Four-Centre, Multi-Electron Bonding in Rare-Earth Germole Sandwich Complexes.

Reduction of the germole-ligated sandwich complexes [(η5-CpGe)M(η5-Cpttt)]2 (1M, M = Y, Gd, Dy) with one or two equivalents of KC8/2.2.2-cryptand produces [(η5-CpGe)M(η5-Cpttt)2]- (2M) and [(η5-CpGe)M(η5-Cpttt)2]2- (3M), respectively, as salts of [K(2.2.2-cryptand)]+ (CpGe = [GeC4-2,5-(SiMe3)2-3,4-Me2]2-, Cpttt = 1,2,4-C5tBu3H2. 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-centre, multi-electron {M2Ge2} bonds, with the reduction increasing the germanium-germanium and metal-germanium bond orders, whilst reducing the metal-Cpttt bond order. Analysis of 2Y by EPR spectroscopy reveals delocalization of the unpaired spin across both yttrium centres. Magnetic measurements on radical-bridged 2Gd show a large exchange coupling constant of -95 cm-1 (-2J formalism). Single-molecule magnet behaviour is found for the dysprosium-germole complexes. Complexes 1Y, 2Y and 3Y can be interconverted by one-electron oxidation or reduction reactions of 2Y, which itself can also be formed by comproportionation of 1Y and 3Y. The masked divalent reactivity of 3Y is demonstrated through one-electron reduction of 2,2'-bipyridyl to give [(η5-CpGe)Y(η5-Cpttt)(2,2'-bipy)]- (4Y) and activation of Ph2Se2 to give [(η5-CpGe)Y(η5-Cpttt)(SePh)]- (5Y).