From Tetragermacyclobutene to Tetragermacyclobutadiene Dianion to Tetragermacyclobutadiene Transition Metal Complexes

The reaction of 3,4-dichlorotetragermetene derivative 2 with Na₂[Fe(CO)₄] in THF produced a (tetragermacyclobutadiene)tricarbonyliron complex, [{η⁴-(^tBu₂MeSi)₄Ge₄}]Fe(CO)₃ 4, which has a slightly folded Ge₄ ring perhaptocoordinated to the Fe center. Structural and spectral characteristics of 4 show a remarkable π-donating ability of the tetragermacyclobutadiene ligand toward the transition metal, surpassing that of tetrasilacyclobutadiene and cyclobutadiene ligands. Reduction of 2 with KC₈ resulted in exclusive formation of the dipotassium salt of the tetragermacyclobutadiene dianion derivative 3²⁻·[K⁺(thf)₂]₂, representing a rare example of a 6π-electron compound that, on the basis of its structural and magnetic properties, was recognized as a nonaromatic species. Reaction of 3²⁻·[K⁺(thf)₂]₂ with CpCoI₂(PPh₃) produced a (cyclopentadienyl)(tetragermacyclobutadiene)cobalt complex, [{η⁴-(^tBu₂MeSi)₄Ge₄}]CoCp 7, as the first example of a sandwich compound featuring an all-germanium-containing cyclic polyene ligand.