Aryl ring twists in tris(2,6-dimethoxyphenyl)-z tripod ethers: X-ray analysis of an isostructural series of triarylpropellers

Tris(2,6-dimethoxyphenyl)amine has been synthesized and its molecular and crystal structure determined by X-ray diffraction. This structure completes the series of isosteric compounds Ar₃Z, where Z=B, C^., N, and Ar=2,6-dimethoxyphenyl. Structures for the tris(2-methoxy-6-methylphenyl) borane and tris(2,6-dimethoxyphenyl)methyl cation triiodide (Ar₃C⁺I₃^-) are also reported. The Ar₃B and Ar₃N structures are isomorphous. The triiodide and the earlier reported tetrafluoroborate salt (Ar₃C⁺BF₄^-) are also quite similar, as are the two boranes above and the known trimesitylborane, which all tend toward D₃ symmetric conformations. In contrast, the radical Ar₃C^., intermediate between Ar₃B and Ar₃N, is markedly unsymmetrical. Taken together, these findings support an earlier conjecture that the solid-state conformation of Ar₃C^. does not represent a minimum energy structure for the free radical in solution. Crystal seeding by radical oxidation products is offered as an explanation for the radical's markedly unsymmetrical crystal geometry.