Deoxygenation of mono-oxo bis(dithiolene) Mo and W complexes by protonation

Article abstract of DOI:10.1021/ic4008747

Protonation-assisted deoxygenation of a mono-oxo molybdenum center has been observed in many oxotransferases when the enzyme removes an oxo group to regenerate a substrate binding site. Such a reaction is reported here with discrete synthetic mono-oxo bis(dithiolene) molybdenum and tungsten complexes, the chemistry of which had been rarely studied because of the instability of the resulting deoxygenated products. An addition of tosylic acid to an acetonitrile solution of [Mo^IVO(S₂C₂Ph₂) ₂]^2- (1) and [W^IVO(S₂C ₂Ph₂)₂]^2- (2) results in the loss of oxide with a concomitant formation of novel deoxygenated complexes, [M(MeCN)₂(S₂C₂Ph₂)₂] (M = Mo (3), W (4)), that have been isolated and characterized. Whereas protonation of 1 exclusively produces 3, two different reaction products can be generated from 2; an oxidized product, [WO(S₂C₂Ph₂) ₂]^-, is produced with 1 equiv of acid while a deoxygenated product, [W(MeCN)₂(S₂C₂Ph₂) ₂] (4), is generated with an excess amount of proton. Alternatively, complexes 3 and 4 can be obtained from photolysis of [Mo(CO)₂(S ₂C₂Ph₂)₂] (5) and [W(CO) ₂(S₂C₂Ph₂)₂] (6) in acetonitrile. A di- and a monosubstituted adducts of 3, [Mo(CO) ₂(S₂C₂Ph₂)₂] (5) and [Mo(PPh₃)(MeCN)(S₂C₂Ph₂) ₂] (7) are also reported.

Full text of DOI:10.1021/ic4008747

Article
pubs.acs.org/IC
Deoxygenation of Mono-oxo Bis(dithiolene) Mo and W Complexes
by Protonation
Junhyeok Seo, Paul G. Williard, and Eunsuk Kim*
Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States
S
* Supporting Information
ABSTRACT: Protonation-assisted deoxygenation of a mono-
oxo molybdenum center has been observed in many
oxotransferases when the enzyme removes an oxo group to
regenerate a substrate binding site. Such a reaction is reported
here with discrete synthetic mono-oxo bis(dithiolene) molybde-
num and tungsten complexes, the chemistry of which had been
rarely studied because of the instability of the resulting deoxygenated products. An addition of tosylic acid to an acetonitrile
solution of [MoIVO(S2C2Ph2)2]2− (1) and [WIVO(S2C2Ph2)2]2− (2) results in the loss of oxide with a concomitant formation of
novel deoxygenated complexes, [M(MeCN)2(S2C2Ph2)2] (M = Mo (3), W (4)), that have been isolated and characterized.
Whereas protonation of 1 exclusively produces 3, two different reaction products can be generated from 2; an oxidized product,
[WO(S2C2Ph2)2]−, is produced with 1 equiv of acid while a deoxygenated product, [W(MeCN)2(S2C2Ph2)2] (4), is generated
with an excess amount of proton. Alternatively, complexes 3 and 4 can be obtained from photolysis of [Mo(CO)2(S2C2Ph2)2]
(5) and [W(CO)2(S2C2Ph2)2] (6) in acetonitrile. A di- and a monosubstituted adducts of 3, [Mo(CO)2(S2C2Ph2)2] (5) and
[Mo(PPh3)(MeCN)(S2C2Ph2)2] (7) are also reported.
