N2–Mo1–N3
101.2(1)
105.7(1)
107.2(1)
N3–Mo1–S1
N1–Mo1–S1
N2–Mo1–Cl1
85.7(1)
76.3(1)
94.3(1)
N3–Mo1–Cl1
N1–Mo1–Cl1
Cl1–Mo1–S1
107.4(1)
137.1(1)
79.7(1)
Table 2 Crystal data and refinement details for complexes 1, 2, 4 and 5
1
2
4
5
Formula
M
C26H44MoN2O2S2Si2
632.87
Triclinic
¯
P1
C26H44N2O2S2Si2W
720.78
Triclinic
¯
P1
C36H57MoN3OS3Si2
796.15
Triclinic
¯
P1
11.834(2)
12.365(3)
15.082(3)
78.65(3)
76.75(3)
76.53(3)
C29H40ClMoN3SSi
622.18
Monoclinic
P21/n
8.982(2)
20.696(3)
16.862(3)
90
101.65(2)
90
Crystal system
Space group
˚
a/A
9.622(3)
9.610(2)
˚
b/A
12.052(3)
13.693(4)
85.06(1)
82.80(1)
88.44(3)
12.010(2)
13.775(3)
85.31(3)
82.58(3)
88.53(3)
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
1569(1)
1571(1)
2065(1)
3070(1)
Crystal size/mm
0.20 × 0.10 × 0.10
0.25 × 0.02 × 0.01
0.05 × 0.05 × 0.02
0.20 × 0.15 × 0.10
Dc/Mg m−3
1.339
1.524
1.280
1.346
Z
2
2
2
4
l/mm−1
5.582
3.26 to 58.92
14417
4334 (0.0347)
0.0259, 0.0613
0.0351, 0.0651
8.975
3.25 to 58.86
14658
4390 (0.0229)
0.0160, 0.0396
0.0168, 0.0400
4.799
3.05 to 58.87
22428
5702 (0.0279)
0.0201, 0.0513
0.0220, 0.0525
5.464
3.42 to 59.05
27816
4397 (0.0471)
0.0276, 0.0729
0.0280, 0.0733
h range/◦
No. observed data
No. indep. reflns (Rint
R1,awR2b (I > 2r(I))
R1,awR2b (all data)
)
ꢀ
ꢀ
ꢀ
ꢀ
2
a R = ꢁFo| − |Fcꢁ/ |Fo|. b wR2 = [ w(Fo − Fc2)2/ w(Fo2)2]1/2
.
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ꢀ
=
Fig. 4 Molecular structure of [Mo( NL )2(OSiMe3)L] (4) showing the
Fig. 2 Molecular structure of [MoO2L2] (1). Hydrogen atoms have been
omitted for clarity.
weak interaction between sulfur and molybdenum.
Fig. 3 Molecular structure of [WO2L2] (2). Hydrogen atoms have been
omitted for clarity.
=
Fig. 5 Molecular view of [Mo( NAr)2LCl] (5).
◦
=
to the oxo groups (trans S–M–O 165.0–166.0 ). The M O bond
˚
˚
lengths are 1.709(2) A (Mo1–O1), 1.701(2) A (Mo1–O2), 1.704(2)
˚
˚
A (W1–O1) and 1.725(2) A (W1–O2), which is in the typical
weak interaction to the thioether sulfur atom. The geometry
of these five ligands about the metal centre is best described
as disto◦rted trigonal bipyramidal (largest angle: N2–Mo1–S3
172.0(1) ), second largest angle: N3–Mo1–O1 123.8(1)◦),43 where
the imido nitrogen atom N2 and the sulfur atom S3 are in the apical
positions (N2–Mo1–S3 172.0(1)◦). The two sulfur atoms within
the imido ligands are not close enough to the metal centre for a
region for group 6 oxo bonds.23,25,41 The M–N bond distances
in 1 and 2 (Mo1–N1 2.044(2) A, Mo1–N2 2.059(2) A, W1–N1
2.038(2) A and W1–N2 2.055(2) A are comparable to another
report of amido complexes of this type (2.079(2)–2.081(2) A).
