Salt-free reducing reagent of bis(trimethylsilyl)cyclohexadiene mediates multielectron reduction of chloride complexes of W(VI) and W(IV)

Article abstract of DOI:10.1021/ja401589a

We developed a salt-free reduction of WCl₆ using 1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene (MBTCD) in toluene to give a low-valent trinulcear tungsten complex involving W(II) and W(III) centers, while in the presence of redox active ligands such as α-diketone and α-diimine the same reduction produced W(IV) complexes with the corresponding redox-active ligands, (α-diketone)WCl₄ and (α-diimine)WCl₄. A W(VI) complex with two α-diketone ligands, (α-diketone)₂WCl₂, was found to be synthetically equivalent to low-valent W(IV) species that trapped azopyridine to give (α-diketone)WCl₂(azopyridine).

Full text of DOI:10.1021/ja401589a

Communication
pubs.acs.org/JACS
Salt-Free Reducing Reagent of Bis(trimethylsilyl)cyclohexadiene
Mediates Multielectron Reduction of Chloride Complexes of W(VI)
and W(IV)
Hayato Tsurugi,† Hiromasa Tanahashi,† Haruka Nishiyama,† Waldemar Fegler,‡ Teruhiko Saito,†
Andreas Sauer,‡ Jun Okuda,‡ and Kazushi Mashima†,*
†Department of Chemistry, Graduate School of Engineering Science, Osaka University, and CREST, JST, Toyonaka, Osaka 560-8531,
Japan
‡Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, D-52056 Aachen, Germany
S
* Supporting Information
method without metal salt formation, several organic-based
reductants have been used for the reduction of WCl6:7
ABSTRACT: We developed a salt-free reduction of WCl6
using 1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene
(MBTCD) in toluene to give a low-valent trinulcear
tungsten complex involving W(II) and W(III) centers,
while in the presence of redox active ligands such as α-
diketone and α-diimine the same reduction produced
W(IV) complexes with the corresponding redox-active
ligands, (α-diketone)WCl4 and (α-diimine)WCl4. A
W(VI) complex with two α-diketone ligands, (α-
diketone)2WCl2, was found to be synthetically equivalent
to low-valent W(IV) species that trapped azopyridine to
give (α-diketone)WCl2(azopyridine).
allyltrimethylsilane and alkenes are effective for generating
W(V) and W(IV) species with the formation of easily removable
organic products or chlorotrimethylsilane. Access to the more
reactive W(II) and W(III) species without any salt contact,
however, is in high demand. Recently, we developed an
organosilicon-based reductant, 1-methyl-3,6-bis(trimethylsilyl)-
1,4-cyclohexadiene (abbr. MBTCD), for reducing tantalum and
niobium pentachlorides.8 Although allyltrimethylsilane did not
reduce TaCl5 and NbCl5, low-valent tantalum and niobium
species were generated by the addition of MBTCD to toluene
solutions of TaCl5 and NbCl5. The high reducing ability of
MBTCD led us to prepare highly valuable low-valent tungsten
species in a salt-free manner by reducing WCl6 using MBTCD
with easy removal of the byproducts, toluene and chlorotrime-
thylsilane. Herein, we report the multielectron reduction of WCl6
by simple treatment with MBTCD to produce a salt-free W(II)−
W(III) species, which, to the best of our knowledge, is the first
example of a trinuclear cluster of tungsten which is free of
additional salts. In situ generated low-valent tungsten species
were immediately trapped by 4,4′-dimethylbenzil (abbr. α-
diketone), N,N′-bis(aryl)-1,4-diaza-1,3-butadiene (abbr. α-dii-
mine), or 1,2-bis(arylimino)acenaphthene (abbr. BIAN) ligands
to form (L)WCl4 complexes. Moreover, the isolated bis(α-
diketone)WCl2 served as a low-valent tungsten source: electron-
accepting molecules such as azopyridine reacted with bis(α-
diketone)WCl2 with the release of one α-diketone ligand to
produce (α-diketone)WCl2(azopyridine).
