Experimental and Theoretical Structure Elucidation of the [2?:?1] Complex Ion of Carbo[n]helicene with n=6, 7?and 8?and Ag+

Gas-phase complexes of [n]helicenes with n=6, 7 and 8 and the silver(I) cation are generated utilizing electrospray ionization mass spectrometry (ESI-MS). Besides the well-established [1:1] helicene/Ag+-complex in which the helicene provides a tweezer-like surrounding for the Ag+, there is also a [2:1] complex formed. Density functional theory (DFT) calculations in conjunction with energy-resolved collision-induced dissociation (ER-CID) experiments reveal that the second helicene attaches via n-n stacking to the first helicene, which is part of the pre-formed [1:1] tweezer complex with Ag+. For polycyclic aromatic hydrocarbons (PAHs) of planar structure, the [2:1] complex with silver(I) is typically structured as an Ag+-bound dimer in which the Ag+ would bind to both PAHs as the central metal ion (PAH-Ag+-PAH). For helicenes, the Ag+-bound dimer is of similar thermochemical stability as the n-n stacked dimer, however, it is kinetically inaccessible. Coronene (Cor) is investigated in comparison to the helicenes as an essentially planar PAH. In analogy to the n-n stacked dimer of the helicenes, the Cor-Ag(+)mor_Cor complex is also observed. Competition experiments using [n]helicene mixtures reveal that the tweezer complexes of Ag+ are preferably formed with the larger helicenes, with n=6 being entirely ignored as the host for Ag+ in the presence of n=7 or 8.

Automated summary

Gas-phase complexes of [n]helicenes with n=6, 7 and 8 and the silver(I) cation were generated using electrospray ionization mass spectrometry (ESI-MS). In addition to the well-established [1:1] helicene/Ag+-complex, a [2:1] complex was also formed, where the second helicene attaches to the first helicene via n-n stacking in the pre-formed [1:1] tweezer complex with Ag+.