Exploring the Concept of Aromaticity on Complexes of a Fourfold Benzannulated Cyclopentadienyl Ligand

• Published in: Chemistry : a European journal Vol. 16; no. 4; pp. 1265 - 1270
• Main Authors: Pammer, Frank, Sun, Yu, Weismann, Daniel, Sitzmann, Helmut, Thiel, Werner R.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 25-01-2010; WILEY‐VCH Verlag; Wiley Subscription Services, Inc
• Subjects: aromaticity; binaphthalene; Chemistry; cyclopropanation; ferrocene; helicenes; Ligands; NICS
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200902003

The ligand dibenzo[c,g]fluorenide (Dbf−), combines a cyclopentadienide ligand and 1,1′‐binaphthyl fragment in one molecule. Preliminary investigations confirmed the special electronic situation in this 6π‐electron donor by way of a series of novel transition‐metal complexes. Herein, the electron delocalization was investigated in detail by means of DFT calculations in combination with calculations on the nucleus‐independent chemical shifts (NICS). These results clearly prove that the Dbf− ion gains the largest aromatic stabilization among all benzannulated fluorenides, and there are two almost olefinic CC bonds in this structure. These bonds undergo cyclopropanation when treated with ZnEt2 and ClCH2I in a modified Simmons–Smith reaction.
Enhanced localization of aromaticity in the terminal rings and in the central five‐membered ring of dibenzo[c,g]fluorenide gives rise to a typical CC double bond type of reactivity at two distinct positions of the ligand. This was evaluated theoretically and by cyclopropanation (see example in structure shown).