Study of the ferrocene–lithium cation interaction by DFT calculations: an in-depth analysis of the existence of a planetary system

Irigoras et al. found two isomers of the ferrocene–lithium cation complex by DFT calculations [Irigoras, A.; Mercero, J. M.; Silanes, I.; Ugalde, J. J. Am. Chem. Soc. 2001, 123, 5040–5043]. The most stable isomer (I) of this complex has Li+ on top of one of the cyclopentadienyls, while in the least stable isomer (II) Li+ binds to the central iron metal. The latter isomer has been characterized as a planetary system in the sense that Li+ has one thermally accessible planar orbit around the central ferrocene moiety. Afterwards, Scheibitz et al. have provided experimental indication for the existence of structure II [Scheibitz, M.; Winter, R. F.; Bolte, M.; Lerner, H.-W.; Wagner, M. Angew. Chem., Int. Ed. 2003, 42, 924–927]. However, their experimental proof is indirect, since it is only based on the synthesis of [3-Li]Li([12]crown-4)2, a crystalline solid, which anion (structure A) has a lithium cation bound to the iron atoms. As these authors have indicated, the existence of structure A could not represent a conclusive proof, because the Li+ placement in this structure could be due to different effects to those of complex II (specifically, the electrostatic field originating from the two anionic dimethylborate bridges). To analyze this subject we have carried out a comprehensive DFT study of the ferrocene–Li+ interaction in this kind of compounds.