We first examined the reduction of WCl6 using excess
MBTCD in toluene at room temperature, which resulted in
the immediate precipitation of black powders from the reaction
mixture. 1H NMR observation of the reaction in C6D6 revealed
the generation of 3.5 equiv of ClSiMe3 and 1.75 equiv of
C6H5CH3, equal to the consumption of 1.75 equiv of MBTCD,
suggesting that the powders contained divalent and trivalent
tungsten atoms (Scheme 1, path a). Although the black powder
could not be dissolved in any nonpolar solvents, it was soluble in
polar coordinating solvents such as THF. In fact, extraction of the
black precipitates with THF afforded greenish black micro-
ow-valent early transition metal species have attracted
Lspecial attention due to their important role in the activation
of small molecules such as dinitrogen and carbon monoxide,
leading to ammonia, tris(trimethylsilyl)amine, and CO-coupling
products.1 Reduction of high-valent early transition metal
complexes is the general strategy for preparing highly reactive
low-valent early transition metal species, and strong reductants
such as amalgams of alkaline and alkaline-earth metals, zinc dust,
and alkylmetal reagents are conventionally used as the
reductants.2 The generation of low-valent metal species using
the conventional reductants, however, is always accompanied by
reductant-derived metal salts as well as side-products through
over-reduction. Thus, such a reducing method sometimes not
only hampers isolation of the desired low-valent metal complexes
but also retards their reactivity or catalytic performance due to
coordination of the metal salt to the catalytically active metal
center.3
Among the low-valent early transition metal complexes, low-
valent tungsten complexes are highly powerful chemical species
that activate not only single covalent bonds but also various C
X double bonds and CC and NN triple bonds. Therefore,
extensive studies have focused on the preparation of low-valent
tungsten species.4,5 Although the versatility of such low-valent
tungsten species is well demonstrated, the reduction of high-
valent tungsten species, including W(VI), W(V), and W(IV)
complexes, requires rather strong reductants, which generate
reductant-derived metal salts, resulting in a low yield of the low-
valent tungsten species in many cases.6 To improve the reducing
Received: February 13, 2013
© XXXX American Chemical Society
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Journal of the American Chemical Society
Communication
C1−C1*, 1.49(3) Å for 4a), whereas the solid-state structure of 3
revealed that 3 adopts a dianionic ene-diolate ligand coordinating
to W(VI) species, based on the elongated C−O bonds (1.317(7)
and 1.322(7) Å) and the shortened C−C bond (1.416(8) Å) of
the ligand motif (Figure 1).10a,b The different coordination mode
Scheme 1. Reduction of WCl6 Using 1-Methyl-3,6-
bis(trimethylsilyl)-1,4-cyclohexadiene
crystals of W3Cl7(THF)3 (1) in 91% yield. The X-ray diffraction
studies revealed the trinuclear structure of the tungsten complex
1, in which three tungsten atoms are arranged in an equilateral
triangle and formal oxidation states are W(II), W(II), and
W(III). One μ3-Cl ligand is located above the plane of the
triangle, and three μ-Cl ligands are in the W3 plane. Each
tungsten atom is further coordinated by one terminal chloride
and one THF ligand. The averaged oxidation state of the
tungsten atoms is +2.33, and to the best of our knowledge, this is
the lowest oxidation state for the triangular group 6 metal
clusters.9 These results indicated that WCl6 was reduced to
[WCl2.33]n aggregates in toluene in the first stage, and the
addition of THF led to the formation of the trinuclear
paramagnetic species 1. In contrast, reduction of WCl6 in THF
using excess MBTCD resulted in the formation of WCl4(THF)2
(2) in quantitative yield;7d its further reduction by MBTCD did
not proceed, however, probably due to the coordination of THF
to the in situ generated low-valent tungsten species accordingly
preventing further reduction (Scheme 1, path b).
