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

Abstract

The synthesis, isolation and full characterization of ion pairs between alkaline metal ions (Li⁺, Na⁺, K⁺) and mono-anions and dianions obtained from <i>5H</i>-dibenzo[<i>a</i>,<i>d</i>]cycloheptenyl (C₁₅H₁₁ = 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[<i>a,e</i>]cyclooctatetraene (C₁₆H₁₂ = dbcot) were prepared, which show classical aromatic character. A d⁸-Rh(I) complex of trop^- was prepared and the structure shows a distortion of the C₁₅H₁₁ 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.