Structural and Electronic Effects of Stepwise Reduction of a Tetraaryl[3]Cumulene

The chemical reduction of a [3]cumulene ([3]TrTol) is explored using alkali metals. Mono‐ and doubly‐reduced forms of [3]TrTol are isolated as solvent‐separated ion pairs (SSIP) with {Na(18‐crown‐6)}+ and {K(18‐crown‐6)THF2}+ counter ions and crystallographically characterized. This allows analysis of structural parameters of the "naked" anions of [3]TrTol without interference from metal binding. The dianion of [3]TrTol is also isolated as a contact‐ion complex with {Cs(18‐crown‐6)}+ cations, adding the effect of metal coordination to the core. Structural comparisons of anions to the neutral molecule, [3]TrTol, outline monotonic increases in bond‐length alternation (BLA) upon step‐wise reduction. The greatest BLA value is found for contact‐ion complex, which shows an alternating sequence of short and long carbon‐carbon bonds consistent with structure of an alkyne. The cumulenic framework remains planar in all derivatives, contrasting studies of tetraphenyl[3]cumulene.

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