Efficient uptake of dimethyl sulfoxide by the desoxomolybdenum(iv) dithiolate complex containing bulky hydrophobic groups

Article abstract of DOI:10.1039/c5dt00075k

A desoxomolybdenum(iv) complex containing bulky hydrophobic groups and NH...S hydrogen bonds, (Et₄N)[Mo^IV(OSi^tBuPh₂)(1,2-S₂-3,6-{(4-^tBuC₆H₄)₃CCONH}₂C₆H₂)₂], was synthesized. This complex promotes the oxygen-atom-transfer (OAT) reaction of DMSO by efficient uptake of the substrate into the active center. The clean OAT reaction of Me₃NO is also achieved. This journal is

Full text of DOI:10.1039/c5dt00075k

Dalton
Transactions
PAPER
Efficient uptake of dimethyl sulfoxide by the
desoxomolybdenum(IV) dithiolate complex
containing bulky hydrophobic groups†
Cite this: Dalton Trans., 2015, 44,
6260
Yuki Hasenaka, Taka-aki Okamura* and Kiyotaka Onitsuka
Received 8th January 2015,
Accepted 23rd February 2015
A desoxomolybdenum(IV) complex containing bulky hydrophobic groups and NH⋯S hydrogen bonds,
IV
t
t
(
Et
motes the oxygen-atom-transfer (OAT) reaction of DMSO by efficient uptake of the substrate into the
active center. The clean OAT reaction of Me NO is also achieved.
4 2 2 6 4 3 2 6 2 2
N)[Mo (OSi BuPh )(1,2-S -3,6-{(4- BuC H ) CCONH} C H ) ], was synthesized. This complex pro-
DOI: 10.1039/c5dt00075k
www.rsc.org/dalton
3
Introduction
Dimethyl sulfoxide reductase (DMSOR), one of the molybdoen-
zymes with two unique dithiolene ligands and an alkoxo
ligand, catalyzes an oxygen-atom-transfer (OAT) reaction of
DMSO via Mo(IV) and Mo(VI) oxidation states. A molybdenum(IV)
center reductively eliminates an oxygen atom from the sub-
strates such as sulfoxides and amine N-oxides, resulting in the
VI
1,2
formation of a Mo vO bond (Chart 1).
DMSOR is involved in anaerobic respiration using dilute
3
DMSO as an energy source in aqueous media. The enzyme
has high affinity for DMSO with K
m
values in the micromolar
4
range. Its molecular crystal structure shows that the molyb-
denum site is located at the base of a large funnel-shaped
2
,5
Chart 1 The structure of the active site and the catalytic cycle of
dimethyl sulfoxide reductase.
depression, and the ligands are not exposed to the surface
6
(
Chart 1). The aromatic residues at the base of the depression
provide a hydrophobic pocket to capture the methyl groups of
1
the substrate. The above results suggest the importance of the
hydrophobic microenvironment for efficient uptake of the
substrates.
for the “desoxo”-molybdenum center of DMSOR, synthesized
by simple modification of the corresponding monooxomolyb-
1
4
denum(IV) complex. Moreover, a structural analogue, alkoxo-
molybdenum(IV) dithiolate complex (8, Chart 2), has been
reported to promote the OAT reaction of DMSO, a biological
substrate; however, because the complex has low reactivity,
A number of molybdenum complexes have been reported as
7
–13
structural and OAT reaction models.
In particular, a siloxo-
molybdenum(IV) complex with benzene-1,2-dithiolate (bdt)
ligands (7, Chart 2) has been reported as a structural model
1
5,16
excess DMSO was used.
The DFT calculations of the models indicated that the tran-
sient distortion of the square-pyramidal geometry that affords
a vacant site cis to the apical ligand is important for the
binding of the substrate. If the active center is covered with
adequately bulky hydrophobic groups, and as a result, dis-
Department of Macromolecular Science, Graduate School of Science,
Osaka University, Toyonaka, Osaka 560-0043, Japan.
