Theoretical Study on the Stability of Metallasilabenzyne and Its
Isomers
Xuerui Wang, and Jun Zhu*
State Key Laboratory of Physical Chemistry of Solid Surfaces, Fujian Provincial Key Laboratory of
Theoretical and Computational Chemistry and Department of Chemistry, College of Chemistry and
Chemical Engineering, Xiamen University, Xiamen 361005(P. R. China).
KEYWORDS: Metallasilabenzyne; Ring strain; Aromaticity.
Supporting Information Placeholder
ABSTRACT: Metallabenzyne has been an interesting research highlight in the chemistry of metallaaromatic complexes.
Particularly, the well-known reluctance to participate in π bonding makes metallasilabenzyne complexes which derived
by taking place of carbyne carbon in metallabenyzne more intriguing and elusive. Inspired by our previous theoretically
study on the aromaticity and stability of osmasilabenzyne and metallasilapentalyne, we wonder how about the relative
stability of isomers with silicon atom at different position in the metallabenzyne ring due to the relative weak and
polarized metal-silicon multiple bond? There is a interesting founding of interplay between aromaticity and ring strain
in determining the relative stability of metallasilabenzyne and metallasilapentalyne. The effect of carbonyl ligand (CO),
the substituent and different metal centers are investigated to tune the relative stability of osmasilabenzyne and its
isomers. Our findings could be a useful take-home message for experimenters to realize the metallasilabenzyne.
INTRODUCTION
Metallabenzyne, as a fantanstic member of
metallaaromatics
family,
has
stirred a
great deal of interests from chemists for the past few
years. And the chemistry of metallabenzyne has
achieved great developments1 such as various synthetic
routs and reported interesting chemical properties since
its first isolation.2 As the analogue of metallabenzyne,
metallasilabenzyne, that derived from replacement of
the carbyne carbon in metallabenzyne by silicon atom,
has been never synthesized up to now. And recently, we
first theoretically reported the stability and aromaticiy of
osmasilabenzyne which would be a helpful guide for
experimentalists.3 That raised a interesting question why
the metallasilabenzyne so challenge to realize? To the
best of our knowledge, only limited transition metal
silylidyne examples have been synthesized till now.4 And
the first X-ray structure of base-stabilized transition
metal silylene complex was synthesized in 19875 while an
intermediate was reported in 1970. 6 The character of
multiple-bond 7 is prominent issue among numerous
differences8 between carbon and silicon. That is to say,
silicon atom is reluctant to participate in π bonding.9
Weak and highly polarized transition metal multiple
bonds give rise to high reactivities toward
nucleophiles. 10 Great efforts to generate these
unsaturated species were focused on the strategies by
taking advantage of exceptionally bulky substituent and
Lewis base.11
Since the weak and highly polarized transition metal
multiple bonds, one may wonder how about the relative
stability of isomers with silicon atom at different
position in the metallabenzyne ring? Here we report
density functional theory (DFT) study on stability of
metallasilabenzyne and its isomers.
1
Figure 1. Proposed structures of osmasilabenzyne and its
isomers.
COMPUTATIONAL METHOD
All structures of the metallasilabenzyne complexes
were fully optimized at the B3LYP level 12 of density
functional theory (DFT). Frequency calculations were
performed at the same level of theory to confirm no
imaginary frequency. The LanL2DZ13 basis set was used
to describe Fe, Ru, Os, Co, Ru, Ir, Si, P, Cl while the
standard 6-311++G** basis set was used for all other
atoms. Polarization functional were added for P (ζ(d) =
0.340), Cl (ζ(d) = 0.514), Si (ζ(d) = 0.262), Fe (ζ(f) =
2.462), Ru (ζ(f) = 1.235), Os (ζ(f) = 0.886), Co (ζ(f) =
2.780), Rh (ζ(f) = 1.350), Ir (ζ(f) = 0.938).14 The NBO
(natural bond orbital analysis)15 method, as implement
of Gaussian 03, was also employed to examine the
character of the chemical bonding. All the optimized
calculations were performed with Gaussian 03 package.16
RESULTS AND DISCUSSION
Relative Stability of Metallasilabenzyne and its
isomers. To begin with, we carried out DFT calculations
to examine the relative thermodynamic stability of
osmasilabenzyne in comparison with its isomers where
silicon atom is at different positions of the metallacycle.
