Silicon-Bridged Alkali-Metal and Alkaline-Earth-Metal Metallocene Complexes

The crystal structures of Si-bridged fluorenyl metallocenes with several alkali or alkaline-earth metals have been determined. Me 2 Si(Fl) 2 Li 2 (Fl = fluorenyl) crystallizes from THF as a solvent-separated ion pair: [Me 2 SiFl 2 Li·2THF] - [Li·4THF] + . The Li + cation encapsulated by Me 2 SiFl 2 2 - is bonded to both fluorenyl rings in an η 1 -fashion. The structure of Me 2 SiFl 2 Ca·3THF shows η 3 -bonding of both fluorenyl rings to Ca. The analogous Ba compound crystallizes as a tetrakis-THF solvate Me 2 SiFl 2 Ba·4THF. The unit cell contains two independent molecules with different Ba coordination geometries, (η 5, η 3 ) in one and (η 3, η 3 ) 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 C ipso −Si−C ipso angles and tilting of the fluorenyl ring toward the metal are observed. Instead, the fluorenyl rings tilt away from the metal in order to create room for a more extended solvation of the cation by THF.