the reductive half-cycle, the MoIV site is restored by removing
1. INTRODUCTION
the oxo group assisted by proton/electron transfer (MoVIO +
Nature utilizes chemistry of high-valent metal oxo species to
carry out a wide range of important catalytic reactions. Many
2 e− + 2 H+ → MoIV + H2O), Scheme 1a. The EPR
spectroscopic studies on DMSOR suggest that the reductive
heme and nonheme iron enzymes (e.g., cytochrome P450) use
half reaction proceeds by two successive one-electron
highly reactive FeIVO for their oxygenase and oxidase
activity.1−3 Photosynthesis relies on high-valent MnO
reduction/protonation via the formation of an EPR active
MoV−OH species.18,24−27 Although naturally occurring
DMSORs exclusively use molybdenum, a W-substituted
isoenzyme (W-DMSOR) has been reported to have com-
parable enzyme activity.28−30
species to carry out water splitting.4,5 In addition to Fe and
Mn, nature also uses a metal-oxo motif with a second and a
third row transition metal ion such as Mo and W which have
considerably different chemical properties from those of Fe-
and Mn-oxo species.6−10 In contrast to the highly reactive Fe/
Mn oxo counterparts, molybdenum and tungsten in high
oxidation state (IV−VI) are stabilized by the oxo ligand. The
high valent MoO or WO species are often considered a
thermodynamic sink in synthesis because they can be easily
generated by a trace amount of O2 or water contamina-
tion.11−14 Nature, however, efficiently utilizes the MoO or
WO unit to achieve difficult multielectron redox catalysis of
oxotransferases, in which the addition and removal of the oxo
group of the Mo/W center is elegantly coordinated as involving
water, protons, and electrons.6−10
The O-atom transfer chemistry in the oxidative half-reaction
has been extensively studied with synthetic bis(dithiolene)
metal (Mo or W) complexes by a group of researchers (Scheme
1b).13,14,31−36,41−43 The comparative studies between Mo and
W complexes with various substrates by Holm and co-workers
showed that the ligand electronic environment as well as the
metal ion can affect the oxo-transfer reaction rate, XO + MIV
→
X + MVIO, where M = Mo, W; X = sulfides, amines, and
others (Scheme 1b).13,14,31,34,35
In contrast to the oxidative half-reaction, synthetic studies on
the deoxygenation chemistry in the reductive half-reaction that
involve proton/electron transfer reactions are rather limited. A
general challenge in the deoxygenation of the MIV−VIO
complexes (M = Mo, W) lies in the highly stable nature of
metal-oxo group when the metal center has the oxidation state
over 4+ and d orbital electrons less than 4.44 With the
exceptions of the enzyme active sites, such a protonation-
dehydration process has been observed mostly in the polyoxo
One of the best studied oxotransferases is dimethylsulfoxide
reductase (DMSOR) from R. sphaeroides.15−24 DMSOR
catalyzes the reduction of dimethylsulfoxide (DMSO) to
dimethylsulfide (DMS). The X-ray structure reveals that the
active site in the reduced state has a penta-coordinate MoIV ion
ligated by two pterin-dithiolene cofactors and a serine side
chain, while the oxidized state has an additional oxo group at
the MoVI center (Scheme 1a).15 The catalytic cycle begins with
an oxygen atom transfer from DMSO to the MoIV center in the
oxidative half-cycle (DMSO + MoIV → DMS + MoVIO). In
Received: April 11, 2013
Published: July 18, 2013
© 2013 American Chemical Society
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Scheme 1. (a) Proposed Catalytic Cycle for DMSOR;15,20,24 (b) Known Synthetic Modeling Chemistry of O-Atom Transfer
from the Substrate to the Metal Center;13,14,31−37 (c) Known Synthetic Modeling Chemistry of O-Atom Transfer from Metal to
Phosphine (e.g. PPh3);37−40 (d) Present Work Demonstrating Protonation-Assisted Deoxygenation Chemistry of MIVO in
Synthetic Models
metal complexes, in which the metal ion can be stabilized with
the remaining oxo ligand(s) after the removal of one of the oxo
ligands by protonation as shown with Re(V),45 W(VI),46−48
and Mo(VI)47,49 polyoxo complexes. Elimination of the oxo
group from mono-oxo Mo/W complexes by protonation
appears to be much more difficult to achieve, for which Sarkar
and co-workers have made considerable efforts.48,50 They found
that [WIVO(mnt)2]2−, where mnt2− = 1,2-dicyanoethylenedi-
thiolate, is not stable in the acidic condition (<pH 4) and
decomposes to [W(mnt)3]2−,48 whereas the presence of thiols
altering the core structure of bis(dithiolene) metal complexes.
The only successful way of removing the oxo group from the
bis(dithiolene) metal site appears to be the use of nonbiological
oxygen abstractors like phosphines (e.g., PPh3) by which the O-
atom, as opposed to oxide (O2−), can be removed from the
metal site with a concomitant reduction of the metal ion
(Scheme 1c).33 Inspired by the biological process of converting
a metal-bound oxo ligand to water, we have studied the
protonation chemistry of discrete molybdenum and tungsten
oxo complexes. Herein, we report for the first time that the oxo-
group of mono-oxo bis(dithiolene) MoIV/WIV complexes can
be removed by simple protonation (Scheme 1d) to yield the
u n p r e c e d e n t e d d e o x y g e n a t e d p r o d u c t s [ M -
(MeCN)2(S2C2Ph2)2].
in the acidic medium converts [WIVO(mnt)2]2− to a thiolate-
50
bridged dimerized compound, [WIV2(SR)2(mnt)4]2−
.
