A diruthenium μ-carbido complex that shows singlet-carbene-like reactivity

Article abstract of DOI:10.1021/ja509364d

Low-temperature deprotonation of the cationic μ-methylidyne complex [(CpRu)₂(μ-NPh)(μ-CH)][BF₄] (Cp = η⁵-C₅Me₅) with KN(SiMe₃)₂ affords a thermally unstable μ-carbido complex [(C

Full text of DOI:10.1021/ja509364d

Communication
pubs.acs.org/JACS
A Diruthenium μ‑Carbido Complex That Shows Singlet-Carbene-like
Reactivity
Shin Takemoto,* Jun Ohata, Kento Umetani, Masahiro Yamaguchi, and Hiroyuki Matsuzaka*
Department of Chemistry, Graduate School of Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan
S
* Supporting Information
(i.e., germylenes, stannylenes, and plumbylenes) have been
reported.10
ABSTRACT: Low-temperature deprotonation of the
cationic μ-methylidyne complex [(Cp*Ru)2(μ-NPh)(μ-
CH)][BF4] (Cp* = η5-C5Me5) with KN(SiMe3)2 affords a
thermally unstable μ-carbido complex [(Cp*Ru)2(μ-
NPh)(μ-C)] (2), as evidenced by trapping experiments
with elemental S or Se and 13C NMR spectroscopic
When a transition metal fragment is used as a substituent on
a carbene center, it will serve as a π-donor substituent if it has
enough d electrons for π-back bonding (Figure 1b). Unlike
electronegative p-block group substituents, which are usually σ-
attractors, metal fragments are generally more electropositive
than carbon and are expected to be σ-donor substituents. These
characteristics of metal fragments may open up a new strategy
for electronic tuning of carbene’s reactivity, especially making
highly nucleophilic carbenes.11 In addition, metal substituents
may allow cooperative reactivity between metal and carbene
centers as well as redox-based reactivity tuning.
We earlier reported a cationic diruthenium μ-methylidyne
complex [(Cp*Ru)2(μ-NPh)(μ-CH)][BF4] (1), in which the
electron-deficient μ-methylidyne ligand is effectively stabilized
by π-back-donation from the Cp*Ru fragments.12 We
envisioned that deprotonation of 1 would produce a neutral
μ-carbido complex [(Cp*Ru)2(μ-NPh)(μ-C)] (2), for which a
singlet-carbene-like structure could be expected. We previously
isolated a Ru2Pt μ3-carbido complex [(Cp*Ru)2(μ-NHPh)(μ-
H)(μ3-C){PtMe(PMe3)2}][OTf],13 in which the diruthenium
μ-carbido fragment {(Cp*Ru)2(μ-NHPh)(μ-H)(μ-C)} is co-
ordinated to the Pt(II) fragment {PtMe(PMe3)2}+ like a π-
donor-stabilized singlet carbene ligand. Herein we report
evidence for the generation of 2 and the singlet-carbene-like
reactivity of this species.
−
observation. The reactivity of 2 toward CO2, Ph2S+CH2 ,
EtOH, and an intramolecular C−H bond indicates that the
μ-carbido carbon in 2 has an ambiphilic (nucleophilic and
electrophilic) nature consistent with the formulation of 2
as the first example of a transition-metal-substituted singlet
carbene. DFT study suggests that the Ru substituents in 2
are stronger σ-donor and weaker π-donor to the carbene
center than amino substituents in N-heterocyclic carbenes.
arbenes are neutral two-coordinate carbon species of the
C
general type CX2, whose chemistry has received much
attention for many years.1 Recently, significant progress has
been made in the chemistry of N-heterocyclic carbenes
(NHCs),2 especially in their application as ligands in metal
complexes and as catalysts or reagents for small molecule
activation. The fascinating properties of NHCs owe much to
the strongly π-donating and moderately σ-withdrawing nature
of the amino substituents,2 which makes these molecules stable
nucleophilic singlet carbenes (Figure 1a). In this respect, much
Initial attempts for the deprotonation of 1 were done by
treating 1 with amide bases MN(SiMe3)2 (M = Li, Na, K) or
LiNPri2 at −80 °C in THF and warming the reaction mixture to
room temperature, which, however, gave a complicated mixture
1
in all cases as monitored by H NMR spectroscopy.
