Structural Control in the Formation of Multidecker Sandwich Anions of Plumbocene:  The Effects of Encapsulating the Alkali Metal Counterions

Addition of CpM (Cp = C₅H₅; M = Li, Na, K, Cs) to Cp₂E (Cp = C₅H₅; E = Sn, Pb) produces π-anions of general formula [Cp_2x+1E_x]^-. Multidecker anions (with x > 1) can be prepared for Pb if crown or cryptand ligands coordinate the alkali metal cations. The syntheses and structures of the new complexes [Cp₃Sn]^-·[Li(12-crown-4)₂]⁺ (2), [Cp₂Pb(μ-Cp)Na·(15-crown-5)] (3), [Cp₅Pb₂]^-[K(2,2,2-crypt)]⁺·THF (4), [Cp₂Pb(μ-Cp)Pb(μ-Cp)Cs(18-crown-6)] (5), and [Cp₅Pb₂]^-[Li(12-crown-4)₂]⁺·2THF (6) are reported. This study, together with that on [Cp₉Pb₄]^-[Cp₅Pb₂]^-[{Li(12-crown-4)₂⁺}₂] (1), which we have communicated previously, indicates that charge separation and lattice energy considerations subtly control the aggregation of the multidecker Pb(II) anions involved. These factors allow the potential control of the anion homologues formed by changing the solvation sphere of the alkali metal cations. However, as is illustrated by the structures of 1 and 6, which contain the same [Li(12-crown-4)₂]⁺ countercation, in certain cases structural modification can be controlled by the reaction employed.