E-mail: tokamura@chem.sci.osaka-u.ac.jp; Fax: +81 6 6850 5474;
Tel: +81 6 6850 5451
17
†
Electronic supplementary information (ESI) available: X-ray crystallographic
1
data, H NMR spectra of 1, resonance Raman spectra of 1 and 3, UV-vis spec- torted by the bulkiness, then both efficient uptake of the sub-
trum of 3, UV-vis spectral change for silylation of 4 and 5, stability of 1 in DMF
strate and enhancement of the reactivity to DMSO are expected
to be achieved.
In our earlier studies, we have demonstrated that the
3
introduction of four bulky CPh CONH substituents onto the
1
and an aqueous micellar solution, H NMR spectra of the reaction mixture of 1
and DMSO, kinetic plots of the reaction of DMSO, UV-vis spectrum of molyb-
denum(V) species. CCDC 1041817 (1·5CH
CH CN·5/2H O). For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c5dt00075k
3 2
CN·2H O) and 1041818 (3·7/
2
3
2
monooxomolybdenum(IV) bdt complex resulted in a dramatic
6260 | Dalton Trans., 2015, 44, 6260–6267
This journal is © The Royal Society of Chemistry 2015

Dalton Transactions
Paper
react with DMSO owing to the strong donation of the oxo
ligand to the Mo(IV) center.
Here, the synthesis and reactivity of a desoxomolybdenum(IV)
IV
t
t
complex, (Et
4 2 2 6 4 3
N)[Mo (OSi BuPh )(1,2-S -3,6-{(4- BuC H ) -
CCONH} C H ) ] (1, Scheme 1), are reported. We showed that
2
6
2 2
simple modification of 4 that maintains the solubility and
coordination structure results in a dramatic enhancement of
the reactivity of the complex to DMSO. Unlike complex 7 with
unsubstituted bdt ligands, 1 promotes the OAT reaction of
DMSO in both polar and nonpolar solvents, thus demon-
strating the efficient uptake of DMSO in the hydrophobic
microenvironment. Monooxotungsten(VI) complex
3
was
also synthesized and isolated as crystals to investigate its
coordination structure in the higher oxidation state.
Experimental
All procedures were performed under an argon atmosphere
by the Schlenk technique. All solvents were dried and
IV
distilled under argon before use. (Et
4
N)
2
[Mo O(1,2-S
2
-3,6-
Chart 2 Designation of molybdenum and tungsten complexes (R =
t
t
VI
C(C
6
H
4
-4- Bu)
3
).
{(4- BuC H ) CCONH} C H ) ] (4), (Et N) [Mo O (1,2-S -3,6-
6
4 3
2
6
2 2
4
2
2
VI
2
t
{
3
(4- BuC
6
H
4
)
3
CCONH}
2
C
6
H
2
)
2
] (5), and (Et
4
N)
2
[W O
2
2
(1,2-S -
t
,6-{(4- BuC
6
H
4
)
3
CCONH}
2
C
6
H
2
)
2
] (6) were prepared by the
2
1
reported method.
3
acceleration of the OAT reaction of Me NO via the “cis-attack”
mechanism. This is due to the stabilization of the distorted
IV
t
t
1
8–20
(Et
4
N)[Mo (OSi BuPh
2
)(1,2-S
2
-3,6-{(4- BuC
6
H
4
)
3
CCONH}
2
-
intermediate.
Furthermore, we have recently reported that the monooxo-
molybdenum(IV) complex containing bulky hydrophobic This compound was synthesized by a modified method
C H ) ] (1)
6
2 2
IV
t
14
dithiolate ligands, i.e. (Et N) [Mo O(1,2-S -3,6-{(4- BuC H ) - reported in the literature. To a solution of 4 (17.5 mg,
4
2
2
6
4 3
t
CCONH}
2
C
6
H
2
)
2
] (4, Scheme 1), was soluble in nonpolar sol- 7.09 μmol) in toluene (0.7 mL) was added BuPh SiCl (3 μL,
2
vents such as toluene. The approach of a polar substrate, 12 μmol), and the reaction mixture was stirred for 1 h to afford
Me NO, to the active center was found to be more efficient in a brownish-yellow suspension. After removal of colorless pre-
3
2
1
hydrophobic media.
