Alkali Metal-Stabilized 1,3,5-Triphenylbenzene Monoanions Synthesis and Characterization of the Lithium, Sodium, and Potassium Complexes

Alkali metal stabilized monoanions of 1 3 5 triphenylbenzene can be obtained by various reactions starting from the alkali metal Reduction of the Grignard compound [{(2 4 6 Ph-3 C6H2)Mg (dme)Br}(2)(mu O O' dme)] (3) in DME (1 2 dimethoxyethane) with alkali metals yields solvent separated ions [(dme)(3)A](+)[C6H2 2 4 6 Ph-3](-) (A = Li Na (4)) with a localized 2 4 6 triphenylphenyl carbanion via metal-metal exchange Contrarily to this observation radical monoanions of 1 3 5 triphenylbenzene are generated via transmetalation of the Grignard reagent [{2 4 6 Ph-3 C6H2}Mg (thf)(2)Br] (2) with alkali metals in THF (including ether cleavage as the dominant reaction step) Reduction of 2 with lithium yields the solvent separated ion pair [(tmta)(4)Li7Br4O](+)[C6H3 2 4 6 Ph-3](-) (6) (tmta = 1 3 5 trimethyl 1 3 5 triazinane) with an oxygen centered lithium cage as complex cation However reactions of 2 with sodium as well as potassium lead to formation of halogen free contact ion pairs [(thf)(3)Na(mu eta(6) C6H3 2 4 6 Ph-3)] (7) and [(thf)(4)K(mu eta(6) C6H3 Ph-3)] (8) respectively Treatment of 1 3 5 triphenylbenzene in THF with lithium leads to fragmentation of the inner arene core whereas usage of the corresponding heavier alkali metals sodium and potassium respectively also generates 7 or 8 Depending on the metal counterion reduction of the propeller shaped 1 3 5 triphenylbenzene system leads to partial planarization in combination with bond elongations of the inner arene Derivatives 7 and 8 consist of a 3 3' di(phenyl)biphenyl moiety with phenyl substituents twisted by 11 degrees in the same direction (7) or contrary to each other (8) The same high symmetry was found in the solid state as well as in solution which was verified by X ray structure determinations and EPR measurements including simulation studies In the solid state the alkali metal stabilized 1 3 5 triphenylbenzene radical monoanions (6 7 and 8) are pyrophoric and strong black opalescent whereas solutions show solvato and thermochromic behavior