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

Reversible and equilibrium isomerization of η 5 -bis(Me 3 Si-fluorenyl)−rare earth metal complexes to η 6 -bis(Me 3 Si-fluorene-AlR 3 )−rare earth metal complexes, which is a novel type of metallotropic tautomerism, was realized by the addition of AlR 3 to the former complexes. Resulting η 6 -complexes also changed to the initial η 5 -complexes reversibly by the addition of excess THF. η 5 -Bis(Me 3 Si-fluorenyl)Sm(THF) 2 ( 1 ) was prepared by reaction of Me 3 Si-fluorenylpotassium with SmI 2 (THF) 2, and its structure was analyzed by X-ray diffraction. η 6 -Bis(Me 3 Si-fluorene-AlMe 3 )Sm ( 2 ) was synthesized by the reaction of 1 with excess AlMe 3 . The corresponding reaction of excess AlEt 3 with 1 gave η 6 -bis(Me 3 Si-fluorene-AlEt 3 )Sm ( 3 ). The structures of 2 and 3 were determined by X-ray analyses. The reaction of bis(Me 3 Si-fluorenyl)Yb(THF) 2 ( 4 ) with an excess amount of AlMe 3 gave η 5 -(Me 3 Si-fluorenyl)-η 6 -(Me 3 Si-fluorene-AlMe 3 )Yb ( 5 ), where one equimolar AlMe 3 coordinates to the Yb atom through its two Me groups via an agostic interaction. One of the Me 3 Si-fluorenyl groups assumes η 5 -coordination, while the other Me 3 Si-fluorenyl group shows η 6 -coordination. The addition of excess THF to 5 produced (fluorenyl)Yb(THF) 4 /AlMe 4 ( 6 ) in low yield. To understand the primary factor for the alternative η 5 - and η 6 -bonding modes, we have synthesized ( i Pr-indenyl) 2 Yb(THF) 2 ( 7 ) without a Me 3 Si group and with a rather small indenyl ring and examined the reaction with AlR 3 .