The addition of 1 equiv of MBTCD to a toluene solution of
WCl6 at low temperature resulted in the formation of a WCl4
species as a dark-gray suspension when the reaction mixture was
warmed to room temperature. Subsequently, redox-active
ligands, such as 4,4′-dimethylbenzil, N,N′-bis(aryl)-1,4-diaza-
1,3-butadiene, and 1,2-bis(arylimino)acenaphthene ligands,
were added to trap the low-valent tungsten species to give the
corresponding tetrachlorotungsten complexes 3, 4a, and 4b in
good yield (eq 1).
Figure 1. Molecular structures of tungsten complexes 3 and 4a.
Hydrogen atoms are omitted for clarity. Selected bond distances (Å):
(a) for 3: W−O1, 1.948(4); W−O2, 1.976(4); C1−C2, 1.416(8); C1−
O1, 1.322(7); C2−O2, 1.317(7). (b) for 4a: W−N1, 2.174(17); C1−
N1, 1.37(3); C1−C1*, 1.49(3).
for 3 in the solid state compared with that of 4a and 4b might be
due to the small dihedral angle between the metallacycle, W−
O1−C1−C2−O2, and 4-methylphenyl rings (26.1°) that
accelerates the reduction of the α-diketone ligand in dianionic
form due to the extended π-conjugation from the metallacycle to
one aromatic ring.11
Although isolation of highly reduced W(II) species without
any supporting ligands and solvents was difficult in pure form due
to facial aggregation in the reaction mixture as described in
Scheme 1, the addition of 2 equiv of the α-diketone ligand to the
reaction mixture of 2 equiv of MBTCD and WCl6 in toluene led
to the isolation of bis(α-diketone)WCl2 (5) as a purple powder
in 93% yield (eq 2).
Complex 5 was alternatively prepared by treating complex 3
with the α-diketone ligand in the presence of MBTCD (1 equiv),
suggesting that the formation of 5 from WCl6 proceeded in a
stepwise manner through the formation of complex 3. The
complex 5 was characterized by the 13C NMR spectrum, which
displayed a singlet signal at δC 162.2 assignable to the dianionic
ene-diolate moiety.10a,b Such a coordination mode of the α-
diketone ligand indicated that four electrons were totally stored
in the two α-diketone ligands to form two dianionic ene-diolate
ligands, giving formal W(VI) species 5. Electrons stored in the
redox-active ligands of metal complexes are utilized for the
activation of small molecules by transferring the electrons from
the ligands to substrates,12 and the bis(α-diketone)WCl2
complex 5 was thus expected to serve as a synthetic equivalent
to low-valent W species.
In their 13C NMR spectra, the resonances for the carbonyl
carbons of the α-diketone ligand or the imine carbons of the α-
diimine ligand were observed at δC 192.6 for 3, δC 150.8 for 4a (a
resonance of the corresponding imine carbons for 4b was not
observed), indicating that the ligands coordinated to the metal
center in a neutral mode to form W(IV) species in solution.8b,10
In the solid state, the neutral coordination modes of α-diimine
and BIAN ligands for 4a and 4b bearing W(IV) were further
confirmed by X-ray diffraction analyses (C1−N1, 1.37(3) Å,
In contrast to the difficulty of the reduction of the THF adduct
2, W(IV) species having redox-active ligands could be reduced,
B
dx.doi.org/10.1021/ja401589a | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
probably due to the relatively electron-deficient character of the
W(IV) metal center. In fact, the reduction of complex 4a using
MBTCD in the presence of the α-diketone ligand afforded
tungsten complex 6 bearing α-diketone and α-diimine ligands
(Scheme 2). Complex 6 was a 1H NMR silent species, and the
molecular structure was clarified by X-ray diffraction study
(Figure 2).
the release of 1 equiv of α-diketone.14 During the reaction, the
transient W(IV) species was immediately trapped by azopyridine
to give tungsten complex 8 (eq 3). The overall molecular
structure was clarified by X-ray diffraction study, which revealed
the coordination of dianionic α-diketone and neutral azopyridine
ligands to the tungsten metal center (Figure 3).16
Scheme 2. Reduction of (α-Diimine)WCl4 (4a) Using
MBTCD
Figure 3. Molecular structure of tungsten complex 8. Hydrogen atoms
and solvent molecules are omitted for clarity. Selected bond distances
(Å): W−O1, 1.969(3); W−O2, 2.005(3); C1−C2, 1.382(6); C1−O1,
1.329(5); C2−O2, 1.332(5); W−N1, 2.120(4); W−N2, 2.063(5); W−
N4, 2.126(4); C21−N2, 1.536(7); N2−N3, 1.213(7); C22−N3,
1.326(7).