The hydrophobic microenvironment cipitates by filtration, the filtrate was evaporated under
formed by bulky hydrophobic moieties was expected to simu- reduced pressure. The resulting brownish-yellow residue was
late the hydrophobic pocket of DMSOR; however, 4 did not recrystallized from toluene–acetonitrile to afford light green
1
blocks. Yield: 17.0 mg, 93%. H NMR (toluene-d
8
): δ 8.54 (s,
4H, NH), 8.10 (s, 4H, 4,5-H), 7.68 (d, J = 8.7 Hz, 24H, Ar), 7.41
(
6
0
(
m, 4H, PhSi), 7.33 (d, J = 8.7 Hz, 24H, Ar), 7.18 (m, 2H, PhSi),
+
t
4
.95 (m, 4H, PhSi), 1.36 (br, 8H, Et N ), 1.25 (s, 108H, Bu),
t
+
.94 (s, 9H, BuSi), 0.03 (br, 12H, Et N ). Absorption spectrum
4
toluene): λmax (ε, M− cm ) 363 (17 000), 420 (sh) (7300) nm.
1
−1
203 5 5 4
Anal. Calcd for C164H N O S
MoSi: C, 76.45; H, 7.94; N, 2.72.
Found: C, 75.17; H, 7.99; N, 2.66.
As described in a previous paper, the disagreement of
elemental analysis for molybdenum complexes is probably
caused by their nanoporous structure in the solid state
2
1
(
CIF†). After removal of the solvent molecules from the crys-
tals in vacuo, the resulting voids might be able to trap a trace
amount of water even under a dry argon atmosphere. Formal
addition of water to the chemical formula improved the
results. The calculated values for
164 203 5 5 4
C H N O S MoSi·
(
H O)2.5: C, 75.13; H, 8.00; N, 2.67 are in agreement with
2
those observed. The water molecules were also detected in
1
H NMR spectra, but the amount of water depended on the
Scheme 1 Synthesis of the complexes 1–3.
conditions.
This journal is © The Royal Society of Chemistry 2015
Dalton Trans., 2015, 44, 6260–6267 | 6261

Paper
Dalton Transactions
VI
t
t
(Et
4
N)[Mo O(OSi BuPh
2
)(1,2-S
2
-3,6-{(4- BuC
6
H
4
)
3
CCONH}
2
-
contains 7/2(CH
3
CN) and 5/2(H
2
O) but the actual crystal must
C H ) ] (2)
contain more solvents in the void. Crystallographic data are
6
2 2
shown in Tables S1.†
This compound was not so thermodynamically stable to be iso-
lated, therefore it was prepared in situ to obtain absorption
Density functional theory (DFT) calculations
spectra. To a solution of 5 (1 mM, 0.3 mL) in toluene in a
t
1
mm UV cell was added a BuPh
2
SiCl solution (128 mM, 4 μL) Geometry optimization and vibrational calculations were per-
in toluene, and the cell contents were quickly mixed by formed using Becke’s three-parameter hybrid functionals
2
6
shaking and allowed to stand for 8.5 h at 27 °C. The product (B3LYP) in the Gaussian 03 program package. The basis set
was used for spectral measurements without further purifi- used for Mo and W was LanL2DZ. For other atoms (H, C, N, O,
−1
−1
cation. Absorption spectrum (toluene): λmax (ε, M cm ) 371 Si, S), a 6-31G** basis set was employed. The coordinates of
IV
t
14
(
8500), 465 (sh) (5000), 583 (4900), 770 (sh) (2100) nm.
the crystal structures, (Et N)[Mo (OSi BuPh )(bdt) ]
and
4
2
2
VI
t
22
(
Et
4
N)[W O(OSi BuPh
2
)(bdt)
2
],
were used for the initial
structures with some modifications.
VI
t
t
(
4 2 2 6 4 3 2
Et N)[W O(OSi BuPh )(1,2-S -3,6-{(4- BuC H ) CCONH} -
C H ) ] (3)
6
2 2
This compound was synthesized by a modified method Kinetic measurements
2
2
reported in the literature. To a solution of 5 (30.9 mg,
3
A reaction system containing 1 and Me NO was monitored
t
1
2.0 μmol) in toluene (1 mL) was added BuPh
2
SiCl (5 μL,
spectrophotometrically in the region 280–1000 nm. The
measurements were carried out in a 1 cm UV cell containing a
toluene solution of 1 (0.1 mM, 3.0 mL) at 27 °C. After thermal
1
9 μmol). The reaction mixture was stirred at r.t. for 10 min.
