5 research outputs found
Aromatic Stabilization Energy and Magnetic Properties in Fulvalenes: Is There a Connection Between These Two Aromaticity Indices?
The nucleus-independent
chemical shift (NICS) and Coulombic energy
of 15 <i>j</i>,<i>k</i>-fulvalenes (<i>j</i>, <i>k</i> = 3, 5, 7, 9, 11) were investigated. It was
found that in all of the hetero derivatives (<i>j</i> ≠ <i>k</i>) charge transfer between the rings occurs. It occurs when
the one ring contains 4<i>n</i> + 1 and the other 4<i>n</i> + 3 π electrons, forming two rings containing numbers
of π electrons closer to 4<i>n</i> + 2, and also when
both rings contain either 4<i>n</i> + 3 or 4<i>n</i> + 1 π electrons, forming a partially aromatized ring and a
partially antiaromatized ring. Both types of charge transfer are associated
with aromatic stabilization energy. The NICS values are consistent
with the above-described partial aromatization and antiaromatization.
A semiquantitative relationship between the aromatic stabilization
energy and NICS is given
Tetraazaacenes Containing Four-Membered Rings in Different Oxidation States. Are They Aromatic? A Computational Study
A symmetrical tetraazaacene incorporating
a central cyclobutadiene
ring was calculated in different oxidation (hydrogenation) states,
displaying different tautomers and conformers. Geometries, thermodynamics,
and electronic properties were computed, and the aromaticity of all
these species was calculated on a per ring basis by NICS-scans and
NICS-X-scans. The results unveil unexpected and fascinating insights
into the complex aromaticity of those compounds, including a formally
aromatic (!) cyclobutadiene ring
Concurrence between Current Density, Nucleus-Independent Chemical Shifts, and Aromatic Stabilization Energy: The Case of Isomeric [4]- and [5]Phenylenes
The 17 isomers of the [4]- and [5]phenylenes
have been studied
with three different computational levels of current-density analysis
(CDA) and by calculation of the out-of-plane contribution to nucleus-independent
chemical shifts (NICS<sub>πzz</sub>). Current-density maps for
these isomeric phenylenes are typically dominated by strong paratropic
ring currents in four-membered rings. The relative energies of the
isomers, which differ only through the effects of differential strain
and aromaticity, were computed at the B3LYP/6-311G* computational
level. It was found that the three levels of CDA correlate well among
themselves and with NICS<sub>πzz</sub>. The latter correlation
is improved when the ring sum ∑NICS<sub>πzz</sub> for
each isomer is correlated to the ring-current sum ∑<i>J</i> extracted from CDA. The strain-corrected relative energies
of the isomers correlate linearly with ∑NICS<sub>πzz</sub>. In particular, the compatibility of different summed quantities
with easily computed Hückel–London ring currents suggests
a simply calculated measure for dealing with global aromaticity of
polycyclic systems
The Planar Cyclooctatetraene Bridge in Bis-Metallic Macrocycles: Isolating or Conjugating?
A minor modification of the reported
procedure for the synthesis
of a corrole dimer that is fused by the cyclooctatetraene (COT) unit,
(H<sub>3</sub>tpfc)<sub>2</sub>COT, allowed for its isolation in 18%
yield. Of the two redox isomers that this interesting macrocycle does
form, the current focus is on the reduced form, in which each subunit
resembles that of monomeric corroles with a trianionic N<sub>4</sub> coordination core. The corresponding bis-gallium(III) complex was
prepared as an entry into the potentially rich coordination chemistry
of (H<sub>3</sub>tpfc)<sub>2</sub>COT. Both X-ray crystallography
and DFT calculations disclosed that the COT moiety is essentially
planar with very unusual nonalternating C–C bonds. The same
holds true for the bis-gallium(III) complexes [(Ga-tpfc)<sub>2</sub>]COT(py)<sub>2</sub> and [(Ga-tpfc)<sub>2</sub>]COT(py)<sub>4</sub>, obtained with one and two pyridine molecules coordinated to each
metal ion, respectively. The electronic spectra of both the free base
and the gallium(III) complexes display an extremely low energy band
(λ<sub>max</sub> at 720–724 nm), which points toward
extensive π delocalization through the COT bridge. This aspect
was fully addressed by examining the interactions between the two
corrole subunits in terms of electrochemistry and DFT calculations
of the oxidized and reduced macrocycle. The new near-IR bands that
appear upon both oxidation (λ<sub>max</sub> 1250 nm) and reduction
(λ<sub>max</sub> 1780 nm) serve as additional supporting evidence
for this conclusion
The Impact of Antiaromatic Subunits in [4<i>n</i>+2] π‑Systems: Bispentalenes with [4<i>n</i>+2] π‑Electron Perimeters and Antiaromatic Character
Three series of stable, neutral,
π-extended bispentalene
derivatives, with two pentalenes fused to a central benzene or naphthalene
moiety, have been prepared through a modified double carbopalladation
cascade reaction. While these chromophores feature skeletons with
[4<i>n</i>+2] π-electron perimeters, the two 8 π-electron
pentalene subunits strongly influence bonding and spectral properties. <sup>1</sup>H NMR spectra showed large upfield shifts of the protons in
the pentalene moieties, comparable to antiaromatic monobenzopentalenes.
Further investigations on magnetic ring currents through NICS-XY-scans
suggest a global paratropic current and a local diatropic current
at the central benzene ring in two of the series, while the third
series, with a central naphthalene ring, showed more localized ring
currents, with stronger paratropic ring currents on the pentalene
moieties. X-ray diffraction analyses revealed planar bispentalene
cores with large double- and single-bond alternation in the pentalene
units, characteristic for antiaromaticity, and small alternation in
the central aromatic rings. In agreement with TD-DFT calculations,
both optical and electrochemical data showed much smaller HOMO–LUMO
energy gaps compared to other neutral, acene-like hydrocarbons with
the same number of fused rings. Both experimental and computational
results suggest that the molecular properties of the presented bispentalenes
are dominated by the antiaromatic pentalene-subunits despite the [4<i>n</i>+2] π-electron perimeter of the skeletons