Although these results indicate that the oxo group must have
been removed by protonation prior to the formation of the
observed products, there has been no literature precedent that
demonstrates that protonation itself can remove the oxide of
the mono-oxo bis(dithiolene) Mo/W complexes without
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Scheme 2
addition, the IR spectrum of the precipitate displays the
ν(CN) stretch frequencies at 2317 and 2282 cm−1 (Supporting
Information, Figure S2) which are in the expected range for the
metal-bound acetonitriles51−53 and are particularly very close to
those found in [MoCl4(MeCN)2] (2320, 2291 cm−1).54 On the
basis of the data obtained from IR spectroscopy and elemental
analysis, we formulate the reaction product as [Mo-
(MeCN)2(S2C2Ph2)2] (3) and propose a six coordination
geometry for the metal ion ligated by two MeCN molecules
and bis(dithiolene). We were unable to obtain an X-ray
structure of 3. However, a six-coordinate geometry for 3 would
be in line with all the known neutral bis(dithiolene)
molybdenum complexes,32,55−59 in addition to our own
analogue with PPh3, [Mo(CH3CN)(PPh3)(S2C2Ph2)2] (7),
that has been crystallographically characterized (see below).
Reaction of [Mo(MeCN)2(S2C2Ph2)2] (3) with CO and
PPh3. Compound 3 is stable as a solid, but its stability in
solution is very much dependent on the solvent. In the absence
of acetonitrile, 3 immediately decomposes to a known60
tris(dithiolene) complex, [Mo(S2C2Ph2)3]; however, 3 is stable
in a solvent mixture that contains at least 1% acetonitrile (e.g,
1% CH3CN: 99% CH2Cl2). The coordinated MeCN ligands in
3 could be easily replaced by a stronger ligand such as CO or
phosphine, which makes 3 a valuable precursor for many
bis(dithiolene) MoIV complexes.
2. RESULTS AND DISCUSSION
Protonation of [MoIVO(S2C2Ph2)2]2− (1) to Form [Mo-
(MeCN)2(S2C2Ph2)2] (3). We examined the protonation of a
bis(dithiolene) molybdenum complex, [MoIVO(S2C2Ph2)2]2−
(1), with p-toluenesulfonic acid (TsOH) as a proton source.
We found that 2 equiv of acid was required to remove the oxo
group of 1 producing deoxygenated bis(dithiolene)-
molybdenum(IV) complex that can be stabilized by solvent
without generating any intermediate (Scheme 2). Addition of 1
equiv of TsOH to an acetonitrile solution of 1 at room
temperature led to an immediate UV−vis spectral change (red
dashed, Figure 1) indicating a fast but incomplete conversion of
Substitution reactions of the MeCN ligand in 3 were first
studied with CO because the spectroscopic characterizations of
the expected reaction product, [Mo(CO)2(S2C2Ph2)2] (5),
were well-known.55,61 When CO was bubbled through an
acetonitrile solution of 3 in dark for ∼10 s, a characteristic
electronic absorption in the visible region (λmax = 555 nm)
known for 5 started to develop, and the complete conversion of
3 to 5 was achieved in an hour (eq 1) (Supporting Information,
Figure S3). The formation of 5 was further supported by IR
spectroscopy, in which two characteristic ν(CO) frequencies of
5 were observed at 1996 and 2035 cm−1 (Nujol).
Figure 1. UV−vis spectral changes during titration of (Et4N)2[MoO-
(S2C2Ph2)2] (1) (black dotted) with tosylic acid (TsOH) in
acetonitrile at room temperature. An addition of 1 equiv of TsOH
to 1 results in the incomplete conversion (red dashed) of 1 to
[Mo(MeCN)2(S2C2Ph2)2] (3). A further addition of extra 1 equiv of
TsOH leads to the complete formation of 3 (purple solid).