After removal of colorless precipitates by filtration, the filtrate
was evaporated. The resulting dark reddish-purple powder was
equilibrium, a Me NO solution (60 mM, 10 μL) in DMF was
3
recrystallized from toluene–acetonitrile to afford dark purple
injected through a silicone rubber cap, and the cell contents
were quickly mixed by shaking. The time course of the reaction
was monitored by using the absorption maximum of 2 at
1
plates. Yield: 23.8 mg, 74%. H NMR (CD CN): δ 7.88 (s, 4H,
3
NH), 7.77 (s, 4H, 4,5-H), 7.63 (d, J = 7.2 Hz, 4H, PhSi), 7.22 (d,
J = 8.5 Hz, 24H, Ar), 7.20 (m, 2H, PhSi), 7.16 (d, J = 8.5 Hz, 24H,
Ar), 7.11 (dd, J = 7.2, 7.8 Hz, 4H, PhSi), 3.15 (q, J = 7.2 Hz, 8H,
583 nm. In the case in a toluene–acetonitrile solution, a solu-
tion (1 mM) of 1 in toluene–acetonitrile (v/v = 1/1) was treated
+
+
Et
4
N ), 1.20 (tt, JH–H = 7.2 Hz, JH–N = 1.9 Hz, 12H, Et
4
N ), 1.19
with an acetonitrile solution of Me NO (60 mM, 10 μL).
3
t
t
(
(
(
s, 108H, Bu), 0.93 (s, 9H, BuSi). Absorption spectrum
toluene): λmax (ε, M cm ) 390 (sh) (6100), 485 (6100), 608
sh) (2600) nm. Anal. Calcd for C164 SiW: C, 73.48;
1
The reaction between 1 and DMSO was monitored by
H
−
1
−1
NMR spectroscopy. A sealed-NMR tube containing 1 (1.2 mM)
and DMSO (5.7 mM) in toluene-d8 (0.6 mL) was heated at
H
203 5 6 4
N O S
H, 7.63; N, 2.61. Found: C, 73.32; H, 7.51; N, 2.57.
50 °C. The measurement was carried out at 30 °C. In another
case, a mixed solution of 1 (0.99 mM) and DMSO (5.4 mM) in
toluene-d –CD CN (v/v = 1/1) was used.
Physical measurements
8
3
The elemental analyses were performed on a Yanaco CHN
1
CORDER MT-5. H NMR spectra were obtained with JEOL
ECA-500 and ECS-400 spectrometers in toluene-d
at 30 °C. UV-visible absorption spectra were recorded using a
8 3
and CD CN
Results
SHIMADZU UV-3100PC spectrometer. Infrared (IR) spectro- Synthesis
scopic measurements were performed on a Jasco FT/IR-6100
Desoxomolybdenum(IV), monooxomolybdenum(VI), and mono-
oxotungsten(VI) complexes, 1–3 (Chart 2), were obtained by the
reactions shown in Scheme 1. First, we monitored the UV-vis
spectral change of the silylation of monooxomolybdenum(IV)
spectrometer. Raman spectra were recorded on a Jasco
NR-1800 laser Raman spectrometer with liq. N2 cooled or a
thermoelectrically cooled CCD detector. Exciting radiation was
+
provided using an Ar ion laser (514.5 nm).
(
4) and dioxomolybdenum(VI) (5) complexes by using a modi-
1
4,22
fied method reported in the literature (Fig. S1†).
Upon
Structural determination
t
adding BuPh
2
SiCl to the complexes in toluene, the absorption
Each single crystal of 1·5CH
3
CN·2H
2 3
O and 3·7/2CH CN·5/ spectra changed with sharp isosbestic points. The silylation of
−
1
−1
2
2
H O was selected carefully and mounted on a MicroMount™ complex 4 (k = 0.18 M
s ) was slower than that of 5 (k =
2
2
2
00 μm with Nujol, which was frozen immediately in a stream 0.29 M−
1
s ), which is probably ascribed to the weak nucleo-
−1
1
7,27
of cold nitrogen at 200 K. Data collection was made on a philicity of the strongly bonded apical oxo ligand.
