Theoretical Studies on the Aromaticity of Organometallics
Abstract
Organometallics is one of the hottest issues in chemistry, it is gaining its
popularity. Cyclobutadiene and pentalene are typical antiaromatic compounds,
chemists usually reduce their antiaromaticity by introducing metal fragments. Now,
many metallacyclobutadiene and metallapentalene have been synthesized, recently
cyclobutadiene and pentalene can be stabilized by just one metal fragment in the same
organometallics. Herein, we proposed organometallic structures containing two
cyclobutadienes and one pentalene, our density function theory calculations indicate
that antiaromaticity of cyclobutadiene and pentalene have been reduced dramatically.
Introduction
Aromaticity is a very important but controversial concept, as it is not an directly
physical observable quantity, until now nobody can define it exactly.1,
9
2, 3, 4, 5, 6, 7, 8,
Since benzene was found by Faraday in 1825,10 aromaticity has always interested
many chemists, both experimentally and theoretically, though it still is an ambiguous
concept, they have devoted themselves to it. Aromatic compounds have many special
properties, according to their unique chemical and physical properties. Such as
reactivity criteria3, bond length equalization
magnetic criteria17,18,
19, 20, 21, 22
11 , 12
,
energetic criteria 13 ,
, introduced π-electron ring currents23,
14 , 15 , 16
24, 25
, these
ring currents can be visualized by the anisotropy of the induced current density
(ACID) method.26,
27, 28, 29, 30
. Besides, Hückel’s rule31,
32
is also frequently be used
to estimate aromaticity of π-electron delocalized materials.
In 1979, Thorn and Hoffmann predicted the existence of metallabenzenes,33 later,
Roper synthesized an osmabenzene, as a result they initiated metalloaromaticity. 34
Most current available criteria are not proper to evaluate metalloaromaticity,
especially HOMA . 35 In 2004, Yang and co-workers adjusted ASE methods to
measure the aromaticity of transition metal heterocyclic complexes, including
osmabenzene,
iridabenzene,
cobaltacyclopentadiene,
iridacyclopentadiene
and
tungstacyclobutadiene.36 Later, Frenking applied energy decomposition analysis to
calculate ASE values of various metallabenzenes, they concluded that ligands, metals,
coordination numbers, charge have no influence on their aromatic stabilization.37
However, ASE values calculated by EDA method depend on the reference acyclic
polyenes.38 As ISE is a more accurate method than ASE, chemists adapt ISE for
evaluation of metalloaromaticity. Geerlings employed adjusted ISE to measure
aromaticity of platinabenzenes (Scheme 1, eq (1)), 39 and verifying that they are
aromatic. Last year, Lin and co-workers employed ISE to quantify the aromaticity of
rhenabenzene, iridabenzene (Scheme 1, eq (2)-(3)).40 NICS41,
42
and ACID 43 are
another two efficient methods to measure metalloaromaticity.
Scheme 1 Isomerization (ISE) method applied to metallacycles. The ISE values are
given in kcal/mol.
Cyclobutadiene (CBD) and pentalene are highly reactive and unstable, however
they can be stabilized by introducing a metal fragment.44,
45, 46, 47
Furthermore, Xia
and Zhu reported that CBD and pentalene could be stabilized simultaneously by just
one metal fragment, the products are air and moisture stable.48 They employed ISE
and NICS to measure their aromaticity/antiaromaticity, theoretical calculations
indicated that their antiaromaticity have been greatly reduced. To some extent, the
dramatic decrease of antiaromaticity may result in their enhanced stability. Now, a
metal fragment can stabilize a CBD and a pentalene in the same molecule, there are
no reports whether three or more antiaromatic rings can be stabilized simultaneously
by introducing one metal fragment into the same molecule. Herein we make a report
on the aromaticity such metallacycles.
Computational details
The molecules were structurally optimized at the B3LYP level of theory and
analytical frequency calculations were performed to ensure real minima, and NICS
values were calculated at the same level.49,50 In the B3LYP calculations, standard
6-311++G** basis set was used for the C, H, O, F atoms, while the effective core
potentials (ECPs) of Hay and Wadt with a double-ζ valence basis set (LanL2DZ) were
used to describe the Os, Ru, P atoms for all ISE and TD-DFT calculations. 51
Polarization functions were added for Os (ζ(f) = 0.886), Ru (ζ(f) = 1.235), P (ζ(d) =
0.340). Structural optimizations were performed with the Gaussian 03 software
package,52 while NICS calculations with the Gaussian 09 software package.53
Results and discussion
Figure 1. Proposed organometallics in this study.
We proposed six organometallics as shown in Figure1.
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