A Joint Experimental and Theoretical Study of Cation−π Interactions:  Multiple-Decker Sandwich Complexes of Ferrocene with Alkali Metal Ions (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺)

A systematic study of cation−π interactions between alkali metal ions and the cyclopentadienyl ring of ferrocene is presented. The alkali metal (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) salts of the ditopic mono(pyrazol-1-yl)borate ligand [1,1‘-fc(BMe₂pz)₂]^2- crystallize from dimethoxyethane as multiple-decker sandwich complexes with the M⁺ ions bound to the π faces of the ferrocene cyclopentadienyl rings in an η⁵ manner (fc = (C₅H₄)₂Fe; pz = pyrazolyl). X-ray crystallography of the lithium complex reveals discrete trimetallic entities with each lithium ion being coordinated by only one cyclopentadienyl ring. The sodium salt forms polyanionic zigzag chains where each Na⁺ ion bridges the cyclopentadienyl rings of two ferrocene moieties. Linear columns [−CpR−Fe−CpR−M⁺−CpR−Fe−CpR−M⁺−]_∞ (R = [−BMe₂pz]^-) are established by the K⁺, Rb⁺, and Cs⁺ derivatives in the solid state. According to DFT calculations, the binding enthalpies of M⁺−η⁵(ferrocene) model complexes are about 20% higher as compared to the corresponding M⁺−η⁶(benzene) aggregates when M⁺ = Li⁺ or Na⁺. For K⁺ and Rb⁺, the degree of cation−π interaction with both aromatics is about the same. The binding sequence along the M⁺−η⁵(ferrocene) series follows a classical electrostatic trend with the smaller ions being more tightly bound.