In summary, we developed a mild reduction method for
tungsten species using a well-designed organosilicon-based
reagent, MBTCD. Low-valent W(II) species were accessible
without conventional metal- or alkylmetal-based reductants, and
low-valent tungsten cluster 1 was isolated as a W(II,II,III)
triangular trinuclear cluster. In contrast, in the presence of α-
diketone and α-diimine ligands, in situ generated W(IV) or
W(II) species were immediately trapped by the redox-active
ligands to afford (α-diketone)WCl4, (α-diimine)WCl4, and (α-
diketone)2WCl2, where electrons stored in the redox-active
ligand backbone were effectively transferred to the tungsten
metal center by adding electron-accepting molecules. Further
extension of the reduction of group 6 metal complexes using
MBTCD and related compounds is ongoing in our laboratory.
Figure 2. Molecular structure of tungsten complex 6. Hydrogen atoms
are omitted for clarity. Selected bond distances (Å): W−O1, 1.956(5);
W−O2, 2.014(5); W−N1, 2.016(6); W−N2, 2.037(6); C1−C2,
1.355(10); C1−N1, 1.371(8); C2−N2, 1.390(8); C27−C28,
1.384(10); C27−O1, 1.362(8); C28−O2, 1.330(8).
In the solid state structure of complex 6, both C−O and C−N
bonds are elongated compared to the original neutral ligands,
and the long−short−long sequences of the ligand backbones
indicate two electron reductions at both α-diketone and α-
diimine ligands to form W(VI) species in the solid state.8b In
contrast, when complex 4a was further reduced by MBTCD in
the presence of the α-diimine ligand as a trial to prepare bis(α-
diimine)WCl2, approximately 0.8 equiv of MBTCD was
consumed to give a triply chloride-bridged dimeric and
paramagnetic tungsten complex 7, in which the additional α-
diimine ligand was not incorporated.13 Geometries of the α-
diimine ligand backbone in 7 indicate the dianionic coordination
of the both α-diimine ligand to the tungsten metal center.14 The
distance of W1−W2 is 2.8948(8) Å, consistent with the
tungsten−tungsten single bond.15
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details, molecular structures of tungsten complexes
1, 4b, and 7, and intermolecular π−π stacking of 3 in the solid
state (PDF) and a CIF file for complexes 1, 3, 4a, 4b, 6, 7, and 8.
These materials are available free of charge via the Internet at
http://pubs.acs.org.
Because of the redox reactivity of α-diketone and α-diimine
ligands from neutral to dianion, we used complex 5 as a low-
valent equivalent for catching small molecules. The reactions of 5
with azopyridine yielded (α-diketone)WCl2(azopyridine) with
AUTHOR INFORMATION
Corresponding Author
mashima@chem.es.osaka-u.ac.jp
■
C
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Journal of the American Chemical Society
Communication
Mindiola, D. J.; Cummins, C. C. J. Am. Chem. Soc. 1999, 121, 10426.
(j) Blackwell, J. M.; Figueroa, J. S.; Stephens, F. H.; Cummins, C. C.
Organometallics 2003, 22, 3351.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
T.S. thanks the Japan Society for the Promotion of Science for a
fellowship and JSPS Japanese-German Graduate Externship.
W.F. and A.S. thank the Deutsche Forschungsgemeinschaft
through the International Research Training Group SeleCa
(GRK 1628) for financial support. This work was supported by
the Core Research for Evolutional Science and Technology
(CREST) program of the Japan Science and Technology Agency
(JST), Japan, and Grant-in-Aid for Scientific Research on
Innovative Areas, “Molecular Activation Directed toward
Straightforward Synthesis”, MEXT, Japan.
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