Synthesis and Characterization of Ion Pairs between Alkaline Metal Ions and Anionic Anti-Aromatic and Aromatic Hydrocarbons with π-Conjugated Central Seven- and Eight-Membered Rings

The synthesis, isolation and full characterization of ion pairs between alkaline metal ions (Li(+), Na(+), K(+)) and mono-anions and dianions obtained from 5H-dibenzo[a,d]cycloheptenyl (C(15)H(11) = trop) is reported. According to Nuclear Magnetic Resonance (NMR) spectroscopy, single crystal X-ray analysis and Density Functional Theory (DFT) calculations, the trop(‒) and trop(2−•) anions show anti-aromatic properties which are dependent on the counter cation M(+) and solvent molecules serving as co-ligands. For comparison, the disodium and dipotassium salt of the dianion of dibenzo[a,e]cyclooctatetraene (C(16)H(12) = dbcot) were prepared, which show classical aromatic character. A d(8)-Rh(I) complex of trop(−) was prepared and the structure shows a distortion of the C(15)H(11) ligand into a conjugated 10π -benzo pentadienide unit—to which the Rh(I) center is coordinated—and an aromatic 6π electron benzo group which is non-coordinated. Electron transfer reactions between neutral and anionic trop and dbcot species show that the anti-aromatic compounds obtained from trop are significantly stronger reductants.