Dodecamethyl-<i>closo</i>-dodecaborate(2−)

Bis(tetraethylammonium) dodecamethyl-closo-dodecaborate(2−), [NEt4]2[closo-B12Me12], [NEt4]22, was prepared employing modified Friedel−Crafts reaction conditions from [NEt4]2[closo-B12H12], [NEt4]21, trimethylaluminum, and methyl iodide. The [NEt4]22 salt provides sufficient solubility in water to allow the synthesis of the important alkali metal salts A22 (A = Li, Na, K, Rb, Cs) using cation-exchange procedures. The solid state structure of colorless [AsPh4]22 reveals a nearly perfect icosahedral B12 cluster with B−B bonds ranging from 1.785(3) to 1.807(3) Å and B−C bonds of 1.597(3)−1.625(3) Å. In contrast, the crystal structure of dark-red [Py2CH2]2 (obtained from [NEt4]22 and [Py2CH2]Br2) contains a distorted icosahedral dianion [B−B = 1.740(13)−1.811(14) Å, B−C = 1.591(13)−1.704(13) Å]. In the [Py2CH2]2 salt, the dianion 22- and its dipositive dipyridiniomethane counterion form a red charge-transfer complex. One-electron oxidation of 22- by ceric(IV) ammonium nitrate affords the blue, air-stable radical [hypercloso-B12Me12]•-, dodecamethyl-hypercloso-dodecaborate(1−), 2•-, isolated as the PPN salt. X-ray crystallography reveals that the geometries of the B12 clusters observed in hypercloso-[PPN]2 and closo-[AsPh4]22 are identical and essentially undistorted icosahedra. The anion in the [PPN]2 structure contains B−B bonds ranging from 1.784(8) to 1.806(7) Å and a range of B−C bonds from 1.596(7) to 1.616(7) Å.