Ligand-to-ligand charge transfer in [(η-C 5 Me 5 )Rh III (η-C 7 H 7 )] 3+ : absorption and emission

Gianetti T, Ostendorf D, Dey S, Mougel V, Grützmacher H. 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 (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[a,e]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.