Experimental Assessment of the Relative Affinities of Benzene and Ferrocene toward the Li⁺ Cation

In order to explore whether benzene or ferrocene presents a more attractive π face to Li⁺ ions, the lithium tetraorganylborate Li[BFc₂Ph₂] has been synthesized, which contains two phenyl rings as well as two ferrocenyl substituents as potential coordination sites (Fc = (C₅H₅)Fe(C₅H₄)). The compound crystallizes from toluene/dibutyl ether as the contact ion pair [Li(OBu₂)][BFc₂Ph₂], with the Li⁺ ion located between the two borylated ferrocenyl cyclopentadienyl rings. This finding indicates ferrocene to be a stronger Li⁺ binder than benzene. In line with this conclusion, the hexaphenyl derivative [(Li(OBu₂))₂][1,1‘-fc(BPh₃)₂] was found to have each of its cyclopentadienyl substituents coordinated to one Li⁺ ion, thereby forming a multiple-decker sandwich complex in the solid state (fc = (C₅H₄)₂Fe).