Disproportionation and Structural Changes of Tetraarylethylene Donors upon Successive Oxidation to Cation Radicals and to Dications
American Chemical Society
Journal of the American Chemical Society
The stepwise (one-electron) chemical oxidation of the tetraphenylethylene donor and its substituted analogues (D) can be carried out by electron exchange with aromatic cations or antimony(V) oxidants to selectively afford the cation radical (D+•) initially and then the dication (D2+). The ready interchange of the latter establishes the facile disproportionation (i.e., 2D+• ⇌ D2+ + D) that was originally examined by only transient electrochemical techniques. The successful isolations of the crystalline salts of the tetraanisylethylene cation radical (1+•) as well as the tetraanisylethylene dication (12+) allow X-ray diffraction analysis (for the first time) to quantify the serial changes in the molecular structure upon successive oxidations. Five structural parameters (d, l, θ, φ, and q) are identified as quantitative measures of changes in bond (CαCβ, Cαanisyl) lengths, dihedral (CαCβ)/torsional (anisyl) angles, and quinoidal (anisyl) distortion attendant upon the removal of first one-electron and then another electron from the tetraanisylethylene framework. The linear variation of all five parameters in Chart 3 point to a strongly coupled relaxation of tetraanisylethylene (involving simultaneous changes of d, l, θ, φ, and q) to a severely twisted dication. Most noteworthy is the structure of the cation radical 1+• with d, l, θ, φ, and q values that are exactly one-half those of the dication. The complex molecular changes accompanying the transformation: D → D+• → D2+ bear directly on the donor properties and the disproportionation processes of various tetraarylethylenes.