Formation and ligand-based reductive chemistry of bridged bis-alkylidene scandium(III) complexes

Article abstract of DOI:10.1039/c7sc02018j

The chemistry of rare-earth carbene and alkylidene complexes including their synthesis, structure and reaction is a challenging issue because of their high reactivity (or instability) and the lack of synthetic methods. In this work, we report the first synthesis of the bridged bis-alkylidene complexes which feature a 2-butene-1,1,4,4-tetraanion and four Sc-C(sp³) bonds by the reaction of 1,4-dilithio-1,3-butadienes with ScCl₃. This reaction proceeds via two key intermediates: an isolable scandacyclopentadiene and a proposed scandacyclopropene. The scandacyclopentadiene undergoes β,β′-C-C bond cleavage to generate the scandacyclopropene, which then dimerizes to afford the bridged bis-alkylidene complex via a cooperative double metathesis reaction. Reaction chemistry study of the bridged bis-alkylidene complex reveals their ligand-based reduction reactivity towards different oxidants such as hexachloroethane, disulfide and cyclooctatetraene.

Full text of DOI:10.1039/c7sc02018j

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Formation and Ligand-Based Reductivity of Bridged Bis-alkylidene
Scandium(III) Complexes
Wangyang Ma,a Chao Yu,a Yue Chi,a Tianyang Chen,a Lianjun Wang,ac Jianhao Yin,a Baosheng Wei,a
Ling Xu,a Wen-Xiong Zhang,*ab and Zhenfeng Xia
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The chemistry of rare-earth carbene and alkylidene complexes including their synthesis, structure and reaction is a
challenging issue because of their high reactivity (or instability) and the lack of synthetic methods. In this work, we will
report the first synthesis of the bridged bis-alkylidene complexes which feature a 2-butene-1,1,4,4-tetraanion and four
Sc−C(sp3) bonds by the reaction of 1,4-dilithio-1,3-butadienes with ScCl3. This reaction proceeds via two key intermediates:
an isolable scandacyclopentadiene and a proposed scandacyclopropene. The scandacyclopentadiene undergoes β,β′-C−C
bond cleavage to generate the scandacyclopropene, which then dimerizes to afford the bridged bis-alkylidene complex via
a cooperative double metathesis reaction. Reaction chemistry study of the bridged bis-alkylidene complex reveals their
ligand-based reductivity towards different oxidants such as hexachloroethane, disulfide and cyclooctatetraene.
www.rsc.org/
Transition metal carbene and alkylidene complexes have been complexes (Ce, Sm, Eu and Yb) supported by redox-inert
extensively studied because of their importance in ligands tend to perform a single electron redox process. The
organometallic chemistry, coordination chemistry and utilization of redox-active ligands at the rare earth metal
synthetic organic chemistry.1 In contrast, rare-earth metal centers is an alternative strategy for affording multi-electron
carbene and alkylidene complexes are very limited mainly due redox reactivity.14 Ligand-based reductive chemistry of
to the energy mismatch between the rare-earth metals and trivalent rare-earth organometallic compounds has received
ligand orbitals.2-12 Since the rare-earth alkylidene complex was much attention. Evans and coworkers have made a great
first postulated in 1979,3 the pioneering works have been progress in reductive reactivity of (C5Me5)3Ln (Ln = La, Nd, Sm,
made to isolate and characterize them. Some pincer-like rare- etc.) and provided a wide variety of new reductive chemistry
earth alkylidene complexes have been reported independently for rare earth metals.13a,15
by Cavell,4 Liddle,5 and Mézailles.6 Very recently, Cui et al.
Herein, we report the first synthesis of the bridged bis-
reported the lutetium methanediide-alkyl complexes,7 and alkylidene complex featuring a 2-butene-1,1,4,4-tetraanion
Chen et al. reported the non-pincer-type mononuclear and four Sc−C(sp3) bonds from 1,4-dilithio-1,3-butadienes and
scandium alkylidene complexes.8 Furthermore, rare-earth ScCl3. This reaction proceeds via two key intermediates:
methylidene complexes were also stabilized by chloride scandacyclopentadiene16,17 and scandacyclopropene.18,19 DFT
bridges9 or Lewis-acid such as AlMe3.10 Interestingly, mixed calculations indicate the dimerization of scandacyclopropenes
methyl/methylidene complexes11 and cubane-like methylidene via the cooperative double metathesis is the key factor for the
complexes12 have been reported. Despite these recent formation of the bridged bis-alkylidene complex. Interestingly,
advances, the chemistry of rare-earth alkylidene complexes is the bridged bis-alkylidene scandium(III) complex shows
still in its infancy, and the bridged bis-alkylidene complex unexpected ligand-based two-electron or four-electron
remains scarce.
