Abstract:Fused benzene rings to antiaromatic compounds generally improve their stability but attenuate their antiaromaticity. The opposite case is now reported. NiII benzonorcorroles were synthesized and the effect of benzo‐fusion on the antiaromaticity was elucidated. The benzo‐fusion resulted in significant decrease of the HOMO–LUMO gaps and enhancement of the paratropic ring current effect. Furthermore, the introduction of the benzo groups induced singlet diradical character in the antiaromatic porphyrinoid.
“…2a), and we have already reported the synthesis of a stable norcorrole Ni(II) complex 18 . The observed reactivity, optical properties, and electrochemical properties of this complex are markedly different from those of conventional aromatic porphyrins 19,20 .Fig. 2Norcorroles and a norcorrole cyclophane.…”
Understanding of interactions among molecules is essential to elucidate the binding of pharmaceuticals on receptors, the mechanism of protein folding and self-assembling of organic molecules. While interactions between two aromatic molecules have been examined extensively, little is known about the interactions between two antiaromatic molecules. Theoretical investigations have predicted that antiaromatic molecules should be stabilized when they stack with each other by attractive intermolecular interactions. Here, we report the synthesis of a cyclophane, in which two antiaromatic porphyrin moieties adopt a stacked face-to-face geometry with a distance shorter than the sum of the van der Waals radii of the atoms involved. The aromaticity in this cyclophane has been examined experimentally and theoretically. This cyclophane exhibits three-dimensional spatial current channels between the two subunits, which corroborates the existence of attractive interactions between two antiaromatic π-systems.
“…2a), and we have already reported the synthesis of a stable norcorrole Ni(II) complex 18 . The observed reactivity, optical properties, and electrochemical properties of this complex are markedly different from those of conventional aromatic porphyrins 19,20 .Fig. 2Norcorroles and a norcorrole cyclophane.…”
Understanding of interactions among molecules is essential to elucidate the binding of pharmaceuticals on receptors, the mechanism of protein folding and self-assembling of organic molecules. While interactions between two aromatic molecules have been examined extensively, little is known about the interactions between two antiaromatic molecules. Theoretical investigations have predicted that antiaromatic molecules should be stabilized when they stack with each other by attractive intermolecular interactions. Here, we report the synthesis of a cyclophane, in which two antiaromatic porphyrin moieties adopt a stacked face-to-face geometry with a distance shorter than the sum of the van der Waals radii of the atoms involved. The aromaticity in this cyclophane has been examined experimentally and theoretically. This cyclophane exhibits three-dimensional spatial current channels between the two subunits, which corroborates the existence of attractive interactions between two antiaromatic π-systems.
“…Large magnetically induced current densities over the pathway in Figure 3a were observed in previous studies of NiNc 15 and Ni II cyanonorcorrole. 16 a NICS contour plot for a NiNc derivative 22 akin to a planar slice through the surface in Figure 1a, b shows deshielding beneath the area of this pathway; it has been argued that in ACID plots for substituted NiNc the main ring current "flows along the inner periphery involving four nitrogen atoms and 14 bonds" 23,24 although inspection of these ACID plots shows significant paratropic currents all over the NiNc unit. The antiaromatic pathway in NiNc (Figure 3a) resembles an antiaromatic pathway with 16 atoms and 16 π electrons in the octaethylporphyrin Zn II dication, 25 established through DFT GIMIC 26 calculations; the absence of two meso-methine units in neutral NiNc produces the same 16 π electron count as that in the octaethylporphyrin Zn II dication.…”
mentioning
confidence: 82%
“…A complication arises from the fact that both molecules are nonplanar. Calculating NICS(0) is less of a problem because the center of a nonplanar ring can be located by averaging the coordinates of its atoms; NICS(0) values have been reported for some norcorrole derivatives. ,,− One way of defining NICS(1) positions for nonplanar rings is to fit a plane to the ring atoms and ring center, and then use the points 1 Å above and below along the normal passing through the ring center . In general, these two points are not equivalent by symmetry and give different NICS(±1) values.…”
Magnetic shielding studies demonstrate that nickel norcorrole (NiNc) and norcorrole (H2Nc) provide unusual examples of stable molecules with high antagonistic levels of antiaromaticity and aromaticity: Both incorporate an antiaromatic "core", a 14-membered cyclic conjugated subsystem with 16 π electrons, surrounded by an aromatic "halo" in the form of a ring of either 14 atoms and 14 π electrons with a new type of homoconjugation (NiNc), or 18 atoms with 18 π electrons (H2Nc).
“…The singlet open‐shell character, which often inhabits o ‐ or p ‐quinoidal subunits in polycyclic hydrocarbons, [6,21–24] is frequently associated with the aromatic or antiaromatic nature due to an inherently small HOMO‐LUMO gap [25–27] . Understanding the interrelation between open‐shell and antiaromatic characteristics is crucial to describe the nature of unique π‐conjugations [28–31] . However, in many neutral 4nπ‐systems, open‐shell and antiaromatic characteristics are incompatible in the same molecule [29,32,33] .…”
The singlet open‐shell character and antiaromaticity are intriguing features in π‐conjugated carbocycles. These two exhibit similar chemical and physical properties. However, they rarely coexist in the same molecule. Understanding the interrelation between the open‐shell and antiaromatic characteristics in the same molecule is crucial to control the electronic properties. Herein we describe the synthesis and characterization of a new member of diareno[a,f]pentalene, benzo[a]naphtho[2,3‐f]pentalene 6. Unlike its isomer 5 with a closed‐shell ground state, 6 exhibits an appreciable open‐shell character and a moderate antiaromatic feature. The behaviors of the open‐shell index (y0) against the difference of the proton chemical signal (Δδ(H1)) between pentalenide dianions/neutral pentalenes for our reported pentalenes 1, 4, 5, and 6 give a thought‐provoking conclusion about the interrelation between open‐shell and antiaromatic characteristics in this series. The mode of the incorporated quinoidal moiety and the formal molecular symmetry are critical to balance these two characteristics.
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