Doubly N-Fused [24]Pentaphyrin Silicon Complex and Its Fluorosilicate: Enhanced Möbius Aromaticity in the Fluorosilicate.

Treatment of nonaromatic N-fused [24]pentaphyrin with trichloromethylsilane in the presence of a base afforded doubly N-fused [24]pentaphyrin and its silicon complex. Addition of fluoride ion to the silicon complex led to the formation of its fluorosilicate as an unprecedented monoanionic six-coordinated Si complex of porphyrinoid. Treatment of the fluorosilicate with acid led to the recovery of the silicon complex. The doubly N-fused pentaphyrin, the silicon complex, and the fluorosilicate were all characterized as distinct Möbius aromatic molecules by spectroscopic measurements and X-ray crystallographic analyses. Importantly, the second N-fusion reaction, Si-incorporation and fluoride addition to the Si-atom enhanced the aromaticity of doubly N-fused [24]pentaphyrins in this order. Tamao-Fleming oxidation of the silicon complex gave β-keto doubly N-fused pentaphyrin and triply fused [24]pentaphyrin, which were nonaromatic and Hückel anti-aromatic, respectively.