Negative Charge as a Lens for Concentrating Antiaromaticity: Using a Pentagonal “Defect” and Helicene Strain for Cyclizations

Abstract

Incorporation of a five‐membered ring into a helicene framework disrupts aromatic conjugation and provides a site for selective deprotonation. The deprotonation creates an anionic cyclopentadienyl unit, switches on conjugation, leads to a >200 nm red‐shift in the absorbance spectrum and injects a charge into a helical conjugated π‐system without injecting a spin. Structural consequences of deprotonation were revealed via analysis of a monoanionic helicene co‐crystallized with {K+(18‐crown‐6)(THF)} and {Cs+2(18‐crown‐6)3}. UV/Vis‐monitoring of these systems shows a time‐dependent formation of mono‐ and dianionic species, and the latter was isolated and crystallographically characterized. The ability of the twisted helicene frame to delocalize the negative charge was probed as a perturbation of aromaticity using NICS scans. Relief of strain, avoidance of antiaromaticity, and increase in charge delocalization assist in the additional dehydrogenative ring closures that yield a new planarized decacyclic dianion.

Structural deformation and host–guest properties of doubly-reduced cycloparaphenylenes, [n]CPPs2− (n = 6, 8, 10, and 12)

“…Each K + ion is fully wrapped by two benzo-15-crown-5 ether molecules with the K/O crown distances (2.752(2)-2.945(2) A) being comparable to those previously reported. [51][52][53][55][56][57][58] Seven interstitial THF molecules are found to ll the structural voids with no close contacts.…”

“…The K/ O crown distances of 2.788(2)-2.832(2) A are comparable to those previously reported. [51][52][53][54][55] Two interstitial THF molecules were found in the crystal structure of 2$2THF, lling the structural voids with no close contacts.…”

“…51 The coordination of K + ion is completed by an axially bound 18-crown-6 ether molecule (K/ O crown : 2.751(2)-2.867(2) A) and a capping THF molecule (K/ O THF : 2.728(2) A), with all K/O crown and K/O THF distances being close to those previously reported. [51][52][53][54][55] Scheme 2 Chemical reduction of [8]CPP, [10]CPP, and [12]CPP.…”

Predi-XY: a python program for automated generation of NICS-XY-scans based on an additivity scheme

“…Though this may not be surprising for molecules such as the [N]phenylenes, which contain both aromatic and antiaromatic moieties, 11 we have also recently reported on similar behavior in an all-benzenoid molecule. 12 Beyond the fundamental interest into their electronic structure, accurate characterization of the aromatic behavior of PASs also has practical significance. Due to their stable low band-gaps, tunability, and structural features, PASs have long been the workhorses of organic electronics.…”

Compressing Double [7]Helicene by Successive Charging with Electrons

“…[9] Comprehensive studies of electron transfer for carbon bowls, [10] belts, [11] and wires [12] revealed a pronounced dependence on p topology and charge, opening a broad exploration of redox chemistry of curved and twisted molecular nanocarbons. [13][14][15] Recently, we have reported the first examples of chemical reduction of double [5]helicenes (Scheme 1 A and B). For the heteroatom-doped helicene A, an increasing twist of the carbon core upon two-electron charging was found along with the reversibility of reduction.…”