Molybdenum–sulfur distances are Mo1–S1 2.805(1) A and Mo1–
S2 2.775(1) A, which are significantly shorter than in 4 pointing to
a stronger Mo–S interaction in the bisamido complex. Tungsten–
˚
˚
˚
˚
23
˚
˚
˚
˚ ˚
bonding interaction (Mo1–S1 4.074(1) A and Mo1–S2 4.487(1) A),
˚
˚
˚
sulfur distances are W1–S1 2.787(1) A and W1–S2 2.766(1) A,
which are similar to those reported in literature.42
whereas the Mo1–S3 is 2.906(1) A, which is significantly longer
than in 1 and 5 (see below) but can be considered as a weak
interaction belonging to the longer bonds reported for this type of
compounds.44
In complex 4, the Mo(VI) centre is coordinated by two imido
ligands, one siloxide and one amido ligand with an additional
1298 | Dalton Trans., 2006, 1294–1301
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˚
prepared according to literature procedures. All other chemicals
mentioned were used as purchased from commercial sources
(Aldrich, Merck).
The Mo1–O1 in 4 is 1.920(1) A, which corresponds to
molybdenum–oxygen single bonds described in literature.45–47
The bond length to the amido nitrogen atom (Mo1–N3)
˚
Samples for mass spectrometry were measured on a BIO-RAD
Digilab FTS-7 mass spectrometer with a Finnigan MAT 95 and
all NMR spectra on a Bruker Avance 500 or 200 MHz. Elemental
analyses were performed by the Analytisches-Chemisches Labo-
ratorium des Instituts fu¨r Anorganische Chemie, Go¨ttingen. IR
spectra were recorded on Bio-Rad Digilab FTS-7 spectrometer as
Nujol mulls between KBr plates.
is 2.009(2) A, that should be compared to the few other
bisimido amido complexes reported in the literature. For example,
[Mo(NAr)2{N(SiMe3)2}Cl] exhibits a metal–amido bond distance
˚
of 1.947(3) A and [Mo(NAr)2{N(SiMe3)2}(NMe2)] 2.012(1) and
48,49
˚
1.953(2) A, respectively.
˚
The imido N1 ligand exhibits a Mo1–N1 distance of 1.747(2) A
and a linear Mo1–N1–C4 angle of 168.0(1)◦ consistent with the
expected sp hybridised nitrogen atom and a M≡N triple bond.40
The other imido ligand shows a longer bond distance (Mo1–N2
X-Ray crystallographic determinations
˚
1.763(2) A) and a significantly less obtuse Mo1–N2–C14 angle of
152.3(1)◦, which is at the low end of the linear coordination. Bent
Crystals of compounds 1, 2, 4 and 5 were taken from the solution,
covered with oil, mounted on glass fibres and placed immediately
in a protective stream of cold nitrogen (100 K). Data were
collected on a Bruker three-circle diffractometer equipped with
Smart 6000 CCD area detector using mirror-monochromated Cu-
structures with angles <150◦ are consistent with M N double
=
bonds and a lone pair at the nitrogen atom.40,48 In compound 4, the
lone pair at nitrogen is likely to be involved in bonding interactions
to the d0 metal due to the otherwise low electron count suggesting
steric reasons for the low metal–nitrogen carbon angle.26,40,50
Comparison of compounds 4 and 5 reveals close overall
geometries, with the latter showing a more pronounced distortion
in the trigonal bipyramid (largest angle: N2–Mo1–S1 167.0(1)◦),
second largest angle: N1–Mo1–Cl2 137.1(1)◦). Again, in 5 both
imido groups exhibit different coordination modes (Mo1–N2
˚
Ka radiation (k = 1.54178 A). The structures were solved by
direct methods using SHELXS-9752 and refined against F2 on
all data by full-matrix least-squares with SHELXL-97.53 All non-
hydrogen were refined anisotropically, while all hydrogen atoms
were included in the model at geometrically calculated positions
and refined using a riding model with Uij tied to the parent atom.
CCDC reference numbers 292971 (1), 286078 (2), 286079 (4)
and 284752 (5).
◦
˚
˚
1.753(2) A; Mo1–N2–C7 168.3(2) and Mo1–N3 1.754(2) A;
Mo1–N3–C8 153.6(2)◦.46,47 However, in compound 4 the more
bent N2 imido ligand is in axial position (trans to the thioether
sulfur atom), whereas in compound 5 the more bent N3 imido
group is found in equatorial position. Apparently, the energy
difference between the linear and bent form is small, so that steric
factors determine the structure.40 This has also been found by
molecular orbital calculations on [Cp*2Ta(NR)H], where the two
forms are found to be only slightly different in energy.51
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b514873a
Syntheses
Synthesis of N-trimethylsilyl-2-tert-butylmercaptoaniline (LH).