1 to [Mo(MeCN)2(S2C2Ph2)2] (3), in which a new band at
446 nm from 3 arose as the 282, 320, and 365 nm absorptions
from 1 decreased. The 446 nm band reached to a maximum
with an additional 1 equiv of TsOH (purple solid, Figure 1),
suggesting that the complete conversion of 1 to 3 requires at
least 2 equiv of acid. Further addition of excess TsOH (a total
of 4 equiv) did not affect the UV−vis features of 3 at λmax (εM)
= 265 (sh, 66000), 375 (16000), 446 (20000), 536 (sh, 9100)
nm.
The isolation of [Mo(MeCN)2(S2C2Ph2)2] (3) was possible
because of differential solubility of [MoO(S2C2Ph2)2]2− (1)
and 3 in acetonitrile. Complex 3 as a dark violet precipitation
was slowly developed from the reaction solution of 1 with
TsOH. The IR spectrum of the isolated precipitate lacks the
characteristic MoIVO stretch frequency of 1 at 881 cm−1,
which indicates the elimination of the molybdenum-oxo group
by protonation (Supporting Information, Figure S1). In
[Mo(MeCN)2(S2C2Ph2)2](3)
2CO
⎯⎯⎯⎯⎯→⎯ [Mo(CO)2(S2C2Ph2)2](5)
(1)
MeCN
Synthesis of a monosubstituted analogue was achieved as we
employed a bulky substituent (Scheme 3). Upon addition of
solid 3 to a solution of PPh3 in CH2Cl2, the reaction mixture
instantly turned into a red brown solution of [Mo(MeCN)-
(PPh3)(S2C2Ph2)2] (7) with UV−vis absorptions at λmax (εM,
CH2Cl2) = 251 (76000), 270 (sh, 63000), 375 (9500), 454
(20000), 543 (sh, 6000) nm (Supporting Information, Figure
S4), which is similar to the UV−vis spectrum of 3 with a
1
noticeable red-shift. The H NMR spectrum of the isolated
product from a CH2Cl2/pentane solution indicates the
presence of coordinated MeCN (1.63 ppm in CD2Cl2) in
addition to the PPh3 ligand (Supporting Information, Figure
S5). Consistent with this, the ν(CN) stretching frequency of
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Scheme 3
Figure 2. Structure (ORTEP view) of Mo(PPh3)(MeCN)(S2C2Ph2)2
(7) with 50% probability thermal ellipsoids.62 Hydrogen atoms were
omitted for clarity. Selected bond distances (Å) and angles (deg) are
as follows: Mo(1)−N(1) 2.155, Mo(1)−P(1) 2.547, Mo(1)−S(1)
2.333, Mo(1)−S(2) 2.357, Mo(1)−S(3) 2.330, Mo(1)−S(4) 2.350,
N(1)−Mo(1)−P(1) 79.5, N(1)−Mo(1)−S(2) 82.7, N(1)−Mo(1)−
S(4) 81.0, S(2)−Mo(1)−S(4) 84.1, P(1)−Mo(1)−S(1) 82.7, P(1)−
Mo(1)−S(3) 80.2, S(1)−Mo(1)−S(3) 87.1.
the MeCN ligand appears at 2264 cm−1 in the IR spectrum
(Supporting Information, Figure S2). Likewise, the molybde-
num-bound PPh3 is supported by a single peak at 62.5 ppm (in
CD2Cl2) in the 31P NMR spectrum (Supporting Information,
Figure S6). All of these spectroscopic features are consistent
with the X-ray structure of 7 (see below). Complex 7 can be
alternatively prepared in a high yield (96%) from the way of
one-pot reaction of [MoIVO(S2C2Ph2)2]2− (1), PPh3, and
TsOH, Scheme 3.
The crystal structure of 7 (Figure 2) reveals a six-coordinate
molybdenum ion ligated by bis(dithiolene), PPh3, and MeCN
in a trigonal prismatic geometry which is a common structure
for six-coordinate bis(dithiolene) complexes such as [Mo-
(CO)2(S2C2Me2)2].55,59 Similar to all the known structures of
neutral six coordinate bis(dithiolene) Mo or W com-
plexes,32,55−59 the PPh3 and MeCN ligands in 7 are cis to
each other, which explains why only one of the MeCN ligands
undergoes substitution by a bulky PPh3 ligand. Conversely, a
six-coordinate bis(dithiolene) W complex coordinated by two
smaller phosphine ligands, [W(PMe3)2(S2C2Me2)2], is
known.59
Figure 3. UV−vis spectra displaying three different products from
reaction between [WO(S2C2Ph2)2]2− (2) (black dotted) with TsOH
in acetonitrile. UV−vis spectra of the reaction of 2 with 1 equiv of
TsOH generating [WO(S2C2Ph2)2]− (blue solid), the reaction of in
situ generated [WO(S2C2Ph2)2]− with additional 2 equiv of TsOH
forming [W(S2C2Ph2)3] (green dashed), and the reaction of 2 with 4
equiv of TsOH generating [W(MeCN)2(S2C2Ph2)2] (4) (purple
solid).
