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 of1. The doubly-reduced helicene1(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-)and1(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 of1, the central axis of helicene becomes more compressed upon electron addition (1.42 angstrom in1(4-)vs. 2.09 angstrom in1). This is accompanied by an extra core twist, as the peripheral dihedral angle increases from 16.5 degrees in1to 20.7 degrees in1(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.