Dications of Fluorenylidenes. The Relationship between Redox Potentials and Antiaromaticity for Meta- and Para-Substituted Diphenylmethylidenefluorenes

Abstract

[reaction: see text] Electrochemical oxidation of meta-substituted diphenylmethylidenefluorenes (3a-g) results in the formation of fluorenylidene dications that are shown to be antiaromatic through calculation of the nucleus independent chemical shift (NICS) for the 5- and 6-membered rings of the fluorenyl system. There is a strong linear correlation between the redox potential for the dication and both the calculated NICS and sigma(m). Redox potentials for formation of dications of analogously substituted tetraphenylethylenes shows that, with the exception of the p-methyl derivative, the redox potentials for these dications are less positive than for formation of the dications of 3a-g and for dications of p-substituted diphenylmethylidenefluorenes, 2a-g. The greater instability of dications of 2a-g and 3a-g compared to the reference system implies their antiaromaticity, which is supported by the positive NICS values. The redox potentials for formation of the dications of meta-substituted diphenylmethylidenes (3a-g) are more positive than for the formation of dications of para-substituted diphenylmethylidenes (2a-g), indicating their greater thermodynamic instability. The NICS values for dications of 3a-g are more antiaromatic than for dications of 2a-g, which is consistent with their greater instability of the dications of 3a-g. Although the substituted diphenylmethyl systems are not able to interact with the fluorenyl system through resonance because of their geometry, they are able to moderate the antiaromaticity of the fluorenyl cationic system. Two models have been suggested for this interaction, sigma to p donation and the ability of the charge on the substituted ring system to affect delocalization. Examination of bond lengths shows very limited variation, which argues against sigma to p donation in these systems. A strong correlation between NICS and sigma constants suggests that factors that affect the magnitude of the charge on the benzylic (alpha) carbon of the diphenylmethyl cation affect the antiaromaticity of the fluorenyl cation. Calculated atomic charges on carbons 1-8 and 10-13 show an increase in positive charge, and therefore greater delocalization of charge in the fluorenyl system, with increasing electronegativity of the substituent. The change in the amount of positive charge correlated strongly with NICS, supporting the model in which the amount of delocalization of charge is related to the antiaromaticity of the species. Thus, both aromatic and antiaromatic species are characterized by extensive delocalization of electron density.

Magnetic criteria of aromaticity

“…8). Using a combination of experimental and computational tools, they have studied the antiaromaticity of these compounds and have found correlations between NICS values and other indices, such as ASE, 114 HOMA, 115 redox potentials, 116 and calculated and measured 1 H and 13 C chemical shifts. 117,118 In addition, Llagostera and Mills 119 studied a wide range of neutral, anionic and cationic systems of varying aromatic, non-aromatic and antiaromatic characteristics, and found that the different species exhibited respective correlations between SNICS(1) ZZ and magnetic susceptibility exaltation (L).…”

Synthesis and aggregation-induced emission properties of pyridine and pyridinium analogues of tetraphenylethylene

“…As illustrated in Scheme 1, the lithium anion derived from trimethylsilyl-substituted diphenylmethane was condensed with a series of isomeric phenyl pyridyl ketones (pyridyl ¼ 4-pyridyl, 3-pyridyl, or 2-pyridyl) to effect Peterson olenation. 22 The TPVP derivatives 6-8 were obtained in comparable yields. Notably, condensation of diphenylmethane with the same phenyl pyridyl ketones followed by acid-catalyzed (p-TSA) dehydration of the putative carbinol products delivered 6-8 in signicantly lower yields.…”

Tetraphenylethylene imidazolium macrocycle: synthesis and selective fluorescence turn-on sensing of pyrophosphate anions

“…As shown in Scheme 1, the TPE derivative 1, a known AIE compound, 12 was used as a starting material and was easily transformed into dialdehyde 2 using a Duff reaction. With dialdehyde 2 in hand, dialcohol 3 and dichloride 4 were obtained by reduction with NaBH 4 followed by a chlorinating reaction with thionyl chloride.…”