One can conclude that silicon atom prefer to straightly
bonded to the metal center for lower energy (Figure 2).
That is consisit with the previous study on the
metallsilapentalynes system17 that the isomers of directly
bonded to the metal centers have greater stability for
metal center has diffuse d orbitals and difference
between the C and Si for size and energy of atomic 3s
and 3p orbitals.18 However, the isomer with silicon atom
singly bongded to the metal center is more stable than
osmasilabenzyne which silicon atom occupy the carbyne
carbon position(-19.1 kcal mol-1 for Gibbs Free energy).
The most stable isomer is silicon atom triply bonded to
the metal center in metallasilapentalyne system, the
results are exactly the opposite. What are the reasons for
this difference in relative stabilities of two bonded types
in different systems? Next, we will perform detailed
calculations on this issue and how to tune the relative
stability of two isomers.
Figure 2. Relatively Stability which silicon at different
positions of the metallacycle ring (kcal/mol ） in
osmasilabenzyne and osmasilapentalyne system.
The evaluation of aromaticity and ring strain.
The isomerization stabilization energy (ISE) method
as one of criteria19 to describe the aromaticity and
antiaromaticy was proposed by Schleyer20, one type is
based on the energy difference between a methyl
derivative of the aromatic system and its nonaromatic
exocyclic methylene isomer which is particular effective
for strained systems, the other type is on account of
erengy difference between indene and its isoindene
isomer which is homodesmotic and all carbon atoms in
the metallabicycle ring are sp2 hybridized. We carried
out this two type ISE methods to evaluate the aromticity
of osmasilabenzyne 1a and its isomer 5a by taken its
convenience and efficiency into consideration. As shown
in the Figure 3, the ISE values of osmasilabenzyne 1a are
relatively smaller than those of its singly bonded isomer
5a. The similar results occurred in osmasilapentalyne
system which triply bonded isomers 1b inclined to have
smaller ISE values. That in line with the fact the silicon
is reluctant to participate in  bonding.
Figure 3. (A)ISEs of osmasilabenzyne and its isomer. (B)
ISEs of osmasilapentalyne and its isomer.
When the carbyne carbon is taken the place of an
silicon atom, the ring strain is apparently reduced
2
possibly attribute to the large size of silicon atom. How
about the ring strain of the isomers with silicon atom
singly bonded to the metal centers? We employed
partial optimization as we previous done for
osmasilabenzyne and osmasilapentalyne to evaluate ring
strain. All the fixed angles are exactly equally to their
optimized metallacycle structures(Figure 4). The general
observation from the Figure 4 can be made : the silicon
atom at the triply bonding position(1a, 1b) have
apparently smaller ring strain in comparison with the
counterparts of silicon singly bonded isomers(5a, 7b).
What’s more, that result is more obvious in
osmasilapnetalyne because five-numbered ring has angle
bending to some extent and the significant mangnitudes
of the bending at Os-C triple bond in five-numbered
ring.
Figure 4. The ring strain of osmasilabenzyne,
osmasilapentalyne and their isomers.
Effect of Metal centers. As the first metallabenzne21,
metallabenzyne,
and
metallanaphthalyne
were
synthesized by osmium complexes, we wonder whether
the metal center counts the relative stability of
metallasilabenzyne against its isomer which Si atom
singly bonded to the metal center. The calculated results
of six transition metals centers of groups 8 and groups 9
were shown in Table 1. With regard to ferrasilabenzyne
and cobaltasilabenzyne, that are too unstable to a local
minimum when we optimized. Since there is a
conversion of metallabenzynes to carbene complexes via
migratory insertion reactions,22metallasilabenzyne took
place similar conversion to generate silene complexes
(Supporting information). Among other four metal
centers, Rh gives the smallest free energy differences and
the electronic energy differences even positive. Because
of the positive charge, the metal centers of group 9 in
comparison with metals of group 8 have contracted d
orbitals. And the metal centers tend to have less diffuse
d orbitals as go up the group. That is, the more
contracted d orbitals of metal center leads to decrease
the energy diffenerces of metallasilabenzyne and its
isomer.
