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.

Stepwise Generation of Mono‐, Di‐, and Triply‐Reduced Warped Nanographenes: Charge‐Dependent Aromaticity, Surface Nonequivalence, Swing Distortion, and Metal Binding Sites

“…These changes are consistent with the molecular dimension calculations (Table S4) showing that the warped nanographene is gradually compressed upon stepwise electron acquisition. A similar compression has been observed upon chemical reduction of the large helical π‐system [12c] …”

Compressing Double [7]Helicene by Successive Charging with Electrons

“…Solvent-free crystals of 1 were obtained by sublimation in vacuo at elevated temperatures (10 days, 300 8C, red blocks isolated in 75 % yield). [17] Unlike the previously reported 1-t (t stands for toluene) crystallized from toluene, [7e] the X-ray diffraction analysis of 1-n (a new "no-solvent" polymorph, Figure 1 a) reveals a tighter packing with the volume per C-atom of 16.2 3 (vs. 19.2 3 in 1-t). As the disordered solvent was squeezed out during refinement of 1-t that left large voids (567 3 ) in the crystal structure (Figure 1 b).…”

Visualizing electron delocalization in contorted polycyclic aromatic hydrocarbons

“…The antiaromatic character of the central rings, and to some extent of the two distal rings of the peropyrene core, in anions D7H 2− , D7H 3− , and D7H 4− could not be perceived from their structural analyses or their electrostatic potential maps, whereas 3D IMS contour maps do this well. The electrostatic potential maps of D7H and its anions 80 showed that the negative charges in the reduced species are likely distributed over the peropyrene cores, while their 3D IMS contour maps show well localized areas of intense negative IMS. Formally, this corresponds to an 8-electron Hückel antiaromatic central ring and poorly delocalized peropyrene core.…”

“…The map of the extended double [7]helicene D7H 79 is comparable showing separated  systems and some non-aromatic rings. The two-, three-, and four-fold reduction of D7H can produce the anions D7H 2-, D7H 3-, and D7H 4-, respectively, 80 the IMS maps of which are shown in Fig. 4b.…”