On com-
Rigaku RAPID II Imaging Plate area detector with Mo-Kα radi- pletion of silylation of 4, we found that the UV-vis spectrum
ation (0.71075 Å) using a MicroMax-007HF microfocus rotating was identical to that of isolated 1. Therefore, we regarded the
anode X-ray generator and VariMax-Mo optics. The structures spectrum of quantitatively silylated complex 5 as that of 2.
2
3
were solved by direct methods (SIR92 ) for 1·5CH CN·2H O Tungsten analogue 3 was obtained by a similar method. All
3
2
2
4
and heavy-atom Patterson methods (PATTY ) for 3·7/ the complexes were soluble in nonpolar solvents such as
CH CN·5/2H O and expanded Fourier techniques using toluene, and pure 1 and 3 were isolated as light green and
SHELXL-97 or SHELXL-2014/7. The chemical formula of 3 dark purple blocks in 93% and 74% yields, respectively.
2
3
2
2
5
6262 | Dalton Trans., 2015, 44, 6260–6267
This journal is © The Royal Society of Chemistry 2015

Dalton Transactions
Paper
Molecular structures in the crystals
moderate (blue) and weak (green) hydrogen bonds were
formed at S3 and S2 owing to the medium and weak trans
influences of the siloxo and thiolate ligands, respectively. The
other amide N4H4 is directed toward the benzene ring of the
neighboring R4 moiety, probably owing to the packing in the
crystal.
The molecular structures of 1 and 3 were determined by X-ray
analysis. These complexes were apparently recrystallized easily,
but their crystallinity was very poor. The result was similar to
that observed in the case of 4, 5, and 6, as previously
2
1
described. Here, we mainly discuss the geometry of the
metal center.
1
H NMR studies
3 2
The molecular structure of 1·5CH CN·2H O is shown in
t
1
Fig. 1a. The C(C
to interlock with each other in the crystal and to create walls NMR signals in toluene-d
similar to those of complex 4 that have been previously four acylamino groups are magnetically equivalent. In the H
6
H
4
-4- Bu)
3
(CAr
3
) moieties are bulky enough Desoxomolybdenum(IV) complex 1 exhibited well-defined
H
(Fig. S2†), which indicated that the
8
1
2
1
reported.
retained both before and after silylation. The square pyramidal of broad signals in a polar solvent, CD
geometry and the linearity of Mo–O–Si (164°) were similar to broad signals were observed in toluene-d
those of the related siloxomolybdenum(IV) complexes (164°– ation and broadening of signals are ascribed to the confor-
The resulting hydrophobic microspace was NMR spectrum of monooxotungsten(VI) complex 3, a single set
CN, and a couple of
(Fig. 2). The separ-
3
8
1
4,19
1
75°).
sulfur atoms of the dithiolate ligand, indicating the presence dioxotungsten(VI) complex 5 in a previous paper. As expected,
of NH⋯S hydrogen bonds.
the signals in toluene-d became sharp upon cooling to
The W center of 3·7/2CH CN·5/2H O shows a slightly dis- −45 °C. The partial assignment using ROESY and COSY tech-
All the amide NH moieties were directed toward the mational change through the Bailar twist, as described for cis-
2
1
8
3
2
torted octahedral geometry, which is similar to that of 6 rather niques showed that the four acylamino groups are nonequiva-
than that of a monooxotungsten(VI) complex with bdt ligands lent like in the crystal. The singlet signal at the lowest field
2
1,22
(Fig. 1b).
The bulky substituents and hydrogen bonds (red a) was reasonably assigned to the amide NH proton trans
2
1
probably maintain the coordination structure. All the four W–S to the oxo ligand, as observed in 6. The protons in the other
bonds in 3 were different, and the order of the bond length dithiolate ligand (blue and pink) were unassignable because of
was W–S1 > W–S3 > W–S4 > W–S2. As described in a previous
paper on complex 6, a strong trans influence of the oxo ligand
makes W–S1 trans to WvO, leading to the most anionic thio-
2
1
late and the strongest (red) N1H1⋯S1 hydrogen bond. The
1
Fig. 2 H NMR spectra of 3 in (a) CD
3 8
CN at 30 °C, (b) toluene-d at
3
0 °C, and (c) toluene-d at −45 °C. The two set of assignable signals
8
and unassignable signals are colored in red, green, and purple, respect-
ively. The asterisks denote solvents as contaminants (*1: acetonitrile; *2:
toluene; *3: water). The double asterisk (**) denotes TMS.