Next, we examined low-temperature trapping of the target
complex 2 with elemental sulfur, which has been used to
demonstrate transient generation of singlet carbenes.2a,3d,14 We
found that the addition of 1 equiv of KN(SiMe3)2 (0.5 M in
toluene) to a stirred slurry of 1 in THF at −90 °C followed by
treatment with solid S8 gave the μ-thiocarbonyl complex
[(Cp*Ru)2(μ-NPh)(μ-CS)] (3a) (Scheme 1), which was
isolated in 61% yield and crystallographically characterized
(Figure 2).15 Similar treatment of 1 with KN(SiMe3)2 and
elemental selenium gave the μ-selenocarbonyl complex 3b,
which was also crystallographically characterized (Figure 2).15
To our knowledge, 3b is the first example of a complex
containing μ−η1:η1-CSe ligand.16
Figure 1. Donor properties of substituents in (a) NHCs and (b)
transition-metal-substituted singlet carbenes.
effort has been devoted to the synthesis and study of stable
carbenes bearing a wider variety of heteroatomic substituents,3
including π-donating alkoxo,4 phosphino,5 and thiolato,6 groups
but also π-accepting silyl5,7 and boryl8 groups. However, the
synthetic design of heteroatom substituents has so far remained
inside the border of p-block in the periodic table. To our
knowledge, no transition metal elements have been used as
substituent atoms directly bonded to a carbene center, although
NHCs bearing metal-functionalized organic substituents9 as
well as transition-metal-substituted heavier carbene analogues
Received: September 11, 2014
Published: November 3, 2014
© 2014 American Chemical Society
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Communication
a
Scheme 1. Generation and Trapping of 2
a
Reagents and conditions: (i) KN(SiMe3)2, THF, −90 °C to −70 °C;
(ii) 1/8S8 or Se, THF, −90 °C to r.t.
Figure 3. 13C{1H} NMR spectra for the reaction of 1′ with
KN(SiMe3)2 (0.5 M in toluene) in THF. The spectrum on the top
was recorded at 20 °C and that on the bottom at −80 °C.
a
Scheme 2. Reactivity of 2
Figure 2. ORTEP drawings of 3a, 3b, 4, and 7 with thermal ellipsoids
drawn at the 30% probability level and all hydrogen atoms omitted for
clarity.
a
−
Reagents: (i) CO2; (ii) Ph2S+CH2 ; (iii) EtOH; (iv) P(OMe)3.
The trapping reactions in Scheme 1 suggest that the target μ-
carbido complex 2 is generated by deprotonation of 1 and
behaves like a nucleophilic singlet carbene. Encouraged by
these results, we next examined the direct observation of 2 by
13C NMR spectroscopy. As shown in Figure 3, when the 13C-
enriched and THF-soluble μ-methylidyne complex
[(Cp*Ru)2(μ-NPh)(μ-13CH)][B(C6F5)4] (1′) was treated
with 1 equiv of KN(SiMe3)2 (as 0.5 M toluene solution) in
THF at −90 °C in an NMR tube, the signal of the μ-13CH
ligand in 1′ at δ = 379 ppm disappeared and a new signal
appeared at δ 675 ppm, which is assignable to the μ-13C ligand
in [(Cp*Ru)2(μ-NPh)(μ-13C)] (2′).17,18 The observed chem-
ical shift for this carbon nucleus shows good agreement with a
computed value of 702 ppm obtained from a GIAO calculation
on a full structural model of 2.15 Although the magnitude of the
downfield shift on going from μ-13CH in 1′ to μ-13C in 2′ is
extremely large, the same trend is known for NHCs and
corresponding imidazolium salts.19 The chemical shift assigned
to the μ-carbido ligand in 2′ is more downfield than any
reported chemical shifts for carbido ligands,20 the most
downfield one being reported for [Tp*W(CO)2CLi] (δ =
556 ppm; Tp* = hydrotris(dimethylpyrazol-1-yl)borate).21 For
reference, the essentially linear and tetravalent μ-carbido ligands
of the types RuCRu22 and Ru−CRu23 resonate at δ 430
and 414 ppm, respectively. The extremely downfield resonance
for the μ-carbido ligand in 2′ compared to these values might
be due to the bent and divalent nature of this carbon center.