Protonation of [WO(S2C2Ph2)2]2− (2) to form [WO-
(S2C2Ph2)2]− or [W(MeCN)2(S2C2Ph2)2] (4). In the case of a
tungsten analogue, we found that the amount of acid played a
critical role in determining the reaction products. This is quite
different from the reactivity observed with [MoO(S2C2Ph2)2]2−
(1) with acid from which the deoxygenated product, [Mo-
(MeCN)2(S2C2Ph2)2] (3), was exclusively generated. Upon
addition of 1 equiv of tosylic acid (TsOH) to an acetonirile
solution of [WIVO(S2C2Ph2)2]2− (2), a clean formation of the
known oxidized product, [WVO(S2C2Ph2)2]−, was observed by
UV−vis spectroscopy with an absorption at 721 nm, Figure 3
and Scheme 4. In fact, this reaction is exactly analogous to the
previously reported reaction of 2 with HBF4·Et2O that yields
[WVO(S2C2Ph2)2]− and H2.63 With additional TsOH (over 2
equiv), the oxidized compound, [WVO(S2C2Ph2)2]−, was
decomposed to become another known complex, [W-
(S2C2Ph2)3], (λmax = 657 nm),60 Figure 3 and Scheme 4. A
similar decomposition reaction to a tris(dithiolene) tungsten
complex in the acidic medium has been reported with
[WIVO(mnt)2]2−.48
To our surprise, an addition of excess (4−10 equiv) amount
of TsOH to [WIVO(S2C2Ph2)2]2− (2), as opposed to a
sequential addition, led to the formation the deoxygenated
product, [W(MeCN)2(S2C2Ph2)2] (4). Complex 4 has similar
UV−vis features to those of its molybdenum analogue,
[Mo(MeCN)2(S2C2Ph2)2] (3), at λmax (εM) = 257 (54000),
360 (8000) 413 (16000), 512 (3000) nm (Figure 3). As with 3,
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may imply that the S-atom is the preferred site for protonation,
which is associated with the reduction of protons to H2 with a
concomitant oxidation of 2, Scheme 4.
Scheme 4
Generation of [Mo(MeCN)2(S2C2Ph2)2] (3) and [W-
(MeCN)2(S2C2Ph2)2] (4) by Photolysis. The deoxygenated
compound, [Mo(MeCN)2(S2C2Ph2)2] (3) and [W-
(MeCN)2(S2C2Ph2)2] (4), can be alternatively generated
from the appropriate dicarbonyl precursors by photolysis (eq
2). Upon irradiation of an acetonitrile solution of the known
[M(CO)2(S2C2Ph2)2](5, 6)
UV
⎯⎯⎯⎯⎯→⎯ [M(MeCN)2(S2C2Ph2)2](3, 4)
(2)
MeCN
dicarbonyl complexes [Mo(CO)2(S2C2Ph2)2] (5) or [W-
(CO)2(S2C2Ph2)2] (6) with a laboratory UV lamp (254−365
nm) for ∼5 min at room temperature, the clean formation of 3
or 4 were observed in the UV−vis spectra (Figure 4 and
two ν(CN) frequencies at 2275 and 2258 cm−1 were observed
for 4 in the IR spectrum (Supporting Information, Figure S2),
and the formulation was further supported by elemental
analysis.