reductivity
towards
different
oxidants
such
as
Reductive reaction of rare-earth organometallic hexachloroethane, disulfide and cyclooctatetraene.
compounds is
a
fundamental process in organometallic
Silyl-substituted 1,4-dilithio-1,3-butadienes 1a-c were
chemistry and coordination chemistry.13 Rare earth metal readily prepared according to our previous procedure.20 When
the 1:1 reaction of 1a and solvated ScCl3 in THF was conducted
at -20 oC, the light yellow crystalline complex 2a could be
a.Beijing National Laboratory for Molecular Sciences, MOE Key Laboratory of
Bioorganic Chemistry and Molecular Engineering, College of Chemistry, Peking
University, Beijing 100871, China.
isolated exclusively in 65% yield (Scheme 1). An X-ray analysis
of 2a revealed that it is a LiCl-ligated scandacyclopentadiene
E-mail: wx_zhang@pku.edu.cn
(Figure 1). The Sc(III) center adopts a distorted octahedral
b.State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin,
fashion bonded with two C(sp2) atoms, two chlorides and two
300071, China
c. School of Chemistry and Chemical Engineering, Hunan Institute of Engineering,
Xiangtan, 411104, China
†Electronic Supplementary
10.1039/x0xx00000x
THF molecules. The C1-C2 (1.348(4) Å) and C3-C4 (1.376(4) Å)
bond lengths are within the range of standard C=C bond
lengths, and the C2-C3 bond length (1.520(3) Å) indicates a
Information
(ESI)
available:
See
DOI:
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typical C-C single bond. These data of bond lengths clearly temperature. However, when the THF-d8 solution of 2a was
o
o
show its butadienyl dianionic structure in 2a
.
heated at 45 C for 3 h or 80 C for 10 min, the TMS proton
resonance at -0.38 ppm completely disappeared in the 1H NMR
spectrum, and two new singlets integrated to the same
numbers of protons appeared at -0.23 ppm and 0.20 ppm (see
SI for more details). The singlet at 0.20 ppm was assigned to
the TMS proton resonance of PhC≡CTMS by comparing with its
standard spectrum. The GC retention time and molecular ion
peak (m/z = 174) detected by GC-MS are also consistent with
Cl
Cl
2 Li(THF)
•
TMS
4
Sc
TMS
R
R
R
Sc
Cl3
THF
TMS
4
Li
Li
C
R
Cl
Cl
4
C
THF, 80 oC, 3 h
C
Sc
Sc
R
C
TMS
THF
R
TMS
1a: R = Ph
TMS
Sc
Cl
1a
1b: R = p-Me-C6H4
Cl
3a
t
: R = Ph, 62% from
1c
: R = p- BuC6H4
≡CTMS. The other new singlet at
-
the standard sample of PhC
quantitative yield from 2a
0.23 ppm was assigned to the TMS groups of a new complex
3a, which was obtained in almost quantitative yield by
thermolysis of 2a. Furthermore, we found the synthesis of 3a
does not require isolation of 2a as the starting material. 3a
could be conveniently prepared by the reaction of 1a with
solvated ScCl3 in THF solution at 80 oC for 3 h. Similarly, 3b and
3c could be prepared from the corresponding 1,4-dilithio-1,3-
butadienes and ScCl3 (Scheme 1).