Having gathered evidence for the formation of 2, we next
examined the reactivity of this species, which is summarized in
Scheme 2. The nucleophilic nature of 2 was demonstrated by
its reaction with CO2, which produced the formally zwitterionic
μ-C−CO2 adduct [(Cp*Ru)2(μ-NPh)(μ-CCO2)] (4) in 95%
yield. A preliminary X-ray analysis for 4 revealed that the CO2
unit in 4 is bent at a bond angle of 130(2)° and perpendicular
to the Ru−C−Ru plane (Figure 2).15 The 13C{1H} NMR
spectrum of 4 displayed resonances assignable to the μ-CCO2
ligand at δ 379 (Cα) and 175 (Cβ) ppm. The former is
consistent with the μ-alkylidyne character of this carbon,12
while the latter is slightly more deshielded than those found in
NHC−CO2 adducts (149−159 ppm).24 Complex 2 also
−
reacted with the nucleophilic ylide Ph2S+CH2 , which produced
the known μ-vinylidene complex [(Cp*Ru)2(μ-NPh)(μ-
CCH2)] (5)12 in 79% yield. Although the initial attack of the
methylide carbon may occur at either a Ru or the μ-carbido
center, this reaction is formally a reaction of the μ-carbido
ligand in 2 with a nucleophile. Thus, these two reactions
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Journal of the American Chemical Society
Communication
demonstrate the ambiphilic nature of the μ-carbido ligand in 2
consistent with its formulation as a singlet carbene-like carbon.
We also observed O−H and C−H activation reactions that
provided additional support for the singlet-carbene-like
reactivity of 2 (Scheme 2). Treatment of 2 with EtOH resulted
in the O−H activation at the μ-carbido center in 2 to give the
μ-ethoxycarbene complex [(Cp*Ru)2(μ-NPh)(μ−CH(OEt))]
(6) in 68% yield. The reaction is obviously analogous to the
1,1-addition of alcohol O−H bonds to NHCs.25 Treatment of 2
with slight excess of P(OMe)3 resulted in the intramolecular
C−H bond activation of a Cp* methyl group at the μ-carbido
carbon center to yield the product 7, which was identified
crystallographically (Figure 2).15 The C−H insertion reaction is
relatively uncommon for nucleophilic NHCs26 but fairly
common for ambiphilic singlet carbenes,5a,27 where the lone-
pair and the empty 2p orbitals on the carbene center can
interact simultaneously with the C−H σ* and σ orbitals,
respectively, to facilitate the C−H bond activation. It seems
likely that the intramolecular C−H insertion that furnished 7
proceeded via the intermediate P(OMe)3 adduct of 2 whose μ-
carbido center would behave like an ambiphilic singlet carbene
to induce the intramolecular C−H insertion.
