Compressing Double [7]Helicene by Successive Charging with Electrons.

Chemical reduction of a benzo-fused double [7]helicene (1) with two alkali metals, K and Rb, provided access to three different reduced states of 1. The doubly-reduced helicene 1 ^2- has been characterized by single-crystal X-ray diffraction as a solvent-separated ion triplet with two potassium counterions. The triply- and tetra-reduced helicenes, 1 ^3- and 1 ^4- , have been crystallized together in an equimolar ratio and both form the contact-ion complexes with two Rb ⁺ ions each, leaving three remaining Rb ⁺ ions wrapped by crown ether and THF molecules. As structural consequence of the stepwise reduction of 1, the central axis of helicene becomes more compressed upon electron addition (1.42 Å in 1 ^4- vs. 2.09 Å in 1). This is accompanied by an extra core twist, as the peripheral dihedral angle increases from 16.5° in 1 to 20.7° in 1 ^4- . Theoretical calculations provided the pattern of negative charge build-up and distribution over the contorted helicene framework upon each electron addition, and the results are consistent with the X-ray crystallographic and NMR spectroscopic data.
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