Fig. 1 Ball-and-stick models (left) and schematic drawings (right) of the
anion part of (a) 1·5CH CN·2H O and (b) 3·7/2CH CN·5/2H O.
3 2 3 2
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Dalton Trans., 2015, 44, 6260–6267 | 6263

Paper
Dalton Transactions
the relatively weak trans influence of the siloxo ligand. The of the molybdenum(IV) complex was assigned to Mo–O stretch-
unassignable signals are shown in purple (Fig. 2). Interest- ing combined with O–Si stretching, and the signals of the
ingly, two distinct d protons were observed at 8.2 and 8.5 ppm tungsten(VI) complex were assigned to symmetric and asym-
t
(
Fig. 2c), indicating that the two phenyl rings of the BuPh
2
SiO metric WO
2
stretchings combined with O–Si stretching. The
ligand in essentially nonequivalent environments can be dis- bands of 1 and 3 involving the M–O bonds were observed at
tinguished owing to the slowing of the conformational change. lower wavenumbers than those of the corresponding bdt com-
plexes; however, it is difficult to evaluate the influence of
IR and resonance Raman spectra
NH⋯S hydrogen bonds on the M–O bonds because of the
The presence of NH⋯S hydrogen bonds was confirmed by IR complicated coupling of the stretching bands, as described
spectroscopy. Amide NH stretching bands for 1 and 3 in the above.
solid state are listed in Table 1, along with the values of related
complexes 4 and 6.
2
1
Oxygen-atom-transfer reactions
The Δν(NH) values, the lower shifts of ν(NH) than that of The OAT reaction between Me NO and complex 1 was moni-
3
the corresponding compound without the NH⋯S hydrogen tored using UV-vis spectroscopy. Upon adding 2 equiv. of
bond (RCONHPh), represent the strength of the hydrogen Me NO in DMF to 1 in toluene, a fast reaction occurred to
3
1
9,21
bond, as described in a previous paper.
afford a dark green solution (Fig. 3). The UV-vis spectrum
−
1
The smaller Δν(NH) values of 1 (−57 cm ) and 3 (−26, obtained after the OAT reaction (red solid line) was identical to
1
−
69 cm− ) than those of 4 and 6 (−97 and −96 cm−1, respect- that of 2 (red dotted line), which indicated the clear and quan-
ively) indicate the presence of weaker NH⋯S hydrogen bonds titative conversion of 1 into 2. The second-order rate constant,
in the complexes with the siloxo ligand, which is consistent k2, of the reaction was calculated to be 98 M−
with the molecular structures in the crystal.
In a polar toluene–acetonitrile solution (v/v = 1/1 with a
In monooxotungsten(VI) complex 3, four ν(NH) values are dielectric constant (ε ) of approximately 25 ), the reaction rate
1 −1
s
.
2
8
r
−
1
−1
expected based on the molecular structure, but only two NH (k = 8.6 M
s ) was about ten times smaller than that in
2
−
1
stretching bands (3377 and 3334 cm ) were observed. The NH toluene, which was consistent with the results of a previous
bands of 3 were considered to be overlapped similar to related study, suggesting that the approach of a polar substrate was
−
1
21
complex 6, which exhibited a single band (3316 cm ) of more efficient in a hydrophobic environment.
−
1
two ν(NH) values (3326 and 3306 cm
by curve-fitting
Notably, these k2 values were smaller in both polar and
nonpolar solvents than those of the OAT reaction between 4
2
1
simulation).
−1
−1
29
Resonance Raman spectra of desoxomolybdenum(IV) and Me NO (k = 26.6 and 384 M
s
r
in DMF (ε = 36.7 )
3
2
2
1
complex 1 excited at 514.5 nm exhibited a single signal at and toluene, respectively). This was ascribed to the steric
19 cm− (Fig. S3a†). In the IR spectrum of the related hindrance of bulky CAr moieties and the BuPh SiO ligand
1
t
−
9
3
2
IV
t
4 2 2
complex, (Et N)[Mo (OSi BuPh )(bdt) ] (7), the corresponding and/or the decrease of the ionicity of the active center by the
band at 947 cm− was assigned to ν(O–Si). In the case of replacement of the dianionic O ligand with the monoanionic
1
14
2−
−
1
t
−
monooxotungsten(VI) complex 3, strong (924 cm ) and weak
BuPh SiO ligand.