10 g (0.055 mol) of 2-tert-butylsulfanylaniline32 were dissolved in
diethyl ether (30 ml), cooled to 0 ◦C and n-BuLi (22 ml of a 2.5 M
solution, 0.055 mol) was added slowly. After 1 h stirring at room
temperature the mixture was cooled to 0 ◦C and 6.9 ml (0.055 mol)
of freshly distilled Me3SiCl were added. The reaction mixture was
allowed to come to room temperature and after 1 h the solvent
volume was reduced in vacuo until a residual oil remained, which
was distilled at high vacuum at 73 ◦C, giving 11.5 g (83%) of the
product as a colourless oil.
1H NMR (200 MHz, C6D6): d 0.16 (s, 9H, Si(CH3)3), 1.20 (s,
9H, SC(CH3)3), 5.56 (br s, 1H, NH), 6.6 (m, 1H, H-Ar) 6.83
(m, 1H, H-Ar), 7.06 (m, 1H, H-Ar), 7.55 (m, 1H, H-Ar); 13C
NMR (500 MHz, C6D6): d −0.2 (Si(CH3)3), 31.0 (SC(CH3)3), 47.5
(SC(CH3)3), 114.7, 117.4, 118.6, 130.9, 140.1, 151.7 (C-Ar).
Conclusion
The work reported here shows that the silyl amido ligand L with a
chelating tert-butylthioether functionality allows the synthesis of
d0 tungsten dioxo complex. The analogous bisamido molybdenum
compound can only be obtained by careful addition of two equiv
of LLi at low temperature. Room-temperature conditions promote
silyl group migration from the nitrogen to the oxygen atom and
subsequent substitution of a siloxide by an amido ligand. This
is in contrast to an earlier report of [MoO2]2+ complexes with
silyl amido ligands where no migration is observed.23 Keeping this
in mind, our results allow the conclusion that a stable bidentate
coordination of the ligand systems prevents migration, which is
relevant for the design of new ligands for high oxidation state
molybdenum and tungsten oxo complexes.
Synthesis of LLi. To a −20 ◦C cold solution of LH (3.2 g, 0.013
mol) in pentane was added n-BuLi (5.0 ml of a 2.5 M solution,
0.013 mol) dropwise via syringe during which a white precipitate
appeared. The mixture was allowed to come to room temperature
and stirred for an additional hour. The white precipitate was
collected on a frit and rinsed with pentane, dried in vacuo to yield
2.6 g (81%) of the product as a white solid.
Experimental
General
All manipulations were carried out under dry nitrogen using
standard Schlenk line or glove box techniques. All solvents were
purified by standard methods and distilled under a nitrogen
1H NMR (200 MHz, C6D6): d 0.30 (s, 9H, Si(CH3)3), 1.07 (s,
9H, SC(CH3)3), 6.60–7.45 (m, 4H, H-Ar); 13C NMR (500 MHz,
C6D6–THF-d8 5 : 1): d 2.3 (Si(CH3)3), 30.9 (SC(CH3)3), 47.8 (quat.
C), 109.3, 117.9, 121.7, 130.5, 139.7, 166.1 (C-Ar); IR (KBr, cm−1):
1574m, 1458s, 1375s, 1283m, 1245m, 1158m, 1034m, 919m, 826m.
atmosphere immediately prior to use. 2-tert-mercaptoaniline,32
39
[WO2Cl2(dme)],34 [MoO2Cl2]33 and [Mo( NAr)2Cl2(dme)] were
=
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Synthesis of [MoO2L2] (1). To a precooled solution of LH
(0.504 g, 0.002 mol) in THF, n-BuLi (0.8 ml of a 2.5 M solution,
0.002 mol) was slowly added via syringe. The mixture was
warmed to room temperature, stirred for 30 min, than cooled to
−5 ◦C. This solution was added dropwise to a pale-yellow solution
of [MoO2Cl2]33 in THF (0.2 g, 0.001 mol) at −5 ◦C. The mixture
was left to stir at −5 ◦C for 30 min. After removing the solvent in
vacuo, the obtained solid was extracted with hexane. The orange
solution was filtered over Celite, concentrated to approx. 5 ml
and left in the freezer at −35 ◦C overnight, where yellow crystals
appeared (0.24 g, 38%).