The observed two different protonation reaction products,
[W(MeCN)2(S2C2Ph2)2] (4) and [WVO(S2C2Ph2)2]−, might
be explained by the presence of two different protonation sites
(i.e., O- vs S-atom) in the complex, Scheme 4. To remove the
oxo ligand of [WIVO(S2C2Ph2)2]2− (2) with acid, generation of
a hydroxo species, [WIV(OH)(S2C2Ph2)2]−, through proto-
nation of the O atom is expected as an intermediate. However,
such a hydroxo species is known to be easily deprotonated to
become [WIVO(S2C2Ph2)2]2− (2),13,63 implying the equilibrium
of the protonation reaction, {W−OH}⇌{WO} + H+, must
lie to the right. Consistent with this, we were able to obtain
[W(MeCN)2(S2C2Ph2)2] (4) only with an excess amount of
acid. Once the {W−OH} species is generated, however, the
dehydration step in the presence of acid appears to be an
efficient and preferred pathway as judged by the high yield of 4
(∼88% as isolated). On the contrary, the oxidized complex,
[WVO(S2C2Ph2)2]−, is generated (∼95%) with an 1 equiv of
acid, which indicates the existence of a different protonation
reaction pathway, Scheme 4. Reduction of protons to
dihydrogen (H2) has been observed with many bis(dithiolene)
complexes possessing Ni,64 Fe,64 or Co65,66 ions. Although the
mechanism of the proton reduction to H2 is not yet fully
understood with these complexes, protonation of the S-atom of
bis(dithiolene) cobalt complexes has been suggested to be a
critical step to achieve the catalytic activity.67 In the case of
[WIVO(S2C2Ph2)2]2− (2), the S-atom of the dithiolne ligand is
known to be susceptible to alkylating agents (e.g., MeI,
PhCH2Br) to produce mono-S-alkylated complexes such as
[WO(MeS2C2Ph2)(S2C2Ph2)2]−.12 Therefore, it is reasonable
to consider the S-atom of 2 as a potent protonation site along
with the O-atom of 2. We cautiously conjecture that
protonation at the different sites (S vs O) may lead to the
formation of different reaction products. The efficient
conversion from 2 to [WVO(S2C2Ph2)2]− by 1 equiv of acid
Figure 4. UV−vis spectra of [Mo(MeCN)2(S2C2Ph2)2] (3) (purple
solid) generated by photolysis of [Mo(CO)2(S2C2Ph2)2 ] (5) (orange
dashed) in acetonitrile at room temperature.
Supporting Information, Figure S7). The generation of 3 or 4
from this method is noteworthy considering that photolysis of a
related monocarbonyl compound, [Mo(CO)(SPh)-
(S2C2Me2)2]−, is known to decompose to a tris(dithiolene)
complex, [Mo(S2C2Me2)3]−.55
3. CONCLUSION
We have reported a novel deoxygenation chemistry of mono-
oxo bis(dithiolene) Mo and W complexes by protonation. It
had been a challenge to achieve deoxygenation chemistry with
this group of complexes because the removal of the oxo ligand
often causes a complex decomposition to tris(dithiolene) Mo/
W species. Our study presented here demonstrates that simple
protonation can efficiently remove the oxo group from the
mono-oxo bis(dithiolene) Mo or W complexes, and the
resulting deoxygenated products can be stabilized by the
solvent. Protonation of [MoIVO(S2C2Ph2)2]2− (1) and [WIVO-
(S2C2Ph2)2]2− (2) in acetonitrile efficiently generates novel
deoxygenated complexes, [Mo(MeCN)2(S2C2Ph2)2] (3) and
[W(MeCN)2(S2C2Ph2)2] (4), in which the coordinated MeCN
ligands can be easily substituted by a stronger ligand such as
CO and PPh3. While CO replaces both MeCN ligands to yield
dicarbonyl products, [M(CO)2(S2C2Ph2)2], a sterically de-
manding incoming ligand like PPh3 results in a monosub-
stitution, [Mo(MeCN)(PPh3)(S2C2Ph2)2] (7). Differential
reactivity between [MoIVO(S2C2Ph2)2]2− (1) and [WIVO-
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(S2C2Ph2)2]2− (2) with protons has been observed. Whereas 3
is the sole product from the reaction between 1 and acid, two
different reaction products, [WO(S2C2Ph2)2]− and [W-
(MeCN)2(S2C2Ph2)2] (4), can be generated from 2 depending
on the amount of acid added. The former is predominantly
generated with 1 equiv of acid while the latter is the main
product with an excess amount of acid.
(0.01−0.03 mM) which was then irradiated with UV light (254/365
nm, 4 W) for 5 min. Upon irradiation, the color of the sample solution
changed from purple to yellow. The conversion yield was estimated as
70−75% based on the molar concentrations deduced from the
absorbance with known extinction coefficients of 3−6.
