Macrocyclic Tetraethynylethene Molecular Scaffolding: Perethynylated aromatic dodecadehydro[18]annulenes, antiaromatic octadehydro[12]annulenes, and expanded radialenes

Tetraethynylethene (3,4‐diethynylhex‐3‐ene‐1,5‐diyne) molecular scaffolding provided access to novel macrocyclic nanometer‐sized C‐rich molecules with unusual structural and electronic properties. Starting from cis‐bis‐deprotected cis‐bis(trialkylsilyl)protected tetraethynylethenes, the per(silylethynyl)ated octadehydro[12]annulenes 1 and 2 and the corresponding dodecadehydro[18]annulenes 4 and 5 were prepared by oxidative Hay coupling. X‐Ray crystal‐structure analyses of (i‐Pr)₃Si‐protected 2 and Me₃Si‐protected 4 showed that both annulene perimeters are perfectly planar. Electronic absorption spectral comparisons provided strong evidence that the macro rings in the deep‐purple‐colored 1 and 2 are antiaromatic (4n π‐electrons), whereas those in yellow 4 and 5 are aromatic ((4n + 2) π‐electrons). Although unstable in solution, the antiaromatic compound 2 gave high‐melting crystals in which the individual octadehydro[12]annulene chromophores are isolated and stabilized in a matrix‐type environment formed by the bulky (i‐Pr)₃Si groups. Electrochemical studies demonstrated that the antiaromatic octadehydro[12]annulene 2 undergoes two stepwise one‐electron reductions more readily that the aromatic chromophore 5. This redox behavior is best explained by the formation of an aromatic (4n + 2) π‐electron dianion from 2, whereas 5 loses its aromaticity upon reduction. The Me₃Si derivative 4 was deprotected with borax in MeOH/THF to give the highly unstable hexaethynyl‐dodecadehydro[18]annulene 6, a C₃₀H₆ isomer and macrocyclic precursor to a two‐dimensional all‐C‐network. Deprotection of 2 did not give isolable amounts of tetraethynyl‐octadehydro[12]annulene 3 due to the extreme instability of the latter. Starting from dimeric and trimeric acyclic tetraethynylethene oligomers, a series of expanded radialenes were obtained. They possess large C‐cores with silylethynyl‐protected peripheral valences and can be viewed as persilylated C₄₀ (7), C₅₀ (8), and C₆₀ (9) isomers. These expanded C‐sheets are high‐melting, highly stable, soluble materials which were readily characterized by laser‐desorption time‐of‐flight (LD‐TOF) mass spectrometry. Due to inefficient macrocyclic cross‐conjugation and/or non‐planarity, the extent of π‐electron delocalization in 7–9 is limited to the longest linearly conjugated π‐electron fragment. In agreement with these properties, all three expanded radialenes exhibited similar redox behavior; they are difficult to oxidize but undergo several reversible one‐electron reductions in similar potential ranges. Presumably, the reduced π‐electron delocalization is also at the origin of the particularly high stability of 7–9.