Synthesis and solid state structures of sterically crowded d0-metallocenes of magnesium, calcium, strontium, barium, samarium, and ytterbium

The base-free metallocenes [1,2,4-(Me3C)3C5H2]2Ca, [1,2,4-(Me3C)3C5H2]2Sr, and [1,2,4-(Me3C)3C5H2]2Ba have been prepared by reaction of MI2and the sodium or potassium salt of the substituted cyclopentadiene anion in THF, while [1,2,4-Me3C)3C5H2]2Mg was prepared from the cyclopentadiene, 1,2,4-(Me3C)3C5H3, and dibutylmagnesium. The structures of the magnesocene and calcocene as well as those of the previously prepared ytterbium and samarium analgoues have been determined by X-ray crystallography. The metallocenes are slightly bent in the solid state with centroid-metal-centroid angles ranging from 163° to 173°. Adducts of the strontocene and samarocene with THF and of the barocene, samarocene, and ytterbocene with 2,6-Me2C6H3NC have also been prepared and characterized. The massive bulk of the [1,2,4-(Me3C)3C5H2]-ligand leads to behavior quite different from that previously observed for base-free metallocenes of the alkaline earths and divalent lanthanides.