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Alternative η5- and η6-Bonding Modes for Bis(fluorenyl)lanthanide Complexes by Reactions with AlR3 and Succesive Addition of THF

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journal contribution
posted on 11.11.2000, 00:00 by Hiroshi Nakamura, Yuushou Nakayama, Hajime Yasuda, Tatsuya Maruo, Nobuko Kanehisa, Yasushi Kai
Reversible and equilibrium isomerization of η5-bis(Me3Si-fluorenyl)−rare earth metal complexes to η6-bis(Me3Si-fluorene-AlR3)−rare earth metal complexes, which is a novel type of metallotropic tautomerism, was realized by the addition of AlR3 to the former complexes. Resulting η6-complexes also changed to the initial η5-complexes reversibly by the addition of excess THF. η5-Bis(Me3Si-fluorenyl)Sm(THF)2 (1) was prepared by reaction of Me3Si-fluorenylpotassium with SmI2(THF)2, and its structure was analyzed by X-ray diffraction. η6-Bis(Me3Si-fluorene-AlMe3)Sm (2) was synthesized by the reaction of 1 with excess AlMe3. The corresponding reaction of excess AlEt3 with 1 gave η6-bis(Me3Si-fluorene-AlEt3)Sm (3). The structures of 2 and 3 were determined by X-ray analyses. The reaction of bis(Me3Si-fluorenyl)Yb(THF)2 (4) with an excess amount of AlMe3 gave η5-(Me3Si-fluorenyl)-η6-(Me3Si-fluorene-AlMe3)Yb (5), where one equimolar AlMe3 coordinates to the Yb atom through its two Me groups via an agostic interaction. One of the Me3Si-fluorenyl groups assumes η5-coordination, while the other Me3Si-fluorenyl group shows η6-coordination. The addition of excess THF to 5 produced (fluorenyl)Yb(THF)4/AlMe4 (6) in low yield. To understand the primary factor for the alternative η5- and η6-bonding modes, we have synthesized (iPr-indenyl)2Yb(THF)2 (7) without a Me3Si group and with a rather small indenyl ring and examined the reaction with AlR3.