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

Abstract

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-Ph3-C6H2)Mg(dme)Br}2(μ-O,O′-dme)] (3) in DME (1,2-dimethoxyethane) with alkali metals yields solvent-separated ions [(dme)3A]+[C6H2-2,4,6-Ph3]− (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-Ph3-C6H2}Mg(thf)2Br] (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)4Li7Br4O]+[C6H3-2,4,6-Ph3]− (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)3Na(μ-η6-C6H3-2,4,6-Ph3)] (7) and [(thf)4K(μ-η6-C6H3-Ph3)] (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° 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.