A series of the extremely crowded triarylpnitogens, tris(2,4,6-triisopropylphenyl)phosphine (1), arsine (2), stibine (3), and bismuthine (4), were synthesized by the reaction of 2,4,6-triisopropylphenylcopper(I) with the corresponding pnictogen trichlorides. Introduction [corrected] of the three bulky aryl groups resulted in the unusual structures and redox properties, which were studied by X-ray crystallography and cyclic voltammetry. The triarylpnictogens 1, 2, and 3 had extremely large bond angles around pnictogen atoms (1: 111.5 degrees , 2: 109.2 degrees , 3: 106.7 degrees ), and not only 1 and 2, but also stibine 3 displayed a reversible redox wave in the cyclic voltammograms at very low potentials (1: 0.16 V, 2: 0.50 V, 3: 0.57 V vs Ag/Ag+), which suggests considerable stability of the corresponding cation radicals.

Significant localization of the unpaired electron on the phosphorus atom of phosphasemiquinone radical anion 1  .- was revealed by EPR spectroscopy. This species was generated by reduction of 1, the first stable p-phosphaquinone. Employment of the 3,5-di-tert-butyl-4-oxocyclohexa-2,5-dien-1-ylidene moiety was essential for the synthesis as well as the effective kinetic protection of 1.

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