mainly centered on the Ru2N unit with Ru−N π-antibonding
character and may explain Lewis acidic behavior of the Ru
centers. An orbital that predominantly represents the empty π-
type orbital on the μ-carbido carbon center is found at LUMO
+3. This orbital has Ru−C π-antibonding character and
indicates that the Ru fragments serve as π-donors to the
carbon center to destabilize the empty carbon 2p orbital and
reduce the electrophilicity of this carbon. The HOMO and
LUMO + 3 of 2 demonstrate that the μ-carbido carbon center
in 2 has a singlet-carbene-like electronic structure with the Ru
units acting as π-donor substituents. To evaluate the donor
ability the of Ru substituents, the energies of HOMO and
LUMO+3 in 2 were compared with the energies of the
corresponding orbitals of an NHC, namely 4,5-dihydro-1,3-
dimethylimidazol-2-ylidene (SIMe). As shown in Figure 4c, the
HOMO of 2 is higher than that of SIMe, which represents
predominantly the nonbonding carbon sp2 orbital. On the
other hand, the LUMO + 3 of 2 is lower than the
corresponding empty carbon 2pz orbital of SIMe (LUMO).
These data suggest that the Ru substituents in 2 has a stronger
σ-donor and weaker π-donor ability than the amino
substituents in SIMe.
In summary, deprotonation of the cationic μ-methylidyne
complex 1 at low temperature produces the μ-carbido complex
2 as evidenced by trapping experiments and 13C NMR
observation. The reactivity of 2 toward electrophile, nucleo-
phile, and O−H/C−H bonds is consistent with its formulation
as a singlet carbene with considerable nucleophilicity and
electrophilicity. The present study demonstrates for the first
time that transition metal fragments can be used as heteroatom
substituents on a carbene carbon center, and the Ru
substituents in 2 appear to be stronger σ-donor and weaker
π-donor compared to amino substituents in NHCs. Further
studies will be directed toward elucidating the ability of 2 as
ligands for metal complexes as well as the design and synthesis
of other types of transition-metal-substituted carbenes.
To gain insights into geometric and electronic structure of 2,
we performed a DFT calculation (B3LYP/6-31G(d,p)+SDD
level), where a singlet state was considered according to the
experimental results. The optimized geometry of 2 is shown in
Figure 4a. It contains a planar NRu2C core analogous to those
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures, characterization data, and computa-
tional details. This material is available free of charge via the
Internet at http://pubs.acs.org.
Figure 4. (a) Optimized structure of 2. Selected distances (Å) and
angle (deg): Ru−C = 1.915 (av), Ru-N = 1.924 (av), Ru−Ru = 2.530,
Ru−C−Ru = 82.64. (b) HOMO (left) and LUMO + 3 (right) of 2.
(c) Frontier orbital energies of 2 and SIMe.
AUTHOR INFORMATION
■
Corresponding Authors
takemoto@c.s.osakafu-u.ac.jp
matuzaka@c.s.osakafu-u.ac.jp
found in complexes of the type [(Cp*Ru)2(μ-NPh)(μ-L)].12
The Ru−C bond distances for the μ-carbido ligand in 2 were
calculated to be 1.915 (av) Å. These distances are slightly
longer than those calculated for the μ-CH ligand in the
protonated precursor [(Cp*Ru)2(μ-NPh)(μ-CH)]+ (1.899
(av) Å), similarly to NHC/NHC-H+ pairs.28 However, the
computed Ru−C distances in 2 are still short enough to suggest
Ru−C multiple bond character. Further insights into the
bonding and electronic structure of 2 were obtained by
molecular orbital analysis. HOMO of 2 is a σ-type orbital that is
predominantly localized on the μ-carbido carbon center and
represents the nonbonding electron pair on this atom (Figure
4b). The LUMO of 2 (not shown)15 is a π-type orbital that is
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Dr. Yuichiro Mutoh for valuable comments on μ-CSe
complexes. This work was supported by Grant-in-Aid for
Scientific Research on Innovative Areas “Molecular Activation
Directed toward Straightforward Synthesis” and “Stimuli-
responsive Chemical Species for the Creation of Functional
Molecules” from the Ministry of Education, Science, Sports,
and Culture of Japan. We also thank financial support from
TOYOTA Motor Corporation.
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