Aromaticity of the cyclobutadiene dianion: Structural characteristics and magnetic properties

The dilithium salts of cyclobutadiene dianions (CBD2-), 1,2-diphenyl-3,4-bis(trimethylsilyl)cyclobutadiene dianion dilithium (3), and 1,3-diphenyl-2,4-bis(trimethylsilyl)cyclobutadiene dianion dilithium (4) have been synthesized by the reaction of the corresponding cyclobutadiene cobalt complexes with lithium metal in tetrahydrofuran (THF). The dilithium salts were isolated as air- and moisture-sensitive crystals, and their molecular structures, containing two molecules of 1,2-dimethoxyethane (DME), were determined by X-ray crystallography. Each of the CBD2- has a monomeric structure and forms contact ion pairs (CIPs) in the crystal. The two Li+ ions are located above and below the plane of the four-membered ring and are bonded to the four quaternary carbon atoms. The cyclobutadienediide ring of the cis-diphenyl CBD2- (3) shows a conspicuous trapezoid structure, whereas that of the trans-diphenyl CBD2- (4) shows a slightly rhomboid structure, The Li-7 NMR spectra of the CBD2- indicate that the two lithium atoms are considerably shielded by a diatropic ring current, resulting from the six-pi-electron system.