1H NMR (200 MHz, C6D6): d 0.71 (s, 9H, NSi(CH3)3) 1.23
(s, 9H, SC(CH3)3), 6.52 (m, 1H, H-Ar), 6.95–7.05 (m, 3H,
H-Ar); 13C NMR (500 MHz, C6D6): d 2.6 (NSi(CH3)3), 28.5
(SC(CH3)3), 55.9 (SC(CH3)3), 119.9, 121.7, 122.5, 130.8, 136.4,
163.3 (C-Ar); MS (EI): m/z 634 (30%) [MoO2L2]+; Anal. Calc. for
C26H44MoN2O2S2Si2: C, 49.34; H, 7.01; N, 4.43. Found: C, 49.10;
H, 6. 89; N, 4.21%.
solution, 0.002 mol) was slowly added via syringe. The mixture
was warmed to room temperature, stirred for 30 min, than added
dropwise via cannula to a cold (−20 ◦C) deep red solution
39
=
of [Mo( NAr)2Cl2(dme)] in THF (0.988 g, 0.002 mol). The
solution was allowed to come to room temperature and stirred
for additional 4 h. The solvent was removed in vacuo giving a
brown solid, which was extracted with toluene. The solution was
filtered over Celite, concentrated to approx. 10 ml and left at
◦
−30 C. Bright orange crystals appeared that were filtered from
the solution, washed thoroughly with hexane and dried in vacuo
giving 0.64 g (52%) of the product.
1H NMR (500 MHz, toluene-d8): d 0.4 (s, 9H, NSi(CH3)3),
1.47 (s, 9H, SC(CH3)3), 1.90 (br s, 6H, CH3–Ar), 2,40 (br s, 6H,
CH3–Ar) 6.55–6.81 (m, 7H, H-Ar), 6.85–6.95 (m, 2H, H-Ar), 7.35
(d, 1H, H-Ar); 13C NMR (500 MHz, C6D6): d 3.4 (NSi(CH3)3),
18.7 (CH3–Ar), 29.8 (SC(CH3)3), 51.7 (quat. C), 121.3, 122.4,
126.1, 129.1, 138.0, 163.0 (C-Ar); MS (EI): m/z 623 (100%)
[Mo(NAr)2ClL]+; Anal. Calc. for C26H44N2O2S2Si2W: C, 55.85; H,
6.42; N, 6.74. Found: C, 55.02; H, 6.32; N, 6.46%; IR (KBr, cm−1):
1577m, 1308m, 1263s, 1246m, 1160m, 1027m, 897m, 871m, 840s,
814s, 764s, 728s.
Synthesis of [WO2L2] (2). To a precooled solution of LH
(0.504 g, 0.002 mol) in THF n-BuLi (0.8 ml of a 2.5 M solution,
0.002 mol) was slowly added via syringe. The mixture was warmed
to room temperature, stirred for 30 min, than added dropwise
to a colourless solution of [WO2Cl2(dme)]34 in THF (0.377 g,
0.001 mol). The mixture became immediately orange and after
40 min the colour changed to brown–orange. After 2 h stirring
at room temperature, the solvent was removed in vacuo and the
product extracted with pentane. The pale-yellow solution was
filtered over Celite, concentrated to approx. 5 ml and left in
the freezer at −35 ◦C overnight. Pale-lemon microcrystals were
collected on a frit, and dried in vacuo giving 0.35 g (55%) of the
product.
1H NMR (500 MHz, C6D6): d 0.74 (s, 9H, NSi(CH3)3), 1.25
(s, 9H, SC(CH3)3), 6.49–7.12 (m, H-Ar); 13C NMR (500 MHz,
C6D6): d 2.6 (NSi(CH3)3), 28.2 (SC(CH3)3), 55.2 (SC(CH3)3),
119.9, 121.1, 123.7, 130.9, 136.4, 163.3 (C-Ar); MS (EI): m/z 720
(100%) [WO2L2]+; Anal. Calc. for C26H44N2O2S2Si2W: C, 43.32; H,
6.15; N, 3.89. Found: C, 43.27; H, 6.32; N, 3.82%; IR (KBr, cm−1):
1580m, 1289m, 1251m, 1158m, 945s, 902s, 859m, 839m, 724m.
Acknowledgements
We would like to thank the Deutsche Forschungsgemeinschaft for
generous financial support (grant MO 963/2).
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◦
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ꢀ
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=
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=
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1300 | Dalton Trans., 2006, 1294–1301
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©