Silicon-Bridged Alkali-Metal and Alkaline-Earth-Metal Metallocene Complexes.
The crystal structures of Si-bridged fluorenyl metallocenes with several alkali or alk.-earth metals were detd. Me2Si(Fl)2Li2 (Fl = fluorenyl) crystallizes from THF as a solvent-sepd. ion pair: [Me2SiFl2Li.2THF]-[Li.4THF]+ (shown as structure I). The Li+ cation encapsulated by Me2SiFl22- is bonded to both fluorenyl rings in an h1-fashion. The structure of Me2SiFl2Ca.3THF shows h3-bonding of both fluorenyl rings to Ca. The analogous Ba compd. crystallizes as a tetrakis-THF solvate Me2SiFl2Ba.4THF. The unit cell contains two independent mols. with different Ba coordination geometries, h5, h3 in one and (h3, h3) in the other. The structures of the Si-bridged fluorenyl metallocenes reported here do not show the distortions that are typical for Si-bridged d- and f-block metallocenes. No squeezing of the Cipso-Si-Cipso angles and tilting of the fluorenyl ring toward the metal are obsd. Instead, the fluorenyl rings tilt away from the metal to create room for a more extended solvation of the cation by THF.
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