2
1
(
889 cm− ) signals were observed (Fig. S3b†), whereas the
Unfortunately, the solubility of complex 1 in acetonitrile
IV
t
corresponding signals of (Et N)[W O(OSi BuPh )(bdt) ] at 933 was not enough to determine the k of the reaction. In another
and 888 cm were assigned to ν(O–Si) and ν(WvO) by IR, polar solvent (DMF), 1 was unstable and hence decomposed
respectively. However, because the sample was excited at the
LMCT absorption band, the signal of ν(WvO) should be more
intense than that of ν(O–Si) owing to the resonance effect
4
2
2
2
−
1
2
2
(Fig. S4†). In order to assign the vibrational mode of these
complexes, we estimated the frequency by DFT calculations
and compared them with the observed values in the resonance
Raman measurements of reported complexes containing bdt
1
4,22
ligands (Fig. S3c, d†).
As illustrated in Fig. S3,† the signal
Table 1 IR bands of ν(NH) (cm− ) in the molybdenum and tungsten
1
complexes in the solid state
a
b
Complexes
Δ(NH)
1
3
4
6
3346
3377, 3334
3306
−57
−26, −69
−97
c
c
3316
−87
Fig. 3 UV-vis spectra of 1 (blue), the reaction mixture of 1 with 2 eq. of
a
Nujol. Differences from the value (3403 cm−1) of the corresponding Me
b
3
NO (red, solid), and in situ synthesized 2 by silylation of 5 (red,
dotted) in toluene.
c
compound, RCONHPh, in solution (10 mM in CH
2
Cl
2
). Ref. 21.
6264 | Dalton Trans., 2015, 44, 6260–6267
This journal is © The Royal Society of Chemistry 2015

Dalton Transactions
Paper
into 4 by the dissociation of the silyl group, and its reaction
with Me NO afforded dioxomolybdenum(VI) complex 5.
3
On the other hand, complex 1 was stable for several hours
in an aqueous micellar solution containing Triton X-100, even
in the presence of Me NO (Fig. S5†), indicating the formation
3
of a robust hydrophobic microenvironment that prevents the
approach of water molecules and polar substrates.
Addition of a large excess (400 eq.) of DMSO to 1 in toluene
at 27 °C resulted in the oxidative dissociation of the dithiolate
ligand to afford a disulfide derivative, L1
2
. These results
suggest that a large excess of the coordinating compound
2
9
with a large donor number (D
ligands of 1.
N
)
tends to dissociate the
Scheme 2 The proposed mechanism of the OAT reaction of DMSO.
Surprisingly, complex 1 could reduce DMSO into Me S in
2
8
toluene-d , while no reaction occurred between monooxomo-
lybdenum(IV) complex 4 and DMSO at 80 °C for 20 h. The reac-
afford the corresponding monooxomolybdenum(V) complex
16,33
and phenol.
The dissociative mechanism is consistent
tion system containing 1.2 mM of 1 and 4.6 equiv. of DMSO in
t
with the ∼60% production of BuPh SiOH (Scheme 2). In
2
1
toluene-d
8
was monitored using H NMR spectroscopy. No sig-
addition, 2 equiv. of DMSO was consumed while ∼20% of
Me S based on the initial amount of 1 was detected, probably
2
owing to the decomposition of the product through a radical
process.
nificant change in the spectrum was observed at r.t. for 24 h;
however, heating the mixture at 50 °C resulted in the slow con-
sumption of DMSO and 1, accompanied by the production of
2
Me S (Fig. 4, S6†). The reaction accomplished within 600 h,
A similar OAT reaction occurred in a mixture of toluene-d
and acetonitrile-d3 (v/v = 1/1, Fig. S7†). The similar reaction
8
and the second-order kinetic constant estimated from the con-
−
4
−1
−1
version of 1 was k
2
= 5.7 × 10
M
s
(Fig. S7a†).
