Four‐Centre, Multielectron Bonding in Rare‐Earth Germole Sandwich Complexes

Reduction of the germole-ligated sandwich complexes [(η⁵-Cp^Ge)M(η⁵-Cp^ttt)]₂ (1_M, M = Y, Gd, Dy) with one or two equivalents of KC₈/2.2.2-cryptand produces [{(η⁵-Cp^Ge)M(η⁵-Cp^ttt)}₂]⁻ (2_M) and [{(η⁵-Cp^Ge)M(η⁵-Cp^ttt₎}₂]²⁻ (3_M), respectively, as salts of [K(2.2.2-cryptand)]⁺ (Cp^Ge = [GeC₄-2,5-(SiMe₃)₂–3,4-Me₂]²⁻, Cp^ttt = 1,2,4-C₅^tBu₃H₂. X-ray crystallography shows that the bond lengths within the central {M₂Ge₂} rings contract markedly with each reduction. Computational analysis reveals the presence of unusual four-center, multielectron {M₂Ge₂} bonds, with the reduction increasing the germanium–germanium and metal–germanium bond orders while reducing the metal-Cp^ttt bond order. Analysis of 2_Y by EPR spectroscopy reveals delocalization of the unpaired spin across both yttrium centers. Magnetic measurements on radical-bridged 2_Gd show a large exchange coupling constant of −95 cm⁻¹ (−2 J formalism). Single-molecule magnet behavior is found for the dysprosium–germole complexes. Complexes 1_Y, 2_Y, and 3_Y can be interconverted by one-electron oxidation or reduction reactions of 2_Y, which itself can also be formed by comproportionation of 1_Y and 3_Y. The masked divalent reactivity of 3_Y is demonstrated through one-electron reduction of 2,2′-bipyridyl to give [(η⁵-Cp^Ge)Y(η⁵-Cp^ttt)(2,2′-bipy)]⁻ (4_Y) and activation of Ph₂Se₂ to give [(η⁵-Cp^Ge)Y(η⁵-Cp^ttt)(SePh)]⁻ (5_Y).