Synthesis of (bis)Silicon Complexes of [38], [37], and [36]Octaphyrins: Aromaticity Switch and Stable Radical Cation

Abstract

Silicon complexation of a [38]octaphyrin (1) was accomplished by reaction with an excess amount of MeSiCl₃ in the presence of N,N-diisopropylethylamine, thus giving an aromatic [38]octaphyrin bis(silicon) complex 2. This complex was interconvertible with an antiaromatic [36]octaphyrin congener (3) by oxidation with MnO₂ and reduction with NaBH₄ . Curiously, mild oxidation of 2 with ferrocenium hexafluorophosphate afforded a [37]octaphyrin bis(silicon) complex 4 as an stable radical cation that can be stored under ambient conditions in the solid state. Owing to the two NNNCC-five-coordinated Si atoms bearing trigonal bipyramidal geometry, these octaphyrin bis(silicon) complexes take on similar and rigid figure-of-eight structures with different consecutive numbers of conjugated π-electrons (38, 37, and 36), and are all stable.

Keywords: antiaromaticity; aromaticity; porphyrinoids; radicals; silicon.