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_πzz). 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_πzz. The latter correlation is improved when the ring sum ∑NICS_πzz 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_πzz. 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, (H₃tpfc)₂COT, 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 N₄ coordination core. The corresponding bis-gallium­(III) complex was prepared as an entry into the potentially rich coordination chemistry of (H₃tpfc)₂COT. 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)₂]­COT­(py)₂ and [(Ga-tpfc)₂]­COT­(py)₄, 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_max 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_max 1250 nm) and reduction (λ_max 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. ¹H 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