−4
−1 −1
rate (k = 6.7 × 10
M
s ) to that in toluene-d indicates
8
2
Accompanied by the decrease in the signals of 1, neither the
that the desoxomolybdenum(IV) complex can reduce DMSO
without depending on the solvent polarity, unlike the case of
signals of amide NH nor the benzenedithiolate protons of the
t
product were observed, whereas a broad signal of Bu protons
Me NO. The polar amine N-oxide is hardly soluble in nonpolar
3
3
in the CAr moiety was observed. The result suggests the for-
solvents and considered to be condensed into the ionic active
center. Thus, the dramatic acceleration of the OAT reaction
mation of paramagnetic species. After the complete con-
sumption of 1, the UV-vis spectrum of the reaction mixture
showed the absorption band at 756 nm (Fig. S8†), which is
characteristic of five-coordinated monooxomolybdenum(V)
21
was achieved, as described in a previous paper. In contrast,
DMSO can diffuse uniformly even in toluene, and only the
hydrophobic microenvironment around the active center
affects the uptake of the substrate. Similar reactivity in both
polar and nonpolar solvents indicates the presence of a hydro-
phobic microenvironment for efficient uptake of the substrate.
3
0–32
complexes.
One-electron oxidation of 4 by 1.1 equiv. of
III
[Fe Cp
2
]BF
4
proceeded quickly and quantitatively in toluene
V
to afford the monooxomolybdenum(V) complex, (Et N)[Mo O-
1,2-S -3,6-{(4- BuC H ) CCONH} C H ) ]. By comparing the
2 6 4 3 2 6 2 2
4
t
(
absorbance at 756 nm, the yield of the Mo(V) species was
found to be approximately 70%. Moreover, sharp signals of
BuPh SiOH were observed. The result is similar to that
2
Discussion
t
observed for monooxomolybdenum(VI) complex, 8, which Desoxomolybdenum(IV) complex 1 containing bulky hydro-
VI
decomposes by a homolytic cleavage of the Mo –OPh bond to phobic substituents could reduce DMSO in both polar and
nonpolar solvents. Although it is difficult to quantitatively
discuss the reaction because of the instability of the resulting
monooxomolybdenum(VI) complex 2 under the reaction con-
1
ditions, the production of Me
2
S was confirmed by H NMR
analysis. Moreover, the clean and quantitative OAT reaction of
1
with Me NO was also observed.
3
In the case of the siloxomolybdenum(IV) complex contain-
ing unsubstituted bdt ligands (7), no appreciable reaction was
observed when a large excess of more reactive tetramethylene
sulfoxide ((CH
2
)
4
SO) was reacted in CH
NO occurred, but it was
not clean. Phenoxomolybdenum(IV) complex 8 slowly reacted
with a large excess of DMSO in CH CN on heating, and the
kinetic parameters ΔH and ΔS were reported to be 14.8(5)
3
CN on heating at
5
0 °C, whereas the OAT reaction of Me
3
1
4
3
‡
‡
Fig. 4 Time course of the conversion of 1 during the reduction of
DMSO in toluene-d
1
−1
16,33
8
.
kcal mol− and −36(1) cal mol , respectively.
The esti-
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Dalton Trans., 2015, 44, 6260–6267 | 6265

Paper
Dalton Transactions
6
mated k
2
at 50 °C from the Eyring plot was 8 × 10− M−1 s−1
,
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which was much smaller than the present system. In another
IV
t
case, a siloxomolybdenum(IV) complex, (Et
(COOCH ], was reported to cleanly react with Me
below −15 °C; however, the reaction with DMSO was too slow
4
N)[Mo (OSi BuPh
2
)-
{
S
2
C
2
3
)
2
}
2
3
NO
1
2
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complex containing 4,5-dimethoxy-1,2-benzenedithiolate
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1
3
Ph
3
P; however, a large excess of DMSO as a solvent was used.
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complex in the present system is considered to be caused by
the stabilization of the DMSO-bound transient structure owing
to bulky hydrophobic substituents. The DFT calculations of
desoxomolybdenum(IV) complexes indicated that the distortion
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1
7
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3
1
9
mechanism.
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The rate of the OAT reaction of DMSO was independent of
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This work was supported by JSPS KAKENHI Grant Number
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