3b: R = p-MeC6H4, 75% from 1b
3c: R = p-tBuC6H4, 68% from 1c
THF
- 20 oC, 2 h
R = Ph
4
TMS
THF
Ph
TMS
80 o
C
+
Li
Cl
Ph
Ph
Cl
2
Sc
Li
(THF)2
4
Cl
THF
TMS
2a
: 65%
Scheme 1 Synthesis of scandacyclopentadiene 2a and bridged bis-
alkylidene scandium(III) complexes 3a-c
An X-ray analysis of 3a reveals it is a bridged bis-alkylidene
complex and adopts a dimeric ate complex via μ2-chloride
bridges (Figure 2). One scandium center (e.g. Sc1) is bonded
with two carbon atoms and two terminal chlorides, while the
other one (e.g. Sc2) is bonded with two carbon atoms, two
bridged chlorides and one THF. The Sc1-Sc2 distance
(3.1366(9)Å) is the shortest length found in the literature,
which is notably shorter than those in dinuclear scandium
hydride complexes (3.20−3.40 Å).21 Two lithium atoms act as
.
counterions, and each lithium atom forms
a distorted
tetrahedron surrounded by four THF molecules. The bond
lengths of C1-C2 (1.468(4) Å) and C3-C4 (1.465(5) Å) in 3a are
significantly longer than those [C1-C2, 1.348(4) Å; C(3)-C(4),
1.376(4) Å] in 2a. The bond length of C2-C3 (1.430(4) Å) in 3a
are significantly shorter than the corresponding C2-C3
(1.520(3) Å) in 2a. Thus, the bond lengths in the C1-C2-C3-C4
moiety in 3a are averaged and not the classical lengths of C-C
single and double bonds. These results show that 3a has a
highly delocalized structure with a tetraanionic ligand. Most
importantly, these results are in striking contrast with what
was observed previously for transmetalation reaction of 1,4-
dilithio-1,3-butadienes with metal salts which gave 1,3-
butadiene-1,4-dianion complexes.20
Figure 1 Molecular structure of complex 2a with thermal ellipsoids at
30% probability. H atoms are omitted for clarity.
TMS
THF
TMS
THF
D5
C6
ꢀ
Ph
Ph
ꢀ
Cl
Cl
Sc
Li
(THF)2
Sc
Li
(THF)2
+
Figure 2 Molecular structure of complex 3a with thermal ellipsoids at
30% probability. H atoms, and two [Li(THF)4]+ counterions are omitted
for clarity.
Cl
Cl
THF
C6D5
THF
TMS
TMS
2a-D10
TMS
2a
TMS
1) THF
TMS
H
H
H
H
H
H
H
H
H
H
Complex 2a is sensitive to air and moisture but stable
under dry N2 atmosphere. In the 1H NMR spectrum in THF-d8, a
singlet at -0.38 ppm was observed and assigned to the proton
resonance of TMS groups. Two β-C(sp2) atoms (C2 and C3)
displayed a singlet at 167.6 ppm in the 13C NMR spectrum,
while two α-C(sp2) atoms (C1 and C4) showed a broad peak at
203.8 ppm, probably due to the coupling with scandium
80 οC, 3 h
2) H2O
D5
D5
C6
C6
Ph
Ph
+
+
Ph
C6D5
Ph
+
TMS
TMS
H
H
TMS
4a-D5
TMS
4a-D10
TMS
4a
C6D5
C22H22D10Si2
m/z: 362.2667
C22H27D5Si2
C22H32Si2
m/z: 352.2037
m/z: 357.2352
Scheme 2 The crossover-reaction between 2a and 2a-D10
.
1
(nuclear spin quantum number I = 7/2). The H NMR spectrum
of 2a in THF-d8 showed no obvious change for 2 weeks at room
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The formation of the asymmetric unit in 3a from two phase optimized structures. Scandacyclopentadiene IM1 will
molecules of 2a along with elimination of two alkynes is a very undergo β,β′-C−C
bond cleavage to generate
interesting process and intrigues us to explore the reaction scandacyclopropene IM2 by release of one equiv of alkyne.
mechanism. The crossover reaction between 2a and 2a-D10 The β,β′-C−C bond cleavage from IM1 to IM2 is the critical step
was carried out. When the reaction mixture was quenched with the highest energy barrier of 13.3 kcal/mol in solution
with H2O, 4a
,
4a-D5, and 4a-D10 could all be detected by HRMS phase,
which
means
that
IM1
is
isolable.