Table 1. Thermodynamic Stability of Metallasilabenzyne
in comparison with its isomer which Si atom singly
bonded to the metal center with different metal centers.
M
Fe
Rh+
Ir+
-19.1
-1.0
-10.8
-17.5
0.5
-9.0
Ru
Os
G
-10.2
E
-8.2
Co+
Effect of Substituent on the metallacycle. We
investigated the substituent effect on the metallacycle
for the sake of tuning the relative stability of
metallasilabenzyne and its isomer which Si atom singly
bonded to the metal center. The introduction of
substituent on the metallacycle gives an effectively
stability of metallabenzene23 and metallabenyne24.In this
work, we focus on phosphonium substituent for there is
a great stabilization role on the iso-osmabenzene
complexes,25,osmapyridine,26the first osmapentalyne and
the first osmapentalene. And when the phosphonium
substituent at the ortho position can delocalize the
electron density from the triple bond to get a better
delocalization on the metallacycle and further
stabilization. As shown in Figure 5A, the energy
differences raised (compared to Figure 2) when
introducing the phosphonium substituent at the ortho
position. In comparison with the ISE values of
phosphonium substituted(Figure 5B) and nonsubstitued(Figure 3A) osmasilabenzyne 1a and its isomer
5a which silicon singly bonded to the metal center, the
singly bonded isomer gives relativity larger increased
aromaticity than osmasilabenzyne. Therefore, it is
understand that the bigger energy differences for larger
increased aromaticity in singly bonded isomer.
3
Figure5. The phosphonium substituent on the relative
stability
and
aromaticity(ISE
method)
of
osmasilabenzyne and its isomer.
Effects of Ligands. Our previous study on the
stabilization of osmasilabenzyne shows CO destabilize
the osmasilabenzyne indicated by a smaller ISE value(5
kcal mol-1 ). When CO is empolyed to take place of one
of the chlorides in osmasilabenzyne 1a and its isomer 5a,
it decreasing the energy difference effectively(Figure 6A).
We found the wiberg bond indices of the Os-Si triple
bond and Os-C single bond on 1c ring are reduced by
9.4% and 17.9%, respectively. And the wiberg bond
indices of the Os-C triple bond and Os-Si single bond on
5c ring are reduced by 7.3% and 11.7%. We desigined
such reaction(Figure 6B) in order to explore the
destabilization extent of 1a and 5a as the strong acceptor ligand CO is introduced. According to the
calculated results, one can easy reason that CO gives a
more remarkable destablization of 5a than 1a. This
explains the reason for the reduction in energy
difference of 1a and 5a. Combining with the previously
mentioned of effect of metal centers, we curiously want
to know how about the relative stability of two isomers.
Very interesting, the stability of two isomers
reversed(Figure 6C).
Figure 6. (A)Ligand effects on the relative stability of
osmasilabenzyne 1c and its isomer 5c(B) Desigined
reaction to investigate the destabilizaiton effect of CO.
(C) Combining the effect of CO with metal center Rh.
CONCLUSION
We have performed density functional calculation to
investigate the relative stability of metallasilabenzyne
and its isomers. The carbonyl ligand (CO) and more
contracted d orbitals of metal center can be used to tune
the relative stability of two isomers. In comparison with
ring strain, aromaticity plays a more important role in
the relative stability of osmasilabenzyne and its isomers.
However, ring strain plays a more significant part than
aromaticity in determing the relative stability of
metallasilapentalyne and its isomers. And our findings
would be a useful take-home message for experimenters.
ASSOCIATED CONTENT
Supporting Information
Cartesian coordinates for all the complexes calculated in this
study are available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
4
*E-mail:
jun.zhu@xmu.edu.cn
ACKNOWLEDGMENT
We acknowledge financial support from the Chinese
National Natural Science Foundation (21103142 and
21133007), the National Basic Research Program of China
(2011CB808504), the Program for Changjiang Scholars
and Innovative Research Team in University and the
Fundamental Research Funds for the Central
Universities (2012121021).
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