The Fluorenyl Cation

Abstract

The fluorenyl cation is a textbook example for a 4π antiaromatic cation. However, contrasting results have been published on how the annelated benzene rings compensate the destabilizing effect of the 4π antiaromatic five-membered ring in its core. Whereas previous attempts to synthesize this cation in superacidic media resulted in undefined polymeric material only, we herein report that it can be generated and isolated in amorphous water ice at temperatures below 30 K by photolysis of diazofluorene. Under these conditions, the fluorenylidene is protonated by water to give the fluorenyl cation, which could be characterized spectroscopically. Its absorption in the visible-light range matches that previously obtained by ultrafast absorption spectroscopy, and furthermore, its IR spectrum could be recorded. The IR bands in amorphous ice very nicely match predictions from DFT and DFT/MM calculations, suggesting the absence of strong interactions between the cation and surrounding water molecules.

Electronic Characterization of Reaction Intermediates: The Fluorenylium, Phenalenylium, and Benz[f]indenylium Cations and Their Radicals

“…Ab road feature at 515 nm, which was observed after photolysis of 9-diazafluorene in amorphous water ice,w as previously assigned to the absorption of FL9 + , [7] and is close to the 518 nm band detected in the present studies.T he 518 nm system is the most intense absorption observed after deposition of C 13 H 9 + ions produced from fluorene,and hence FL9 + is proposed to be the carrier of the 518 nm system. However,t here are two other cationic origins at 538 and 392 nm, and for acomplete structural assignment, theoretical calculations on the plausible isomers of C 13 H 9 + were carried out.…”

“…[3] Recently,a ni nvestigation of the photochemical products of 9-DAFL in amorphous water ice in the infrared and optical domain confirmed that the broad absorption at 515 nm originates from FL9 + . [7] TheFL9 À anion was the focus of gas-phase photoelectron studies that provided vibrational frequencies for the ground state and the first excited state of neutral FL9. [8,9] This radical, which was produced by electron bombardment and UV photolysis of fluorene and trapped in solid argon, has also been studied by infrared, Raman, and UV/Vis spectroscopy.…”

Use of Charge‐Charge Repulsion to Enhance π‐Electron Delocalization into Anti‐Aromatic and Aromatic Systems

“…In ar ecent report aboutt he unsubstituted 9fluorenyl cation, Sander et al notedt he difficulties associated with generating and observing the unstablef luorenyl cation. [10] Our studies show that acid-promoted cyclizationso fb iaryl ketones (5)p rovide convenient access to these cationic species.…”

Activation of Molecular Hydrogen by Arylcarbenes

“…Thus, the ground state of 1 is switched from triplet to singlet.T his is not only observed in cryogenic matrices at low temperature, [10,11] but also in solution at room temperature. [12] Carbene 2 shows as imilar behavior, [13] whereasi n3 the singlet-triplet splitting is too large to be overcome by hydrogen bonding.H alogen bondings tabilizes the singlet stateso fc arbenes even more than hydrogen bonding, [14] and BF 3 formss tronglys tabilizedd onor-acceptor complexes, [15,16] similart oN H 3 -BF 3 .T hese resultss tress the high basicity of the closed-shell singlet stateso fc arbenes.…”