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.