Reaction of [Mo(MeCN)2(S2C2Ph2)2] (3) with CO. A dilute
solution (23 μM) of [Mo(MeCN)2(S2C2Ph2)2] (3) was prepared in a
Schlenk cuvette. (Caution! complex 3 has poor solubility in acetonitrile
and this experiment is possible only with a dilute sample) Gaseous CO
was bubbled through the solution of 3 for 10 s in the dark. The
solution was allowed to sit in the dark for 1 h to complete the
conversion from 3 to [Mo(CO)2(S2C2Ph2)2] (5), which was
monitored by UV−vis spectroscopy. After the UV−vis monitoring,
the final sample was saved, and the solvent was removed by vacuum.
The IR spectrum (Nujol) of the final reaction product showed two
strong ν(CO) peaks at 1996, 2035 cm−1 suggesting the formation of 5.
Synthesis of [Mo(MeCN)(PPh3)(S2C2Ph2)2]·2H2O (7·2H2O).
Method 1. To a mixture of (Et4N)2[MoIVO(S2C2Ph2)2] (1) (20 mg,
0.023 mmol) and PPh3 (20 mg, 0.069 mmol) in 4 mL of MeCN was
added a solution of TsOH·H2O (16 mg, 0.092 mmol) in 4 mL of
MeCN with stirring. The color of the reaction mixture changed
instantly from orange brown to red brown upon addition of TsOH.
The resulting mixture was allowed to stir for 1 h, during which a red
brown precipitate was slowly developed. The red brown solid was
isolated, washed with MeCN (3 × 2 mL), and dried under vacuum.
The solid was redissolved in 1 mL of CH2Cl2, and the solution was
layered with 10 mL of pentane. The red brown crystals were obtained
overnight to yield 18 mg (96%) of [Mo(MeCN)(PPh3)-
(S2C2Ph2)2]·2H2O (7). Absorption spectrum (DCM): λmax (εM) 251
(76000), 270 (sh, 63000), 375 (9500), 454 (20000), 543 (sh, 6000)
nm. IR (KBr): 2264 cm−1 (w, vCN). 1H NMR (CD2Cl2): δ 1.63 (s, 3),
7.43−7.24 (m, 35) ppm. 31P NMR (CD2Cl2): δ 62.5 ppm (referenced
by H3PO4). Anal. Calcd for C48H42NO2PS4Mo: C, 62.66; H, 4.60; N,
1.52. Found: C, 63.27; H, 4.41; N, 1.32.
4. EXPERIMENTAL SECTION
Materials and Methods. Solvents were purified by passing
through alumina columns under an argon atmosphere (MBraun
solvent purification system) and stored over 4 Å molecular sieves.
Carbon monoxide (99.0+%), triphenylphosphine, Bu4NPF6, and p-
toluenesulfonic acid (TsOH·H2O) were purchased from Aldrich and
used as received. Complexes of (Et4N)2[MoO(S2C2Ph2)2] (1),14
(Et4N)2[WO(S2C2Ph2)2] (2),12 [Mo(CO)2(S2C2Ph2)2] (5),55 and
[W(CO)2(S2C2Ph2)2] (6)12 were synthesized following literature
procedures. All syntheses and manipulations were performed under an
inert atmosphere using an MBraun glovebox (<0.1 ppm O2, <0.1 ppm
H2O) or standard Schlenk techniques. Elemental analyses were carried
out by Columbia Analytical Services (Tucson, AZ). UV−vis spectra
were recorded on a Varian Cary 50 Bio spectrometer. Infrared spectra
were recorded on a Bruker Tensor 27 FT-IR spectrometer. X-ray
crystallographic data were collected on a Bruker Smart Apex I
diffractometer. A single crystal was mounted on glass capillary fibers in
grease and cooled in a stream of dinitrogen. The structure was solved
and refined using the Bruker SHELXTL Software Package. Electro-
chemical data were collected with a CV-50W, Version 2.3, from
Bioanalytical Systems, Inc. A 3-electrode system composed of a glassy
carbon working electrode (circular, 3 mm diameter), a Ag/Ag+
reference electrode, and a platinum wire auxiliary electrode was
employed. All experiments were conducted with 0.10 M Bu4NPF6 as
supporting electrolyte at room temperature under N2 atmosphere, and
the data were referred to ferrocene/ferrocenium couple at 0.00 V.
