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 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 trop2−• 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.

Heteroleptic Rare-Earth Tris(metallocenes) Containing a Dibenzocyclooctatetraene Dianion

“…KCp tet was synthesized by deprotonation of HCp tet with K­[N­(SiMe 3 ) 2 ] . KC 8 , (HNEt 3 )­(BPh 4 ), Cp tet 2 Y­(BPh 4 ), and K 2 dbCOT , were prepared according to literature procedures. All NMR spectra were recorded on a 500 MHz Agilent DirectDrive2 500 spectrometer at 25 °C in THF- d 8 .…”

“…To determine a suitable method for the characterization of [Cp tet 2 Y­(η 2 -dbCOT)] − , five functionals and three basis set combinations were tested. The B3LYP and PBE functionals were considered due to their previous implementation in characterizing the dianionic (dbCOT) 2– ligand. , As a result, the hybrid PBE0 functional was assessed. The TPSS and TPSSh functionals were also evaluated due to their performance with transition-metal complexes. , The three basis set combinations included (a) a polarized split valence basis set (def2-SV­(P)) , description for all atoms, (b) the def2-SV­(P) , basis set for Y and the 6-31G­(d,p) − basis set for all C and H atoms, and (c) the def2-SV­(P) , basis set for Y and the 6-31+G­(d,p) − basis set for C and H atoms.…”

“…The neutral 16-π-electron antiaromatic dbCOT ligand has been employed to synthesize complexes, mainly with precious second- and third-row d-block metals . A two-electron reduction of the neutral dbCOT ligand engendered the planar dianion, (dbCOT) 2– , where its aromaticity was quantified through experimental and computational methods. , The 18-π-electron-containing (dbCOT) 2– satisfies Huckel’s rule, providing an inherent stability to the compound, similar to that of the parent (COT) 2– , and has only been crystallographically confirmed as a salt with alkali metals, such as K 2 [dbCOT]. − Aside from group I compounds, molecules comprising (dbCOT) 2– dianions are unknown for the remaining elements, and the redox chemistry of dbCOT has not been exploited in coordination chemistry. With regard to the f-block metals in particular, coordination complexes with dbCOT in any oxidation state have yet to be discovered.…”

“…28 A two-electron reduction of the neutral dbCOT ligand engendered the planar dianion, (dbCOT) 2− , where its aromaticity was quantified through experimental and computational methods. 29,30 The 18-π-electron-containing (dbCOT) 2− satisfies Huckel's rule, providing an inherent stability to the compound, similar to that of the parent (COT) 2− , and has only been crystallographically confirmed as a salt with alkali metals, such as K 2 [dbCOT]. 29−31 Aside from group I compounds, molecules comprising (dbCOT) 2− dianions are unknown for the remaining elements, and the redox chemistry of dbCOT has not been exploited in coordination chemistry.…”

Carbon Ligands: From Fundamental Aspects to Applications

“…In the same family of carbon species characterized by the presence of a sp 3 -carbon atom, selenonium ylides were described from synthetic and structural aspects to synthetic applications by Drabowicz et al [ 25 ]. Anionic carbon ligands were also of primary interest in this issue, as illustrated with the report by Grützmacher et al of alkaline metal salts based on trop (C 5 H 11 ) and dbcot (C 16 H 12 ) moieties and the characterization of a d 8 -Rh(I) complex of the anionic trop ligand [ 26 ].…”