Compressing Double [7]Helicene by Successive Charging with Electrons

Abstract

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 12− 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, 13− and 14−, 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 14− 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 14−. 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.

Chiroptical Properties of Symmetric Double, Triple, and Multiple Helicenes

“…Recently, a stepwise reduction of 27 with alkali metals such as potassium and rubidium was demonstrated to impose more helical twist induced by compression of the central axis. 143 Such structural variation should affect the corresponding chiroptical responses but has not yet been demonstrated experimentally. The π-extended version of 27, which is pyrene- fused [X]-type double [7]helicene 28, was also reported with the experimental and theoretical CD spectra (Figure 16).…”