Synthesis of [Mo(MeCN)2(S2C2Ph2)2]·H2O (3·H2O). To a
solution of TsOH·H2O (54 mg, 0.284 mmol) dissolved in 4 mL of
acetonitrile was added a solution of (Et4N)2[MoO(S2C2Ph2)2] (1) (60
mg, 0.07 mmol) in 4 mL of MeCN. The solution color changed
immediately from orange brown to dark yellow brown. The reaction
mixture was allowed to stir vigorously for an hour during which time a
dark violet solid precipitated. The supernatant fluid was removed by
decanting. The excess TsOH and [Et4N][OTs] were removed by
washing the solid with MeCN (5 × 2 mL). The resulting solid was
dried in vacuo to yield 32 mg (69%) of [Mo(MeCN)2(S2C2Ph2)]·H2O
(3·H2O). UV−vis (MeCN): λmax (εM) 265 (sh, 66000), 375 (16000),
446 (20000), 536 (sh, 9100) nm. IR (Nujol): 2317, 2282 cm−1 (w,
vCN). Redox couples (CH2Cl2/toluene =1:1 v/v): E1/2 = −0.56 and
Method 2. To a solution of PPh3 (12 mg, 0.045 mmol) in 3 mL of
CH2Cl2 was added powder [Mo(MeCN)2(S2C2Ph2)2] (3) (10 mg,
0.015 mmol). The reaction mixture instantly became a red brown
color. After stirring the reaction mixture for 1 h, the solvent was
removed in vacuo to a half of the original volume. An addition of
pentane to the concentrated reaction solution led to the formation of a
red brown precipitate that was isolated and was washed with pentane
(3 × 2 mL). The obtained precipitate was spectroscopically identical
to the product of method 1.
ASSOCIATED CONTENT
* Supporting Information
■
S
IR spectra of 1 and 3, UV−vis spectra of 3−7, IR ν(CN)
comparisons of 3, 4, 7, and free acetonitrile, 1H- and 31P- NMR
spectra of 7, and X-ray crystallographic data of 7. This material
is available free of charge via the Internet at http://pubs.acs.org.
+
−1.04 V vs FeCp2/FeCp2 . Anal. Calcd for C32H28N2OS4Mo: C,
56.45; H, 4.15; N, 4.11. Found: C, 55.99; H, 3.81; N, 4.11.
Synthesis of [W(MeCN)2(S2C2Ph2)2] (4). To a solution of
TsOH·H2O (40 mg, 0.21 mmol) dissolved in 4 mL of acetonitrile
was added a solution of (Et4N)2[MoO(S2C2Ph2)2] (1) (60 mg, 0.07
mmol) in 4 mL of MeCN. The solution color changed immediately
from red brown to dark yellow. The reaction mixture was allowed to
stir vigorously for an hour during which time a brown solid
precipitated. The supernatant fluid was removed by decanting. The
excess TsOH and [Et4N][OTs] were removed by washing the solid
with MeCN (5 × 2 mL). The resulting solid was dried in vacuo to
yield 35 mg (88%) of [W(MeCN)2(S2C2Ph2)] (4). UV−vis (MeCN):
λmax (εM) 257 (54000), 360 (8000) 413 (16000), 512 (3000) nm. IR
(Nujol): 2275, 2258 cm−1 (w, vCN). Redox couples (CH2Cl2/toluene
=1:1 v/v): E1/2 = −0.61 and −1.10 V vs FeCp2/FeCp2 . Anal. Calcd
for C32H26N2S4W: C, 51.20; H, 3.49; N, 3.73. Found: C, 51.13; H,
3.30; N, 3.48.
Photolysis of [Mo(CO)2(S2C2Ph2)2] (5) and [W(CO)2(S2C2Ph2)2]
(6). The formation of [Mo(MeCN)2(S2C2Ph2)2] (3) or [W-
(MeCN)2(S2C2Ph2)2] (4) via photolysis was studied by in situ UV−
vis spectroscopy. An acetonitrile solution of [Mo(CO)2(S2C2Ph2)2]
(5) or [W(CO)2(S2C2Ph2)2] (6) was prepared in a Schlenk cuvette
AUTHOR INFORMATION
Corresponding Author
*E-mail: Eunsuk_Kim@brown.edu.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Brown University (E.K.), the ACS-PRF
(E.K. 50840-DNI3), and the NSF (P.G.W. 1058051) for
financial support.
+
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