(Scheme 2). This result unambiguously reveals that the 2- Metallacyclopropenes, as an important class of reactive
butene-1,1,4,4-tetraanion moiety in 3a should be originated intermediates, have been isolated and characterized in
from two molecules of scandacyclopentadienes instead of a transition and main group organometallic chemistry.18,19 The
simple reduction of
scandacyclopentadiene. Thus, the crossover experiment dimerization to give
excludes two possible pathways involving two eliminated contrast, rare-earth metallacyclopropenes are unknown. IM2
alkynes from the same scandacyclopentadienes: i) cooperative is the first optimized structure of rare-earth
a
diene moiety in one metallacyclopropene, e.g. aluminacyclopropene can undergo
a
1,4-dialuminacyclohexadiene.25 In
intermolecular redox process, and ii) stepwise intermolecular metallacyclopropenes by DFT calculations. Then we tried to
redox via scandacyclopropene process (see SI for more optimize the dimeric structure of IM3′ which is similar to 1,4-
details).
Based on the above information, we proposed
dialuminacyclohexadiene. However, the optimization of the
a
structure of IM3′ to a local energy minimum failed, probably
mechanism involving the scandacyclopropene intermediate. because of its high energy and instability. Rather than giving
For a better understanding of the formation of 3a, DFT IM3′, a new intermediate IM3 as a result of two IM2
calculations were carried out using Gaussian 09 (Figure 3).22 approaching to each other via the weak Sc−C interacꢀon was
We chose the LiCl-free scandacyclopentadiene IM1 as a optimized to a local minimal energy, 2.7 kcal/mol lower than
starting model compound and the THF-ligated monomer 3a-M IM2. Surprisingly, a cooperative double metathesis of IM3
as a targeted compound for simplization.23 The structure of all gives 3a-M via the transition state TS2
the minima and transition states were optimized at the scandacyclopropene rings locate in
.
In TS2, two
a
triangular prism
B3LYP24/LANL2DZ (for Sc)/6-31+G*(for other elements) level in geometry, in which each Sc atom is coordinated to another
gas phase. The effect of solvent was examined by performing carbon neighbouring TMS group. This geometry of TS2 could
single-point self-consistent reaction field (SCRF) calculations also explain the selectivity of C(Ph)−C(Ph) coupling.
based on the polarizable continuum model (PCM) for gas-
Figure 3 DFT calculated energy profiles of related intermediates and transition-states in the generation of 3a-M (red lines: broken bonds; blue
lines: newly formed bonds).
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alkylidene scandium(III) complex 3a can act as an efficient two-
electron or four-electron reductant.
Conclusions
In summary, we have developed a simple and efficient
synthetic method for the first series of well-defined bridged
bis-alkylidene scandium(III) complexes from 1,4-dilithio-1,3-
butadienes and ScCl3. This reaction proceeds via two key
intermediates: an isolable scandacyclopentadiene and
proposed scandacyclopropene. A mechanistic pathway of C-C
bond recombination through dimerization of
a
scandacyclopropene intermediate are well elucidated by DFT
calculations. Bridged bis-alkylidene scandium(III) complex are
found to show ligand-based reductivity towards different kinds
of oxidants. Further reaction chemistry of bis-alkylidene
scandium(III)
complexes
and
characterization
of
scandacyclopropenes are in progress.
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
This work was supported by the Natural Science Foundation of
China (nos. 21572005, 21372014).
Notes and references
1
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−
−
Scheme 3 Ligand-based reductivity of 3a towards different oxidants.
−
The structure of 3a features the 2-butene-1,1,4,4-
tetraanion moiety and thus we thought it could be oxidized to
generate the diene moiety in 2a as illustrated in Scheme 3. As
we expected, 2a was generated by treatment with two
equivalents of hexachloroethane as an oxidant (Scheme 3a).
This reaction resulted in the formation of ScCl3 which can be
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mixture was heated at 80 oC, 3a was transformed to
PhC≡CTMS and ScCl3 (Scheme 3b). Furthermore, when
3
2
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5
served as an oxidant,26 the reaction of 3a with
(see SI for X-ray structure of , Scheme 3c)
5
provided complex
6
6
along with the formation of PhC≡CTMS. When 3a was treated
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23 The Sc-O interaction is much stronger than Sc-Cl interaction
because of the oxophilicity of rare-earth elements. In THF,
LiCl in 2a will be easily replaced by THF to yield a LiCl-free
complex IM1. Based on the effects of THF atmosphere, we
excluded LiCl from the calculation. In THF, 3a tends to be a
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DOI: 10.1039/C7SC02018J
Table of Contents
Bridged bis-alkylidene Sc(III) complexes featuring a 2-butene-1,1,4,4-tetraanion are synthesized and
show unexpected ligand-based two-electron or four-electron reductivity towards different oxidants.